Antioxidants for male subfertility

Wiep de Ligny; Roos M Smits; Rebecca Mackenzie-Proctor; Vanessa Jordan; Kathrin Fleischer; Jan Peter de Bruin; Marian G Showell

doi:10.1002/14651858.CD007411.pub5

Antioxidants for male subfertility

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Versión publicada: 04 mayo 2022 Historial de versiones

https://doi.org/10.1002/14651858.CD007411.pub5

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Abstract

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Background

The inability to have children affects 10% to 15% of couples worldwide. A male factor is estimated to account for up to half of the infertility cases with between 25% to 87% of male subfertility considered to be due to the effect of oxidative stress. Oral supplementation with antioxidants is thought to improve sperm quality by reducing oxidative damage. Antioxidants are widely available and inexpensive when compared to other fertility treatments, however most antioxidants are uncontrolled by regulation and the evidence for their effectiveness is uncertain. We compared the benefits and risks of different antioxidants used for male subfertility.

Objectives

To evaluate the effectiveness and safety of supplementary oral antioxidants in subfertile men.

Search methods

The Cochrane Gynaecology and Fertility (CGF) Group trials register, CENTRAL, MEDLINE, Embase, PsycINFO, AMED, and two trial registers were searched on 15 February 2021, together with reference checking and contact with experts in the field to identify additional trials.

Selection criteria

We included randomised controlled trials (RCTs) that compared any type, dose or combination of oral antioxidant supplement with placebo, no treatment, or treatment with another antioxidant, among subfertile men of a couple attending a reproductive clinic. We excluded studies comparing antioxidants with fertility drugs alone and studies that included men with idiopathic infertility and normal semen parameters or fertile men attending a fertility clinic because of female partner infertility.

Data collection and analysis

We used standard methodological procedures recommended by Cochrane. The primary review outcome was live birth. Clinical pregnancy, adverse events and sperm parameters were secondary outcomes.

Main results

We included 90 studies with a total population of 10,303 subfertile men, aged between 18 and 65 years, part of a couple who had been referred to a fertility clinic and some of whom were undergoing medically assisted reproduction (MAR). Investigators compared and combined 20 different oral antioxidants. The evidence was of 'low' to 'very low' certainty: the main limitation was that out of the 67 included studies in the meta‐analysis only 20 studies reported clinical pregnancy, and of those 12 reported on live birth. The evidence is current up to February 2021.

Live birth: antioxidants may lead to increased live birth rates (odds ratio (OR) 1.43, 95% confidence interval (CI) 1.07 to 1.91, P = 0.02, 12 RCTs, 1283 men, I² = 44%, very low‐certainty evidence). Results in the studies contributing to the analysis of live birth rate suggest that if the baseline chance of live birth following placebo or no treatment is assumed to be 16%, the chance following the use of antioxidants is estimated to be between 17% and 27%. However, this result was based on only 246 live births from 1283 couples in 12 small or medium‐sized studies. When studies at high risk of bias were removed from the analysis, there was no evidence of increased live birth (Peto OR 1.22, 95% CI 0.85 to 1.75, 827 men, 8 RCTs, P = 0.27, I² = 32%).

Clinical pregnancy rate: antioxidants may lead to increased clinical pregnancy rates (OR 1.89, 95% CI 1.45 to 2.47, P < 0.00001, 20 RCTs, 1706 men, I² = 3%, low‐certainty evidence) compared with placebo or no treatment. This suggests that, in the studies contributing to the analysis of clinical pregnancy, if the baseline chance of clinical pregnancy following placebo or no treatment is assumed to be 15%, the chance following the use of antioxidants is estimated to be between 20% and 30%. This result was based on 327 clinical pregnancies from 1706 couples in 20 small studies.

Adverse events
Miscarriage: only six studies reported on this outcome and the event rate was very low. No evidence of a difference in miscarriage rate was found between the antioxidant and placebo or no treatment group (OR 1.46, 95% CI 0.75 to 2.83, P = 0.27, 6 RCTs, 664 men, I² = 35%, very low‐certainty evidence). The findings suggest that in a population of subfertile couples, with male factor infertility, with an expected miscarriage rate of 5%, the risk of miscarriage following the use of an antioxidant would be between 4% and 13%.

Gastrointestinal: antioxidants may lead to an increase in mild gastrointestinal discomfort when compared with placebo or no treatment (OR 2.70, 95% CI 1.46 to 4.99, P = 0.002, 16 RCTs, 1355 men, I² = 40%, low‐certainty evidence). This suggests that if the chance of gastrointestinal discomfort following placebo or no treatment is assumed to be 2%, the chance following the use of antioxidants is estimated to be between 2% and 7%. However, this result was based on a low event rate of 46 out of 1355 men in 16 small or medium‐sized studies, and the certainty of the evidence was rated low and heterogeneity was high.

We were unable to draw conclusions from the antioxidant versus antioxidant comparison as insufficient studies compared the same interventions.

Authors' conclusions

In this review, there is very low‐certainty evidence from 12 small or medium‐sized randomised controlled trials suggesting that antioxidant supplementation in subfertile males may improve live birth rates for couples attending fertility clinics. Low‐certainty evidence suggests that clinical pregnancy rates may increase. There is no evidence of increased risk of miscarriage, however antioxidants may give more mild gastrointestinal discomfort, based on very low‐certainty evidence. Subfertile couples should be advised that overall, the current evidence is inconclusive based on serious risk of bias due to poor reporting of methods of randomisation, failure to report on the clinical outcomes live birth rate and clinical pregnancy, often unclear or even high attrition, and also imprecision due to often low event rates and small overall sample sizes. Further large well‐designed randomised placebo‐controlled trials studying infertile men and reporting on pregnancy and live births are still required to clarify the exact role of antioxidants.

PICO

Population

Intervention

Comparison

Outcome

El uso y la enseñanza del modelo PICO están muy extendidos en el ámbito de la atención sanitaria basada en la evidencia para formular preguntas y estrategias de búsqueda y para caracterizar estudios o metanálisis clínicos. PICO son las siglas en inglés de cuatro posibles componentes de una pregunta de investigación: paciente, población o problema; intervención; comparación; desenlace (outcome).

Para saber más sobre el uso del modelo PICO, puede consultar el Manual Cochrane.

Plain language summary

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Antioxidants for male subfertility

Review question
Do supplementary oral antioxidants compared with placebo, no treatment or another antioxidant improve fertility outcomes for subfertile men?

Background
A couple may be considered to have fertility problems if they have been trying to conceive for over a year with no success. Many subfertile men undergoing fertility treatment also take dietary supplements in the hope of improving their fertility. Fertility treatment can be a very stressful time for men and their partners. It is important that these couples have access to high‐certainty evidence that will allow them to make informed decisions on whether to take a supplemental antioxidant. This is especially important as most antioxidant supplements are uncontrolled by regulation. This review aimed to assess whether supplements with oral antioxidants, taken by subfertile men, would increase the chances of a couple to achieve a (clinical) pregnancy confirmed by ultrasound and ultimately the birth of a baby (live birth). This review did not examine the use of antioxidants in men with normal sperm.

Study characteristics

Cochrane authors conducted a review including 90 randomised controlled trials comparing 18 different antioxidants with placebo, no treatment or another antioxidant in a total population of 10,303 subfertile men. The age range of the participants was 18 to 65 years; they were part of a couple who had been referred to a fertility clinic and some were undergoing fertility treatment. The evidence is current to February 2021.

Main results
Antioxidants may be associated with an increased live birth and clinical pregnancy rate. Based on the studied population for live birth, we would expect that out of 100 subfertile men not taking antioxidants, 16 couples would have a baby. In subfertile men taking antioxidants, between 17 and 27 per 100 couples would have a baby. If studies with high risk of bias were removed from the analysis, there was no evidence of increased live birth in the population taking antioxidants. In the people who were studied for clinical pregnancy, we would expect that out of 100 subfertile men not taking antioxidants, 15 couples would have a clinical pregnancy. In subfertile men taking antioxidants, between 20 and 30 per 100 couples would have a clinical pregnancy. Adverse events were poorly reported. Only six studies reported miscarriage. In these studies, miscarriage did not occur more often in the group using antioxidants when compared with the group with placebo or no treatment. However, there is insufficient evidence to draw conclusions about antioxidant use and the risk of miscarriage. The use of antioxidants may be associated with more mild stomach discomfort, with a frequency of 2% in subfertile men not taking antioxidants, and between 2% and 7% in men taking antioxidants. The oral supplements may cause discomforts such as nausea or stomach ache.

Authors' conclusion and certainty of the evidence
Antioxidant supplementation taken by subfertile males of a couple attending a fertility clinic may increase the chance of a live birth, however the overall certainty of evidence was very low from only 12 small to medium‐sized randomised controlled trials. Low‐certainty evidence suggests that clinical pregnancy rates may increase. Overall, there is no evidence of increased risk of miscarriage. Evidence of low certainty suggests that antioxidants may be associated with more gastrointestinal discomfort. Subfertile couples should be advised that overall the current evidence is inconclusive due to the poor reporting of methods, failure to report on live birth and clinical pregnancy rate, imprecision due to low event rates, high number of dropouts and small study group sizes. Large well‐designed randomised placebo‐controlled trials studying infertile men and reporting on pregnancy and live births are still required to clarify the exact role of antioxidants.

Authors' conclusions

Implications for practice

In this review, there is very low‐certainty evidence suggesting that antioxidant supplementation in subfertile males may improve live birth rates for couples attending fertility clinics. Low‐certainty evidence suggests that clinical pregnancy rates may increase as well. Overall, there is no evidence of increased risk of miscarriage. Based on low‐certainty evidence, antioxidants may be associated with more gastrointestinal discomfort. Subfertile couples should be advised that the current evidence is inconclusive based on serious risk of bias.

Implications for research

As opposed to previous updates of this review, we have now included several recently published clinical trials with live birth as an outcome. This shows that investigators acknowledge the need for more trials with clinical outcomes in this field. However, the proportion of well‐powered trials with low risk of bias remains small. Hence, large well‐designed placebo‐controlled randomised trials, focusing on male factor infertility and with live birth as primary outcome, are urgently needed. Researchers should make an effort to register and report important confounding factors including the use of other supplements, lifestyle factors (e.g. diet, physical activity, smoking habits, and alcohol consumption), and living environment.

There is insufficient evidence supporting one type or dose of antioxidants versus another, or a single antioxidant versus a combination of antioxidants.

The side‐effect profile of antioxidant supplements appears to be low and mild. However, conclusions cannot be drawn based on the limited research reporting this outcome. Future trials should include predefined adverse events of antioxidants, with a focus on clinical outcomes such as miscarriage, stillbirth and ectopic pregnancy.

Summary of findings

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Summary of findings 1. Antioxidants compared to placebo or no treatment for patients with male subfertility

Outcomes	*Anticipated absolute effects^ (95% CI)**		Relative effect (95% CI)	№ of participants (studies)	Certainty of the evidence (GRADE)	Comments
Antioxidants compared to placebo or no treatment for patients with male subfertility
Patient or population: patients with male subfertility Setting: clinic Intervention: antioxidants Comparison: placebo or no treatment
Outcomes	Risk with placebo or no treatment	Risk with antioxidants	Relative effect (95% CI)	№ of participants (studies)	Certainty of the evidence (GRADE)	Comments
Live birth rate per couple randomised	162 per 1000	216 per 1000 (171 to 269)	OR 1.43 (1.07 to 1.91)	1283 (12 RCTs)	⊕⊝⊝⊝ VERY LOW ^{1 2 3}
Clinical pregnancy rate per couple randomised	146 per 1000	245 per 1000 (199 to 297)	OR 1.89 (1.45 to 2.47)	1706 (20 RCTs)	⊕⊕⊝⊝ LOW ^{1 3}
Adverse events ‐ Miscarriage	48 per 1000	68 per 1000 (36 to 125)	OR 1.46 (0.75 to 2.83)	664 (6 RCTs)	⊕⊝⊝⊝ VERY LOW ^{1 3 4}
Adverse events ‐ Gastrointestinal	15 per 1000	39 per 1000 (22 to 71)	OR 2.70 (1.46 to 4.99)	1355 (16 RCTs)	⊕⊕⊝⊝ LOW ^{1 3}
*The risk in the intervention group (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). CI: Confidence interval; OR: Peto Odds ratio;
GRADE Working Group grades of evidence High certainty: We are very confident that the true effect lies close to that of the estimate of the effect. Moderate certainty: We are moderately confident in the effect estimate; the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different. Low certainty: Our confidence in the effect estimate is limited; the true effect may be substantially different from the estimate of the effect. Very low certainty: We have very little confidence in the effect estimate; the true effect is likely to be substantially different from the estimate of effect.
¹ Downgraded one level for serious risk of bias: lack of blinding and incomplete accounting of patients and outcome events ² Downgraded one level for suspected publication bias based on the funnel plot ³ Downgraded one level for serious imprecision: less than 400 events ⁴ Downgraded one level for serious imprecision: crossing the line of no effect

Background

Description of the condition

It is believed that 48.5 to 186 million people worldwide are affected by the inability to have children (Boivin 2007; Inhorn 2015; Mascarenhas 2012), with delayed conception affecting 10% to 15% of couples trying to conceive (Evers 2002). The International Glossary on Infertility and Fertility Care (Zegers‐Hochschild 2017) defines infertility as a disease characterised by the failure to establish a clinical pregnancy after 12 months of regular, unprotected intercourse and is used interchangeably with the term subfertility (Zegers‐Hochschild 2017). Subfertility generally describes any form or grade of reduced fertility in couples trying to conceive (Gnoth 2005).

In 2010, it was stated in a World Health Organization (WHO) report, based on data from 190 countries (Mascarenhas 2012), that worldwide 1.9% of women trying to conceive were unable to have a first live birth (primary infertility) and 10.5% with a prior live birth were unable to have an additional live birth (secondary infertility). However, the distribution of male and female causes of infertility has not been well‐defined. Based on a WHO multicentre study from the 1980s, it is suggested that 20% of cases are solely attributed to the male, 38% to the female, 27% to both, and 15% not clearly to either (Comhaire 1987).

In the literature, it is suggested that a male factor is indeed involved in up to 50% of infertility cases (Irvine 1998; Winters 2014). An epidemiological study in the USA showed a mean prevalence of 17.1% of isolated male factor infertility (infertility exclusively caused by a male factor) and 34.6% of total male factor infertility (infertility exclusively or partially caused by a male factor) (Odisho 2014). The true extent of male infertility is likely to be underestimated due to the lack of male evaluation in infertile couples and the heterogeneity of studies (Barratt 2017; Eisenberg 2013). Oxidative stress (OS) has been commonly investigated and found to play a role in 25% to 87% of male factor subfertility (Aitken 1987; Aitken 1989; Aitken 1992; Iwasaki 1992; Mazzilli 1994; Shekarriz 1995; Zini 1993).

In all cells using oxygen to survive, toxins are produced as a consequence. These toxic end‐products are better known as free radicals. Some free radicals are characterised by having higher reactive activity than molecular oxygen, and are therefore called reactive oxygen species (ROS). Excessive production of ROS can lead to cell damage. Therefore, the human body has developed a defence system in which antioxidants play an important role. Antioxidants are capable of reducing the production of free radicals, slowing or preventing the oxidation, and repairing the damage (Mirończuk‐Chodakowska 2018).

The increased levels of ROS are thought to be due to either exogenous or endogenous factors. Exogenous factors could be environmental such as high temperatures, pesticides and pollution, or related to lifestyle such as alcohol consumption, smoking, poor nutrition, and obesity. Endogenous factors are infections, chronic disease, autoimmune disease, and in the male reproductive tract the occurrence of leukocytes (white blood cells) and immature spermatozoa, and varicocele (Alvarez 2003; Tremellen 2008).

Spermatozoa are especially vulnerable to ROS due to the lack of cytoplasm containing antioxidants (Aitken 1994; Ebisch 2007). Also, spermatozoal membranes are rich in polyunsaturated fatty acids (PUFAs) which makes them susceptible for lipid peroxidation by ROS, resulting in decreased flexibility of the sperm membrane and reduction of tail motion (Jones 1973).

This means that OS can lead to impaired male fertility firstly by damaging the sperm membrane, thus affecting the sperm motility and ability to break down the oocyte membrane, and secondly by apoptosis and direct alteration of the sperm DNA (Kodama 1997; Lewis 2013). Deceivingly, men with sperm DNA damage can still have normal seminal parameters but have a poor chance of natural conception (Aktan 2013; Intasqui 2015). Sperm DNA damage or integrity can be measured in several ways, either direct or indirect (Agarwal 2017). Direct tests measure the actual DNA strand breaks, and indirect tests measure the susceptibility of the damaged DNA to denaturation or fragmentation.

The most current sperm DNA fragmentation (SDF) tests used are the terminal deoxynucleotidyl transferase‐mediated dUTP nick‐end labelling (TUNEL) test, the comet assay, and the sperm chromatin structure assay (SCSA). Other options are measurement of 8‐hydroxydeoxyguanosine (8‐OHdG), a by‐product of DNA oxidation, or chemoluminescence assays.

Multiple studies and meta‐analyses show an association between low SDF and clinical pregnancy and live birth rate after intrauterine insemination (IUI), in vitro fertilisation (IVF) or intracytoplasmic sperm injection (ICSI) treatment (Bungum 2004; Sugihara 2020; Collins 2008; Evenson 2006; Li 2006; Osman 2015; Zhang 2015; Zhao 2018). However, Cissen and colleagues found that this association does not imply that SDF tests have a predictive value (Cissen 2016). The test used in these studies are heterogenic and most of them are expensive, complex and lack standardisation and validation (Borini 2017; Cissen 2016).

All the above suggests a leading role of OS in the evaluation and management of male factor infertility. Agarwal and colleagues have even proposed the introduction of a novel condition that comprises subfertile men with abnormal semen characteristics and seminal OS: Male Oxidative Stress Infertility (MOSI) (Agarwal 2019). There are also studies suggesting that sperm DNA damage and OS do not exist in male idiopathic infertility (Hughes 1996; Verit 2006).

Description of the intervention

Antioxidants are substances that inhibit or delay the oxidation of biologically‐relevant molecules, either by directly scavenging free radicals or by chelation of redox metals (Valko 2006). However, the definition is very general and does not specify how a compound may act as an antioxidant (Huang 2018). Antioxidants can be categorised as enzymatic and non‐enzymatic. Enzymatic antioxidants prevent the reaction of ROS with bodily substances and repair cellular damage. Non‐enzymatic antioxidants, which include exogenous or dietary antioxidants, act to modify or deactivate ROS (Mirończuk‐Chodakowska 2018).

The predominant supplementary antioxidants that are studied in male subfertility clinical trials are carnitines, carotenoids, coenzyme Q10 (ubiquinol), cysteine, the micronutrients folate, selenium and zinc, vitamin C, and vitamin E (Eskenazi 2005; Majzoub 2017). Antioxidants can be administered orally as a single or combined supplement. They are widely available and inexpensive when compared to other fertility treatments. However, cost‐benefit analysis is beyond the scope of this review.

Substances with direct antioxidant action

Arginine
Arginine, or L‐arginine, is an amino acid that is required for normal spermatogenesis. It plays a role in the inflammatory response and directly protects against oxidative damage by being a free radical scavenger. Arginine can be derived from meat products, dairy, nuts and seeds. Significant adverse events have not been observed, however arginine is contraindicated for people with a history of genital or oral herpes, asthma or cancer (Appleton 2002).

Carnitines
Carnitine is an antioxidant, with the two most important isomers being called l‐carnitine (LC) and its active form l‐acetylcarnitine (LAC). In the male genital tract carnitines are found in the epididymis, seminal plasma and in spermatozoa (Bøhmer 1978). Carnitines assist sperm metabolism by positively affecting sperm motility and maturation. There might be an association between the concentration of LAC and male fertility (Agarwal 2004a). Animal products like meat, fish, poultry and dairy are the best sources for carnitines. Doses above 3 g/day can give gastrointestinal side effects and malodorous effects (Annals of the New York Academy of Science 2004).

Carotenoids
Carotenoids are pigments found in plants. One of the most important carotenoids is β‐carotene (Ross 2006), a provitamin A, which can directly scavenge ROS. Other carotenoids found in food are lycopene, lutein, and zeaxanthin, however these are not converted into vitamin A. Both in vivo and in vitro, β‐carotene has been shown to protect isolated lipid membranes from peroxidation (Bendich 1989). Healthy young men with a higher carotenoid intake have higher sperm motility, and higher lycopene intake is associated with better sperm morphology (Zareba 2013). However, a review by Grune and colleagues (Grune 2010) stated that there are conflicting results whether β‐carotene has antioxidant properties. Carotenoids come from leafy green vegetables, fruits, and some vegetable oils (Ross 2006). Excess intake of preformed vitamin A can lead to toxicity (hypervitaminosis A). However, excessive ingestion of provitamins such as carotenoids are not associated with vitamin A toxicity, the only side effect is carotenaemia (yellow‐tinged skin).

Coenzyme Q10
Coenzyme Q10 (CoQ10) is a fat‐soluble antioxidant synthesised endogenously and an essential component of the mitochondrial energy metabolism. In its reduced form, CoQH2, ubiquinol, it inhibits protein and DNA oxidation and lipid peroxidation (Littarru 2007). CoQ10 seminal fluid levels are significantly correlated to sperm count and motility, except in men with varicocele (Mancini 1994). Meat, fish, nuts and some oils are the most important dietary sources of CoQ10 due to their relatively high level of fats and mitochondria (Pravst 2010). Reported side effects are mild gastrointestinal symptoms (Bhagavan 2006).

Cysteine
Cysteine plays an important role in glutathione synthesis. N‐acetylcysteine (NAC) is a precursor of the amino acid cysteine and a direct scavenger of ROS. Glutathione becomes depleted when there is OS, and this can be reversed by NAC supplementation (Atkuri 2007). NAC is less toxic and less susceptible to oxidation compared to cysteine itself. Oral administration of NAC up to 8000 mg/day is not known to cause significant adverse events (Atkuri 2007). Less is known about ethylcysteine, however in vivo and animal studies have shown anti‐oxidative effects (Hsia 2016).

Micronutrients (folate, selenium, zinc)
Folate, also known as vitamin B9, is a micronutrient important for the synthesis of DNA, transfer RNA and the amino acids cysteine and methionine. Folic acid, the synthetic form, can scavenge oxidising free radicals, and it inhibits lipid peroxidation (Joshi 2001). Folate is present in green‐leafy vegetables, liver, bread, yeast and fruits (Ebisch 2007). Folic acid doses of 5 mg/day and over can cause abdominal cramps, diarrhoea and rash. Higher doses can even cause altered sleep patterns, irritability, confusion, exacerbation of seizures and nausea (Rogovik 2009).

Zinc is involved as a cofactor in DNA transcription and protein synthesis and has extensive antioxidants properties (Ebisch 2007). Zinc has an important role in testes development, sperm physiological functions and decrease of zinc in seminal plasma is associated with sperm quality (Colagar 2009a). Zinc, like selenium, is absorbed from the soil into plants. Dietary sources rich of zinc are meat products, wheat and seeds.

Magnesium and selenium are different from other antioxidant nutrients because they are involved in the mechanisms of cellular antioxidant defence by increasing the activity of the antioxidant enzyme glutathione peroxidase, and not by directly reacting with oxidant molecules (Burk 2002; Yavuz 2013). It is suggested that both magnesium and selenium deficiency would make humans more susceptible to oxidative injury. Selenium is furthermore essential for normal spermatogenesis (Boitani 2008). Selenium is derived from fish, meat products, dairy, and soil absorption by plants (Navarro‐Alarcon 2008). Early indicators of excess intake are a garlic odour in the breath and a metallic taste in the mouth. The most common clinical signs of chronically high selenium intakes are gastrointestinal symptoms, fatigue, hair loss, joint pain, and nail problems (MacFarquhar 2010). Magnesium is derived from green leafy vegetables, nuts, beans, and cereals (McNeill 1985).

Vitamin E
Vitamin E, also known as the bioactive form α‐tocopherol, has a principal role by being the first defence against oxidant‐induced membrane injury (Traber 2007). Vitamin E is found in vegetable oils and there is a given upper daily limit based on the possible increased bleeding risk (Institute of Medicine 2000).

Vitamin C
Vitamin C, also known as ascorbic acid, is able to diminish DNA damage directly by scavenging free radicals and decreasing formation of lipid hydroperoxides (Padayatty 2003). Ascorbic acid concentrations are 10‐fold higher in seminal plasma compared to blood plasma. Low levels of seminal plasma ascorbic acid are directly related to decreased number of spermatozoa with normal morphology and increased sperm DNA damage (Colagar 2009). Vitamin C is mainly found in fruits and vegetables.

Substances with antioxidant properties

Myo‐inositol
Inositol is a polyalcohol, naturally occurring as nine stereoisomers including myo‐inositol. Myo‐inositol, a "pseudovitamin" and previously known as vitamin B8, plays an important role in cell membrane formation and lipid synthesis. The highest concentration in the genital tract is within the seminiferous tubules. Myo‐inositol is produced by Sertoli cells in response to follicle‐stimulating hormone (FSH) (Lewin 1976). Myo‐inositol is a precursor for the phosphatidyl‐inositol signalling pathway and directly involved in regulation of sperm motility, capacitation and acrosome reaction (Bevilacqua 2015). Myo‐inositol has a role as a possible antioxidant agent by increasing endogenous antioxidant enzymes and directly affecting the mitochondria leading to an increase of the membrane potential (Colone 2010; Condorelli 2017). Corns, beans, fruits, and nuts are the main dietary sources of myo‐inositol (Vazquez‐Levin 2020)

Polyunsaturated fatty acids (PUFAs)
Polyunsaturated fatty acids (PUFAs) are subdivided into omega‐3 (docosahexaenoic acid, DHA), omega‐6 and omega‐9. Omega‐9 is synthesised by animals, but omegas‐3 and ‐6 needs to be supplemented in the diet. The main sources of these are vegetables and fish oils (Wathes 2007). PUFAs increase the plasma fluidity of the sperm membrane. However, this fluidity makes the sperm susceptible to ROS and lipid peroxidation that can damage the sperm. Wathes and colleagues state that "It appears that PUFAs are a two‐edged sword ‐ some are essential, but too many are potentially harmful" (Wathes 2007, page 198). It seems to be that PUFAs have a pro‐oxidant rather than a direct antioxidant effect. Although it is suggested that omega 3 might have a free radical‐scavenging potential (Giordano 2014; Richard 2008).

Resveratrol
Resveratrol is a natural phytoalexin with antioxidant properties. Several in vitro studies with human cryopreserved sperm and in vivo studies in animal models suggest that resveratrol improves sperm motility and enhances antioxidant defences (Branco 2010; Collodel 2011; Ourique 2013). It is naturally found in our diet in the form of grapes, berries, several nuts, and wine (Ourique 2013). Worldwide, resveratrol is better known from research on the effect of daily intake of red wine, "the Mediterranean diet", in cardiovascular disease (Bertelli 2009). Reversible gastrointestinal side effects are reported, however evidence on side effects is limited (Hausenblas 2014).

Vitamin B (complex)
Vitamin B is a water‐soluble vitamin and consists of several precursors and coenzymes such as thiamine (B1), riboflavin (B2) and cobalamin (B12). Vitamin B plays an important role in the homocysteine metabolism. It is suggested that total plasma homocysteine may have a pro‐oxidant effect and may play a role in the release of ROS (Hankey 1999). Increased intake of vitamin B has a homocysteine‐lowering effect, with folate (also known as vitamin B9) shown to have the strongest effect, however vitamins B6, B12, and B2 have all been shown to be independently predictive of plasma homocysteine (Hankey 1999). Vitamin B is mainly found in meat products, other food sources are beans, potatoes, bananas, and mushrooms.

Vitamin D
Vitamin D is a fat‐soluble vitamin, with the natural main source being dermal synthesis (sunlight). The active form of vitamin D is 1,25‐dihydroxyvitamin D, also called vitamin D3. Halicka and colleagues suggest that vitamin D3 has antioxidant activity, mainly by inducing the antioxidant protein superoxide dismutase (Halicka 2012). However, there are no other studies about the antioxidant properties of vitamin D in male fertility. Clearly, vitamin D plays an important role in male fertility and serum levels of vitamin D are positively associated with semen quality (de Angelis 2017). However, most of the studies do not mention the antioxidant properties of vitamin D, but rather relate the effect to the synthesis of sex steroids or the regulation of calcium.

How the intervention might work

It must be noted that a low production of reactive oxygen species (ROS) is physiological and required for adequate sperm function by supporting capacitation, maturation and hyperactivation (Aitken 1994; Du Plessis 2015). However, OS occurs when the balance between ROS production and antioxidant defence is disturbed. This applies to sperm cells in particular.

If OS at the heart of the increased sperm DNA damage and the decrease of pregnancy and live birth rates, then supporting the antioxidant defence system with exogenous antioxidants would seem logical. An extra dietary intake of antioxidants or a healthy diet in general has shown to be strongly associated with semen quality in healthy men (Eskenazi 2005; Irvine 1998; Lewis 1997; Mendiola 2010; Pasqualotto 2001; Salas‐Huetos 2017; Zareba 2013). In conclusion, there is a fine balance between preventing OS by antioxidants, removing excessive amounts of ROS, and maintaining a small amount of ROS for their physiological effect on sperm functions. Since "reductive stress" as a rebound effect of antioxidants has been reported, large or high doses of antioxidants might better be avoided (Dattilo 2016; Ghyczy 2001; Henkel 2019).

Why it is important to do this review

In an effort to enhance fertility, couples are increasingly offered treatment with assisted reproductive techniques (ART). However, these techniques are expensive and do not cure the causes of subfertility, but rather overcome some of its barriers. Since integrity of sperm DNA is one of the major determinants of normal fertilisation and embryo growth in natural and assisted conception (Agarwal 2003; Aitken 2010; Evenson 2006), there is a clear rationale for antioxidant therapy.

One of the other reasons for this review, apart from finding out if antioxidant therapy can overcome some of the barriers of subfertility, is that the global vitamin and supplement market has grown exponentially over the last years. The market value is expected to reach 278 billion USD by 2024 (Grand View Research 2016). The low costs and low apparent risks of supplements are appealing to both, patients and healthcare providers. However, most antioxidants are uncontrolled by regulation and the evidence for their effectiveness is not based on randomised controlled trials (RCTs). Vitamins and supplements are dispensed through various retail outlets, including health food shops, online retailers, health centres, fitness clubs, supermarkets, and pharmacies (Showell 2017).

The purpose of this Cochrane Review is to assess the effectiveness and safety of different antioxidants and dosages used by men of subfertile couples, through evaluation of live birth rates, clinical pregnancy rates and adverse events. This is an update of a review first published in 2011 (Showell 2011), updated in 2014 (Showell 2014), and in 2019 (Smits 2019).

Objectives

To evaluate the effectiveness and safety of supplementary oral antioxidants compared with placebo, no treatment or another antioxidant in subfertile men.

Methods

Criteria for considering studies for this review

Types of studies

Inclusion criteria

Randomised controlled trials (RCTs).
Cross‐over trials are included: however, we only used first‐phase data in the analysis. Achieving outcomes such as pregnancy and live birth would preclude entry of couples into the next trial phase (Dias 2006).

Exclusion criteria

Any quasi‐randomised trials.

Types of participants

Inclusion criteria

Studies that included subfertile men (male factor subfertility),part of a couple who had been referred to a fertility clinic and might or might not be undergoing assisted reproductive techniques (ART), such as in vitro fertilisation (IVF) and intracytoplasmic sperm injection (ICSI), or intrauterine insemination (IUI).
Male factor subfertility was defined as men who were part of a couple referred to a fertility clinic with abnormal semen parameters, including elevated sperm DNA fragmentation or other seminal biomarkers of oxidative stress. Men with subfertility and varicocele were also included

In situations where individuals were randomised again following failed cycles, the data would not be pooled in a meta‐analysis unless individual data could be excluded.

Exclusion criteria

Studies enrolling only men attending a fertility clinic exclusively as the result of female partner or idiopathic infertility.
Studies enrolling men taking any other fertility‐enhancing drugs.
Studies enrolling men who had chemotherapy treatment in the past.

Types of interventions

Inclusion criteria

Any type or dose of oral antioxidant supplementation (individual or combined) that can be obtained without prescription and is not regulated as a pharmaceutical drug, versus placebo or no treatment.
Any type or dose of oral antioxidant supplementation (individual or combined) versus another type or dose of oral antioxidant (head‐to‐head).

Interventions were considered 'combined antioxidants' if they included three or more antioxidants in the intervention arm.

Exclusion criteria

Interventions that included plant extracts (for example garlic) or herbal substances.

Studies that included antioxidants plus a plant extract (for example garlic) were included if the antioxidant agent was the main focus of the investigation.

Definition of antioxidant in male fertility: a substance that has the ability to protect spermatozoa against endogenous oxidative damage by directly neutralising hydroxyl, superoxide, and hydrogen peroxide radicals, chelation of redox metals or by functioning as a component of an antioxidant enzyme.

Types of outcome measures

Primary outcomes

Live birth rate per couple randomised, defined as delivery of a live fetus after 20 completed weeks of gestation. Live births are counted as birth events, i.e. twin live birth is counted as one live birth event.

Secondary outcomes

Clinical pregnancy rate per couple, defined as a viable intrauterine pregnancy, diagnosed by ultrasonographic examination of at least one fetus with a discernable heartbeat. A twin pregnancy is counted as one pregnancy event.
Any adverse event (including miscarriage) reported by the study
Level of sperm DNA fragmentation, defined as percentage (%) of sperm with abnormal DNA integrity estimated by either toluidine blue (TB) staining, sperm chromatin structure assay (SCSA) or terminal transferase dUTP nick end labelling (TUNEL) assay.
Total sperm motility: any sperm movement in any direction (progressive plus forward plus non‐progressive motility), provided as percentage (%).
Progressive sperm motility: sperm with forward progression, defined as WHO category A + B, provided as percentage (%)
Sperm concentration: number of sperm (10⁶)/mL.

Search methods for identification of studies

We searched for all published and unpublished RCTs investigating oral antioxidant supplementation for subfertile men, without language restriction and in consultation with the Gynaecology and Fertility Group (CGF) Information Specialist (MGS).

Electronic searches

We searched the following electronic databases for relevant trials:

The Cochrane Gynaecology and Fertility Group's (CGF) Specialised Register of Controlled Trials, ProCite platform (searched 15 February 2021) (Appendix 1);
the Cochrane Central Register of Controlled Trials (CENTRAL; 2021, issue 2 on 15 February 2021) in the Cochrane Library (now containing records from CINAHL), (Appendix 2);
MEDLINE, Ovid platform (searched from 1946 to 15 February 2021) (Appendix 3);
Embase, Ovid platform (searched from 1980 to 15 February 2021) (Appendix 4);
PsycINFO, Ovid platform (searched from 1806 to 15 February 2021) (Appendix 5);
AMED, Ovid platform (searched from 1985 to 15 February 2021) (Appendix 6);
Epistemonikos, Web platform (searched 18 February 2021) (Appendix 7).

The MEDLINE search was limited by the Cochrane highly sensitive search strategy filter for identifying randomised trials which appears in the Cochrane Handbook of Systematic Reviews of Interventions (Version 5.1.0, Chapter 6, 6.4.11) (Higgins 2011). The Embase and PsychINFO searches were combined with trial filters developed by the Scottish Intercollegiate Guidelines Network (SIGN) (https://www.sign.ac.uk/what‐we‐do/methodology/search‐filters/).

Searching other resources

The following other resources were searched (last search February 2021):

International trial registers: the ClinicalTrials database, a service of the US National Institutes of Health (clinicaltrials.gov/ct2/home) and the and the World Health Organization International Trials Registry Platform search portal (ICTRP) (https://trialsearch.who.int/Default.aspx)) (Appendix 8; Appendix 9);
Google scholar, using the keywords 'antioxidants male infertility' and 'antioxidants sperm random';
Database for Abstracts of Reviews of Effects (DARE) for other reviews on this topic;
'Grey' literature (unpublished and unindexed), through the openGREY database (www.opengrey.eu/) (Appendix 10);
ProQuest Dissertations and Theses (http://search.proquest.com.ezproxy.auckland.ac.nz/pqdtft/advanced?accountid=8424) was also searched (Appendix 11);
Web of Knowledge for conference proceedings and published trials (Appendix 12);
Appropriate journals were handsearched for trial conference abstracts in consultation with the CGF Information Specialist.

We handsearched reference lists of relevant trials and systematic reviews retrieved by the search and contacted experts in the field to obtain additional trials.

Data collection and analysis

Selection of studies

Review authors WL and RS did an initial screen of titles and abstracts retrieved by the search. The search was conducted by MGS and WL. We retrieved the full texts of all potentially eligible studies. Two review authors (WL and RM‐P) independently examined these full‐text articles for compliance with the inclusion criteria and selected eligible studies. We corresponded with study investigators as required, to clarify study eligibility. Disagreements were resolved by discussion. If any reports required translation, we described the process used for data collection. We documented the selection process with a “PRISMA” flow chart (see Figure 1).

Figure 1

Data extraction and management

Three review authors (WL, KF and JB) independently extracted data from eligible studies using a data extraction form designed and pilot‐tested by the authors. Any disagreements were resolved by discussion. Data extracted included study characteristics and outcome data (see data extraction table for details, Characteristics of included studies and Characteristics of excluded studies). Where studies had multiple publications, the review authors collated the multiple reports under a single study ID with multiple references.

We corresponded with study investigators for further data on methods and/or results, as required.

Assessment of risk of bias in included studies

Three review authors (WL, KF and JB) independently assessed the included studies for risk of bias using the Cochrane risk of bias assessment tool to assess: selection (random sequence generation and allocation concealment); performance (blinding of participants and personnel); detection (blinding of outcome assessors); attrition (incomplete outcome data); reporting (selective reporting); and other potential sources of bias (Higgins 2011). Judgements were assigned as recommended in the Cochrane Handbook for Systematic Reviews of Interventions Section 8.5 (Higgins 2011). Disagreements were resolved by discussion; when needed we consulted a third party to achieve agreement (MGS, VJ or RM‐P). We described all judgements fully and present the conclusions in the risk of bias table (Characteristics of included studies), which is incorporated in the interpretation of review findings by means of sensitivity analyses (see below). We sought published protocols.

We took care to search for within‐study selective reporting, for example, trials failing to report outcomes such as live birth or reporting them in insufficient detail to allow inclusion. Where protocols were available, we assessed studies for differences between study protocols and published results.

In cases where included studies failed to identify the primary outcome of live birth, but did report pregnancy rates, we carried out an informal assessment to determine whether pregnancy rates were similar to those in studies that reported live birth.

We considered that the blinding status of participants could influence findings for the outcomes of live birth, pregnancy and adverse events, as antioxidants are easily available, and it would be possible for participants to self‐medicate. Therefore, if the participants were not blinded or the study was not placebo‐controlled, or both, we considered the study to be at high risk of bias.

Measures of treatment effect

We collected dichotomous data for live birth, pregnancy rate, miscarriage and adverse events and for the continuous data for sperm quality measurements we collected mean differences (MDs) and the associated standard deviations (SDs).

Sperm parameter outcomes, if reported, were analysed at the time points of three, six and nine months post‐randomisation. All studies were analysed in this way regardless of whether the participants were treated for three, six or nine months.

Unit of analysis issues

The primary analysis of the outcomes of live birth, pregnancy and adverse events was per couple randomised, counting multiple births as one live birth event. The sperm outcome analyses were per man randomised. Only the first‐phase data from cross‐over trials were included.

Dealing with missing data

We analysed the data on an intention‐to‐treat (ITT) basis as far as possible (i.e. including all randomised participants in analyses, in the groups to which they were randomised). Attempts were made to obtain missing data from the original trialists and the results of author contact are reported in Characteristics of included studies. When data were unobtainable, we undertook imputation of individual values for live birth only; live birth was assumed not to have occurred in participants without a reported outcome. For other outcomes, we analysed only the available data. Any imputation undertaken was subjected to sensitivity analysis (see below).

If studies reported sufficient detail to calculate MDs but gave no information on an associated SD, we assumed the outcome to have a SD equal to the highest SD from other studies within the same analysis.

Assessment of heterogeneity

We considered whether the clinical and methodological characteristics of included studies were sufficiently similar for meta‐analysis to provide a clinically meaningful summary. We assessed statistical heterogeneity by the measure of the I². If an I² was 50% or higher, we assumed high heterogeneity, and conducted a sensitivity analysis. A high I² statistic suggests that variations in effect estimates may be due to differences between trials rather than to chance alone (Higgins 2011).

Assessment of reporting biases

In view of the difficulty of detecting and correcting for publication bias and other reporting biases, we aimed to minimise their potential impact by ensuring a comprehensive search for eligible studies and by being alert for duplication of data. If there were 10 or more studies in an analysis, we used a funnel plot to explore the possibility of small‐study effects (a tendency for estimates of the intervention effect to be more beneficial in smaller studies).

Data synthesis

We conducted statistical analysis of the data using Review Manager 5 (RevMan 2014). We expressed the dichotomous data for live birth, pregnancy rate, miscarriage and adverse events as Peto odds ratios (ORs) with 95% confidence intervals (CIs) and combined them in a meta‐analysis with Review Manager 5 software using the Peto method and a fixed‐effect model (Higgins 2011). Continuous outcomes, i.e. sperm parameters, provided as median and interquartile range (IQR) or median and range were adjusted to mean and SD (Wan 2014). A fixed‐effect model was used on sperm outcomes. The Peto OR has mathematically sound properties that are consistent with benefit or harm and work well in small samples with rare events. This effect measure is appropriate when considering subfertility. For continuous data (for example sperm quality measurements) MDs between treatment groups were calculated with associated SDs and 95% CIs. The results were displayed on forest plots where possible.

We considered pregnancy outcomes to be positive, and higher pregnancy rates of benefit. We considered the outcomes of miscarriage and adverse events to be negative effects, and higher numbers harmful. We combined data for the following comparisons.

Antioxidants versus placebo or no treatment
Antioxidants versus antioxidants (head‐to‐head)

Adverse events as reported in the studies were included in the two comparisons above.

The total sperm motility, progressive sperm motility and concentration outcomes were divided into three groups: measurement after starting treatment, at three, six and nine months or more, as reported by the studies. Studies were analysed together if they reported these outcomes at the same point in time, for example a study that stopped treatment at three months but measured at six or nine months was measured in the same analysis as those that were treated for six or nine months.

We displayed increases in the odds of a particular outcome, which may be beneficial (e.g. live birth) or detrimental (e.g. adverse events), graphically in meta‐analyses to the right of the centre line, and decreases in the odds of a particular outcome to the left of the centre line.

The aim was to define analyses that were comprehensive and mutually exclusive, so that we could slot all eligible study results into one stratum only. We specified comparisons so that any studies falling within each stratum could be pooled for meta‐analysis. Stratification allowed for consideration of effects within each stratum, as well as or instead of an overall estimate for comparison.

If individuals had been randomly re‐assigned after failed cycles, we did not pool the data in a meta‐analysis.

Statistical analysis was performed using Review Manager 5.4.1 (RevMan 2014).

Subgroup analysis and investigation of heterogeneity

Where data were available, we conducted subgroup analyses to determine the separate evidence within the following subgroups.

Studies that included different types of antioxidant
Studies that included couples who were also receiving IVF/ICSI treatment (for the outcomes of live birth and clinical pregnancy)
Over time analysis for sperm outcomes of motility and concentration, at three, six and nine months

If we detected substantial heterogeneity, we explored possible explanations in subgroup analyses (e.g. differing populations) and/or sensitivity analyses (e.g. differing risk of bias). We took any statistical heterogeneity into account when interpreting the results, especially if there was any variation in the direction of effect.

Sensitivity analysis

We conducted sensitivity analyses (using the fixed‐effect model in RevMan software) on the primary outcomes if we detected a high degree of heterogeneity (I² = 50% or more), excluding studies to assess if there is a change in effect:

for studies with a high risk of bias, or
for studies using no treatment as a control group instead of placebo (for outcomes of live birth and clinical pregnancy), or
for studies enrolling men who are part of a couple undergoing IUI, or
enrolling men with varicocele, or
for studies that reported both live birth and clinical pregnancy rate in order to assess any overestimation of effect and reporting bias, or
for studies where results had been imputed, or
for studies that reported remarkably low SDs as the review authors considered that these data were potentially erroneous (a post hoc sensitivity analysis).

Summary of findings and assessment of the certainty of the evidence

We prepared a summary of findings; table using GRADEpro (GRADEpro GDT 2015) and Cochrane methods (Higgins 2011). This table evaluates the overall certainty of the body of evidence for the main review outcomes (live birth, clinical pregnancy, and the adverse events) for the main review comparison (antioxidant compared with placebo or no treatment). We assessed the certainty of the evidence using GRADE criteria: risk of bias, consistency of effect, imprecision, indirectness and publication bias. Judgements about evidence certainty (high, moderate, low or very low) were made by three review authors (WL, KF and JB) working independently, with disagreements resolved by discussion. Judgements were justified, documented, and incorporated into reporting of results for each outcome.

We extracted study data, formatted our comparisons in data tables and prepared a summary of findings table before writing the results and conclusions of our review.

Results

Description of studies

Results of the search

2011 version of review

We assessed 590 abstracts for inclusion from the title and abstract found in a search dated from inception to August 2010. The MEDLINE search produced 406 abstracts; there were six abstracts from CENTRAL, three from CINAHL, 62 from Embase, 107 from the Cochrane Gynaecology and Fertility Group' (CGF) database and three from PsycINFO. Two conference abstracts were found from handsearching the conference proceedings of the European Society for Human Reproduction and Embryology (ESHRE) and the American Society for Reproductive Medicine (ASRM). One title was found from reference lists in reviews. After removal of inappropriate and duplicate studies, we retrieved the full texts of 53 studies. Five non‐English studies were assessed for inclusion: two Chinese, one Bulgarian, one Japanese and one Iranian. The two Chinese studies (Li 2005; Li 2005a), the Japanese study (Akiyama 1999), and the Iranian study (Peivandi 2010) were included in the analysis. The Bulgarian study (Nikolova 2007) was excluded as it did not use random allocation (see Characteristics of excluded studies). We excluded 15 articles and found four ongoing studies in searches of the clinical trial registers.

A total of 34 studies were included in the 2011 version of the review (Showell 2011).

2014 update

We assessed 483 abstracts for inclusion from the title and abstract found in a search dated from 1 August 2010 to 30 January 2014. After duplicates were removed 338 remained. We assessed 34 of these papers in full text.

Eleven of the full‐text reports assessed studies were in a language other than English and required translation, five of these were in Chinese, two in Persian and one each in Japanese, Russian, Italian, and Portuguese (see Acknowledgements for those who helped with translation). Five of the Chinese studies were excluded: three (Chen 2012; Tang 2011; Wang 2010a) due to an inappropriate intervention, one was not randomised (Wu 2012), and one had an inappropriate population (Lu 2010). The Portuguese study (Verzeletti 2012) was excluded as it used a herbal intervention. Five non‐English studies were included: one in Persian (Eslamian 2013), one Japanese (Kumamoto 1988), one Italian (Morgante 2010), one Russian (Sivkov 2011), and one Chinese (Wang 2010).

We excluded 20 articles, and included 14 articles. An updated search was run in August 2014 where six studies (Anarte 2013; Gopinath 2013; Iacono 2014 Nadjarzadeh 2014; Nashivochnikova 2014; Nematollahi‐Mahani 2014) were placed in 'Studies awaiting assessment'. There were six ongoing studies found in the new searches.

We included 14 new trials in the 2014 update: Attallah 2013; Azizollahi 2013; Dimitriadis 2010; Eslamian 2013; Kumamoto 1988; Martinez‐Soto 2010; Morgante 2010; Nadjarzadeh 2011; Poveda 2013; Pryor 1978; Safarinejad 2011; Safarinejad 2012; Sivkov 2011; Wang 2010.

A total of 48 studies were included in the 2014 update (Showell 2014).

2018 update

We assessed 979 abstracts for inclusion from the title and abstract found in a search dated from January 2014 until February 2018. One extra study was found through the grey literature search. After duplicates were removed, 718 articles remained. We assessed 58 of these papers in full text. One of the full‐text articles assessed studies was in Chinese (Deng 2014) and one in Russian (Gamidov 2017); both required translation. We excluded 22 studies (28 articles), and included 19 studies (29 articles). Twelve studies were classified as ongoing studies. One study was placed in 'Studies awaiting assessment' (Goswami 2015).

We removed and excluded four pentoxifylline studies that were previously included in the 2014 update and the original review (Merino 1997; Micic 1988; Safarinejad 2011; Wang 1983). Furthermore, we removed two previously included studies due to the discovery that the population did not meet the inclusion criteria: they included men with idiopathic infertility with normal sperm parameters, and no male factor infertility. (Ciftci 2009; Keskes‐Ammar 2003).

We included 19 new trials in the 2018 update: Barekat 2016; Blomberg Jensen 2018; Boonyarangkul 2015; Busetto 2018; Cyrus 2015; Deng 2014; Ener 2016; Exposito 2016; Gamidov 2017; Gopinath 2013; Haghighian 2015; Haje 2015; Martinez 2015; Mehni 2014; Micic 2019; Pourmand 2014; Raigani 2014; Sharifzadeh 2016; Sofikitis 2016.

A total of 61 studies were included in the 2018 update (Smits 2019).

2021 update

We assessed 1445 abstracts for inclusion from the title and abstract found in a search dated from February 2018 until February 2021. After duplicates were removed, 1055 articles remained. We assessed 42 of these papers in full text.

Three of the full‐text articles assessed studies were in Chinese (Cheng 2018; Sun 2018; Zhou 2016) and three were in Russian (Gamidov 2019; Popova 2019; Vinogradov 2019); all required translation. One study was found eligible through reference checking and was included (Safarinejad 2011b). In total, we excluded nine articles and included 29 studies (34 articles). One study was placed in "Studies awaiting classification", because of unclear study population (Kuzmenko 2018). See the PRISMA flow chart (Figure 1).

One previously excluded study was added as a sub‐study to an included study (Raigani 2014).

In the current update, six of the 12 previously ‘ongoing studies’ were included (Amini 2020; Bahmyari 2021; Eslamian 2020; Joseph 2020; Kumalic 2020; Steiner 2020). One study remained as an ongoing study (NCT03337360). The manuscript of one trial was submitted, but not yet published and was therefore placed in “Studies awaiting classification” (NCT01407432). Three other former ongoing studies were placed in “Studies awaiting classification” with a status of “completed” and “recruitment stopped” in the trial registry (DRKS00011616; NCT00975117; NCT01828710). One former ongoing study was excluded, because of withdrawal on the trial registry website (NCT03104998).

The authors from the one study placed in "Studies awaiting assessment" in the previous update (Goswami 2015) were contacted and confirmed that the study was a randomised clinical trial.

We added 11 new ongoing studies (CTRI/2019/03/018303; IRCT20120215009014N322; IRCT20140622018187N9; IRCT20190406043177N1; IRCT20190714044209N1; IRCT20200911048689N1; NCT03634644; NCT04193358; NCT04256278; NCT04509583; PACTR201802003076341).

We included 29 new studies (34 articles) in this update: Abbasi 2020; Alahmar 2019; Alahmar 2020; Amini 2020; Ardestani 2019; Bahmyari 2021; Cheng 2018; Eslamian 2020; Gamidov 2019; Gonzalez‐Ravina 2018; Goswami 2015; Huang 2020; Joseph 2020; Kizilay 2019; Kopets 2020; Korshunov 2018; Kumalic 2020; Lu 2018; Nouri 2019; Popova 2019; Saeed Alkumait 2020; Safarinejad 2011b; Schisterman 2020; Steiner 2020; Stenqvist 2018; Sun 2018; Tsounapi 2018; Vinogradov 2019; Zhou 2016.

A total of 90 studies have been included in this update (Characteristics of included studies). A total of 67 studies were excluded (Characteristics of excluded studies).

Included studies

Study design and setting

The studies came from 31 different countries. Twenty‐one studies were from Iran (Abbasi 2020; Amini 2020; Ardestani 2019; Azizollahi 2013; Bahmyari 2021; Barekat 2016; Cyrus 2015; Eslamian 2013; Eslamian 2020; Haghighian 2015; Mehni 2014; Nadjarzadeh 2011; Nouri 2019; Peivandi 2010; Pourmand 2014; Raigani 2014; Safarinejad 2009; Safarinejad 2009a; Safarinejad 2011b; Safarinejad 2012; Sharifzadeh 2016). Ten studies were based in Italy (Balercia 2005; Balercia 2009; Biagiotti 2003; Busetto 2018; Cavallini 2004; Galatioto 2008; Lenzi 2003; Lenzi 2004; Lombardo 2002; Morgante 2010). Nine studies were from China (Cheng 2018; Deng 2014; Huang 2020; Li 2005; Li 2005a; Lu 2018; Sun 2018; Wang 2010; Zhou 2016). Six were from Russia (Gamidov 2017; Gamidov 2019; Korshunov 2018; Popova 2019; Sivkov 2011; Vinogradov 2019), four from Iraq (Alahmar 2019; Alahmar 2020; Haje 2015; Saeed Alkumait 2020), and four from the USA (Dawson 1990; Schisterman 2020; Sigman 2006; Steiner 2020). Three studies each were from India (Gopinath 2013; Goswami 2015; Joseph 2020), Japan (Akiyama 1999; Dimitriadis 2010; Kumamoto 1988), the UK (Kessopoulou 1995; Pryor 1978; Scott 1998) and Spain (Exposito 2016; Gonzalez‐Ravina 2018; Martinez‐Soto 2010). Two studies each were from Kuwait (Omu 1998; Omu 2008), Greece (Sofikitis 2016; Tsounapi 2018) and Turkey (Ener 2016; Kizilay 2019). A single study was set in each of the following countries: Australia (Tremellen 2007), Belgium (Zalata 1998), Canada (Conquer 2000), Denmark (Blomberg Jensen 2018), Egypt (Attallah 2013), France (Greco 2005), Germany (Rolf 1999), Hungary (Zavaczki 2003), Mexico (Martinez 2015), the Netherlands (Wong 2002), Panama (Poveda 2013), Saudi Arabia (Suleiman 1996), Serbia (Micic 2019), Slovenia (Kumalic 2020), Sweden (Stenqvist 2018), Thailand (Boonyarangkul 2015), Tunisia (Nozha 2001), and Ukraine (Kopets 2020).

All included studies were randomised. Five studies had a randomised cross‐over design (Akiyama 1999; Kessopoulou 1995; Lenzi 2003; Peivandi 2010; Pryor 1978). In the meta‐analysis only the first phase data were used as all studies reported first and second phase data separately. The remaining 85 studies used a randomised parallel group design. One study (Li 2005) had a large imbalance between the intervention and control groups at the randomisation stage; 150 men were randomised, 90 into the treatment group and 60 into the control group. This appeared to be a blocked 3:2 allocation ratio. This method of randomisation was not explained in the report. Attempts were made to contact the author, but there has been no reply. Fifteen studies (Biagiotti 2003; Cavallini 2004; Conquer 2000; Dawson 1990; Gamidov 2017; Gopinath 2013; Goswami 2015; Kumamoto 1988; Martinez 2015; Mehni 2014; Raigani 2014; Saeed Alkumait 2020; Scott 1998; Sofikitis 2016; Zalata 1998) were three‐armed, 11 (Azizollahi 2013; Balercia 2005; Boonyarangkul 2015; Cheng 2018; Eslamian 2020; Gonzalez‐Ravina 2018; Haje 2015; Omu 2008; Poveda 2013; Safarinejad 2009; Wong 2002) were four‐armed and one study was five‐armed (Tsounapi 2018).

The duration of the treatment period ranged from three weeks with a three‐week follow up (Dawson 1990) to 12 months treatment (Ener 2016). The longest follow‐up periods were in the studies by Blomberg Jensen and Safarinejad with respectively a five‐month (Blomberg Jensen 2018) and six and a half‐month (Safarinejad 2009a) treatment duration and both with 14 months of follow‐up. Ten studies reporting on either live birth rate or clinical pregnancy rate, only mentioned follow‐up consultations during their treatment, however they did not report the length of follow‐up after treatment (Azizollahi 2013; Attallah 2013; Barekat 2016; Busetto 2018; Gamidov 2019; Kessopoulou 1995; Omu 1998; Suleiman 1996; Tsounapi 2018; Zhou 2016).

Funding sources were stated by 36 studies (Abbasi 2020; Amini 2020; Bahmyari 2021; Barekat 2016; Blomberg Jensen 2018; Busetto 2018; Cheng 2018; Conquer 2000; Deng 2014; Eslamian 2013; Eslamian 2020; Haghighian 2015; Joseph 2020; Kessopoulou 1995; Kopets 2020; Kumalic 2020; Lenzi 2003; Lombardo 2002; Martinez‐Soto 2010; Mehni 2014; Micic 2019; Nadjarzadeh 2011; Nouri 2019; Omu 1998; Peivandi 2010; Poveda 2013; Raigani 2014; Rolf 1999; Saeed Alkumait 2020; Safarinejad 2012; Schisterman 2020; Sharifzadeh 2016; Steiner 2020; Stenqvist 2018; Wang 2010; Zavaczki 2003). Eight of these studies stated that funding was from a commercial source (Abbasi 2020; Busetto 2018; Conquer 2000; Kumalic 2020; Martinez‐Soto 2010; Micic 2019; Safarinejad 2012; Stenqvist 2018), and the remaining 28 obtained funding through non‐commercial avenues or university grants. Nine studies specifically reported no funding (Cyrus 2015; Gonzalez‐Ravina 2018; Gopinath 2013; Haje 2015; Huang 2020; Lombardo 2002; Popova 2019; Pourmand 2014; Safarinejad 2011b). Forty‐five studies did not mention any funding sources.

Participants

The 90 studies included 10,303 subfertile men, 6262 in the intervention groups and 4041 men in the control groups. The age range of the participants was 18 to 65 years. Studies included couples who had attended a fertility clinic, with a fertile partner and had been trying to conceive with regular intercourse for over one year. Most men in the included studies had a deficient level of spermatozoa in the seminal fluid (oligospermia) or a low motility of sperm in the seminal fluid (asthenospermia). Five studies included men with an increased level of DNA fragmentation or oxidative stress (Akiyama 1999; Gamidov 2019; Goswami 2015; Greco 2005; Stenqvist 2018), and one study included men with low acrosin activity (Sun 2018). Three studies also included fertile (Wong 2002) or normospermic men (Exposito 2016, Schisterman 2020) with subgroup analysis. Studies excluded men with any inflammatory disease, antibody problems or chromosomal problems; and most studies stated that they did not enrol men who smoked, took any additional medication or drank alcohol.

Two studies enrolled men with varicocele (Busetto 2018; Cavallini 2004), 10 studies enrolled men post‐varicocelectomy (Abbasi 2020; Ardestani 2019; Azizollahi 2013; Barekat 2016; Cyrus 2015; Ener 2016; Gamidov 2017; Kizilay 2019; Lu 2018; Pourmand 2014), and one study enrolled men with chronic prostatitis (Sivkov 2011). Eight studies (Exposito 2016; Joseph 2020; Kessopoulou 1995; Kumalic 2020; Popova 2019; Schisterman 2020; Sigman 2006; Tremellen 2007) enrolled men who, as part of a couple, were undergoing in vitro fertilisation (IVF)/intracytoplasmic sperm injection (ICSI). One study specifically enrolled men who were undergoing ICSI with sperm obtained with testicular extraction (TESE) (Korshunov 2018). Three studies enrolled men who were part of a couple undergoing intrauterine insemination (IUI) (Attallah 2013; Schisterman 2020; Steiner 2020).

Further details of inclusion and exclusion criteria are available in Characteristics of included studies.

Interventions

A wide variety of antioxidants were used in the included studies. Comparisons covered antioxidants versus placebo or no treatment and head‐to‐head comparisons (antioxidant versus antioxidant).

The comparison 'antioxidants versus placebo or no treatment' included the following antioxidants: arginine, carnitines (L‐carnitine, L‐acetyl carnitine, L‐carnitine plus L‐acetyl carnitine), carotenoids (β‐carotene), coenzyme Q10 (CoQ10), cysteines (ethylcysteine and N‐acetylcysteine (NAC)), folic acid, magnesium, melatonin, polyunsaturated fatty acids (PUFAs) (alpha‐lipoic‐acid and docosahexaenoic acid (DHA)), resveratrol, selenium, vitamin B, vitamin C, vitamin D with calcium, vitamin E and zinc.

Combined antioxidants were used in 23 studies. They were labelled as Proxeed Plus (Busetto 2018; Micic 2019), Menevit (Tremellen 2007), Selznic (Sivkov 2011), SpermActin‐forte (Gamidov 2017; Gamidov 2019), Spermotrend (Poveda 2013), Androdos (Popova 2019), Androferti (Stenqvist 2018), Profertil (Tsounapi 2018), and Brudy Plus (Vinogradov 2019). Eleven of these 23 studies used combined antioxidants without any brand name or labelling; vitamin E combined with selenium and folic acid (Ardestani 2019, Bahmyari 2021), a combination of vitamin E, C and zinc (Joseph 2020), l‐carnitine, acetyl‐Lcarnitine, vitamin C, folic acid, selenium, coenzyme Q10 and vitamin B12 (Kizilay 2019), “Verum TDS”: l‐carnitine, l‐acetyl‐carnitine, l‐arginine, glutathione, coenzyme Q10, zinc, vitamin B9, vitamin B12 and selenium (Kopets 2020), an antioxidant supplement containing vitamin E, vitamin C, selenium and l‐carnitine (Korshunov 2018), vitamin C/D/E, selenium, L‐carnitine, zinc, folic acid and lycopene (Steiner 2020), "N‐acetylcysteine (NAC) with vitamins and micronutrients" (Galatioto 2008), selenium plus vitamin A/C/E (Scott 1998), a fixed dose combination (FDC) of coenzyme Q10, L‐carnitine, lycopene and zinc (Gopinath 2013), and "essential fatty acid (EFA) mixture combined with α‐tocopherol (vitamin E) and β‐carotene, acetylcysteine and other antioxidants" (Zalata 1998). Goswami 2015 did not specify the brand name or content of the "combined oral antioxidant".

The second comparison, head‐to‐head, included t26 studies. The head‐to‐head comparisons were included in an attempt to assess whether one antioxidant was more effective than another. They looked at effects of ethylcysteine versus vitamin E (Akiyama 1999), 200 mg versus 400 mg of coenzyme Q10 (Alahmar 2019), coenzyme Q10 versus selenium (Alahmar 2020), zinc versus folic acid versus zinc plus folic acid (Azizollahi 2013; Raigani 2014; Wong 2002), L‐carnitine versus L‐acetyl carnitine versus L‐carnitine plus L‐acetyl carnitine (Balercia 2005), l‐carnitine versus coenzyme Q10 versus l‐carnitine plus coenzyme Q10 versus vitamin B1 (Cheng 2018), 400 mg versus 800 mg of DHA (Conquer 2000), 1000 mg versus 200 mg of vitamin C (Dawson 1990), vitamin D plus calcium versus vitamin C plus vitamin E (Deng 2014), DHA plus vitamin E versus DHA versus vitamin E (Eslamian 2020), SpermActin Forte versus SpermActin Forte plus "vitamin complex" (Gamidov 2017), 0.5 g versus 1 g versus 2 g of DHA (Gonzalez‐Ravina 2018), L‐carnitine plus acetyl‐L‐carnitine versus vitamin E plus vitamin C (Li 2005), L‐carnitine versus vitamin E plus vitamin C (Li 2005a), vitamin E plus selenium versus vitamin B (Nozha 2001), zinc versus zinc plus vitamin E versus zinc plus vitamin E and vitamin C (Omu 2008), glutathione versus coenzyme Q10 (Saeed Alkumait 2020), N‐acetylcysteine versus selenium versus selenium plus N‐acetylcysteine (Safarinejad 2009), selenium versus combined antioxidants (Scott 1998), l‐carnitine versus vitamin E (Sun 2018), Profertil (combined antioxidant) versus l‐carnitine (Tsounapi 2018), L‐carnitine plus vitamin E versus vitamin E (Wang 2010), acetyl‐cysteine versus essential fatty acid (EFA) plus α‐tocopherol (vitamin E) plus β‐carotene versus acetylcysteine plus EFA plus antioxidants (Zalata 1998), and vitamin E versus vitamin E plus amino acids (Zhou 2016).

In summary:

42/90 studies compared antioxidants with placebo;
10/90 studies compared antioxidants with no treatment;
11/90 studies compared one antioxidant with another antioxidant (head‐to‐head);
27/90 multi‐arm studies: 19 of these compared antioxidants versus placebo, six compared antioxidants versus no treatment, one study compared antioxidants versus a diet rich in antioxidants versus placebo, and one study compared different types of antioxidants without use of a placebo or no treatment group.

Outcomes

The primary outcome for this review was as follows.

Live birth per couple. Fourteen studies reported data for live birth in the antioxidant versus placebo or no treatment comparison (Balercia 2005; Balercia 2009; Blomberg Jensen 2018; Gamidov 2019; Huang 2020; Joseph 2020; Kessopoulou 1995; Korshunov 2018; Kumalic 2020; Omu 1998; Schisterman 2020; Steiner 2020; Suleiman 1996; Tremellen 2007). One of these studies could also be included in the head‐to‐head comparison of live birth rate (Balercia 2005). In one study, the unpublished data on live births following ICSI treatment were used (Kumalic 2020). The data from Schisterman 2020 and Huang 2020 could not be used in the meta‐analysis, as the number of patients in whom the outcome was assessed was not reported.

Secondary outcomes for this review were as follows.

Clinical pregnancy rate per couple, as reported by 22 studies in the antioxidant versus placebo or no treatment comparison (Attallah 2013; Azizollahi 2013; Balercia 2005; Balercia 2009; Barekat 2016; Busetto 2018; Gamidov 2019; Huang 2020; Joseph 2020; Kessopoulou 1995; Kizilay 2019; Kopets 2020; Korshunov 2018; Omu 1998; Popova 2019; Schisterman 2020; Steiner 2020; Stenqvist 2018; Suleiman 1996; Tremellen 2007; Tsounapi 2018; Zavaczki 2003). Two of these studies could also be included in the head‐to‐head comparison of clinical pregnancy rate (Balercia 2005; Tsounapi 2018); two more studies in the head‐to‐head comparison reported on clinical pregnancy rate (Cheng 2018; Deng 2014). From one study, the unpublished data on clinical pregnancy following ICSI treatment were used (Kumalic 2020). The data from Schisterman 2020 and Huang 2020 could not be used in the meta‐analysis, as the number of patients with male subfertility in whom the outcome was assessed, was not reported. Data for biochemical and undefined pregnancy can be seen in Table 1.
Adverse events (miscarriage, ectopic pregnancy, stillbirth, gastrointestinal discomfort, euphoria, headache, upper respiratory infection and nasopharyngitis) were reported by 23 studies (Busetto 2018; Cavallini 2004; Gamidov 2017; Gamidov 2019; Gopinath 2013; Joseph 2020; Kessopoulou 1995; Kizilay 2019; Kopets 2020; Korshunov 2018; Kumalic 2020; Omu 1998; Pourmand 2014; Safarinejad 2009a; Safarinejad 2011b; Schisterman 2020; Sharifzadeh 2016; Sigman 2006; Steiner 2020; Stenqvist 2018; Suleiman 1996; Tremellen 2007; Zavaczki 2003) in the antioxidant versus placebo or no treatment comparison. Safarinejad 2011b and Steiner 2020 reported different types of gastrointestinal discomfort separately, which made the data unuseable for meta‐analysis. Adverse events were not reported as an outcome in any of the studies in the head‐to‐head comparisons, except that the study by Li (Li 2005) reported that no side effects were found in either the treatment or control groups.
DNA fragmentation at three months or less was reported by 13 studies (Abbasi 2020; Barekat 2016; Boonyarangkul 2015; Gamidov 2017; Gamidov 2019; Gonzalez‐Ravina 2018; Greco 2005; Huang 2020; Kumalic 2020; Martinez‐Soto 2010; Raigani 2014; Steiner 2020; Stenqvist 2018), comparing antioxidants versus placebo or no treatment. One study in the head‐to‐head comparison reported on DNA fragmentation (Cheng 2018). Data from one study were not usable as the investigators used the Comet assay and reported DNA tail length, which is not a percentage and can therefore not be pooled with the other results (Boonyarangkul 2015)(Analysis 1.8).
DNA fragmentation at six months was reported by four studies (Gamidov 2019; Micic 2019; Schisterman 2020; Stenqvist 2018), comparing antioxidants versus placebo or no treatment.
Total sperm motility at three months or less was reported by 30 studies in the antioxidants versus placebo or no treatment comparison (Abbasi 2020; Azizollahi 2013; Bahmyari 2021; Balercia 2005; Barekat 2016; Conquer 2000; Dimitriadis 2010; Ener 2016; Eslamian 2020; Gopinath 2013; Greco 2005; Kumalic 2020; Lenzi 2003; Lu 2018; Martinez‐Soto 2010; Morgante 2010; Nadjarzadeh 2011; Nouri 2019; Omu 2008; Peivandi 2010; Raigani 2014; Scott 1998; Sigman 2006; Sivkov 2011; Steiner 2020; Stenqvist 2018; Tsounapi 2018; Vinogradov 2019; Zavaczki 2003; Zhou 2016) and by 14 studies in the head‐to‐head comparison (Akiyama 1999; Alahmar 2019; Alahmar 2020; Azizollahi 2013; Balercia 2005; Cheng 2018; Conquer 2000; Dawson 1990; Eslamian 2020; Li 2005; Omu 2008; Scott 1998; Tsounapi 2018; Zhou 2016).
Total sperm motility at six months was reported by 19 studies in the antioxidants versus placebo or no treatment comparison (Ardestani 2019; Azizollahi 2013; Balercia 2005; Balercia 2009; Blomberg Jensen 2018; Busetto 2018; Ener 2016; Gopinath 2013; Kizilay 2019; Lenzi 2004; Safarinejad 2009; Safarinejad 2009a; Safarinejad 2012; Schisterman 2020; Sigman 2006; Steiner 2020; Stenqvist 2018; Suleiman 1996; Wong 2002). Four studies reported this in the head‐to‐head comparison (Azizollahi 2013; Balercia 2005; Safarinejad 2009; Wong 2002).
Total sperm motility at nine months or more was reported by five studies in the antioxidants versus placebo or no treatment comparison (Balercia 2005; Balercia 2009; Ener 2016; Safarinejad 2009a; Safarinejad 2012). One study reported this in the head‐to‐head comparison (Balercia 2005).
Progressive sperm motility at three months or less was reported by 26 studies in the antioxidants versus placebo or no treatment comparison (Abbasi 2020; Amini 2020; Attallah 2013; Azizollahi 2013; Bahmyari 2021; Balercia 2005; Boonyarangkul 2015; Cyrus 2015; Dawson 1990; Eslamian 2020; Gonzalez‐Ravina 2018; Haghighian 2015; Huang 2020; Joseph 2020; Kumalic 2020; Martinez‐Soto 2010; Mehni 2014; Morgante 2010; Nadjarzadeh 2011; Nouri 2019; Peivandi 2010; Popova 2019; Rolf 1999; Sharifzadeh 2016; Tsounapi 2018; Vinogradov 2019). Thirteen studies reported this in the head‐to‐head comparison (Alahmar 2019; Alahmar 2020; Balercia 2005; Cheng 2018; Deng 2014; Eslamian 2020; Gonzalez‐Ravina 2018; Li 2005; Li 2005a; Sun 2018; Tsounapi 2018; Wang 2010; Zhou 2016).
Progressive sperm motility at six months was reported by 13 studies in the antioxidants versus placebo or no treatment comparison (Ardestani 2019; Azizollahi 2013; Balercia 2005; Balercia 2009; Blomberg Jensen 2018; Boonyarangkul 2015; Cavallini 2004; Gamidov 2019; Kizilay 2019; Micic 2019; Saeed Alkumait 2020; Safarinejad 2011b; Stenqvist 2018). Two studies reported this in the head‐to‐head comparison (Balercia 2005; Saeed Alkumait 2020).
Progressive sperm motility at nine months or more was reported by two studies in the antioxidants versus placebo or no treatment comparison (Balercia 2005; Balercia 2009). One study reported this in the head‐to‐head comparison (Balercia 2005).
Sperm concentration at three months or less was reported by 34 studies in the antioxidants versus placebo or no treatment comparison (Abbasi 2020; Amini 2020; Attallah 2013; Azizollahi 2013; Bahmyari 2021; Balercia 2005; Barekat 2016; Boonyarangkul 2015; Conquer 2000; Cyrus 2015; Dimitriadis 2010; Ener 2016; Eslamian 2020; Gonzalez‐Ravina 2018; Gopinath 2013; Greco 2005; Haghighian 2015; Huang 2020; Joseph 2020; Kumalic 2020; Lu 2018; Martinez‐Soto 2010; Mehni 2014; Morgante 2010; Nadjarzadeh 2011; Nouri 2019; Peivandi 2010; Rolf 1999; Scott 1998; Sharifzadeh 2016; Steiner 2020; Tsounapi 2018; Vinogradov 2019; Zavaczki 2003), and 14 in the head‐to‐head comparison (Alahmar 2019; Alahmar 2020; Akiyama 1999; Azizollahi 2013; Balercia 2005; Cheng 2018; Conquer 2000; Eslamian 2020; Gonzalez‐Ravina 2018; Li 2005a; Scott 1998; Sun 2018; Tsounapi 2018; Wang 2010).
Sperm concentration at six months was reported as an outcome by 20 studies in the antioxidants versus placebo or no treatment comparison (Ardestani 2019; Azizollahi 2013; Balercia 2005; Balercia 2009; Blomberg Jensen 2018; Boonyarangkul 2015; Busetto 2018; Cavallini 2004; Ener 2016; Gamidov 2019; Gopinath 2013; Kizilay 2019; Lenzi 2004; Safarinejad 2009; Safarinejad 2009a; Safarinejad 2011b; Safarinejad 2012; Schisterman 2020; Stenqvist 2018; Wong 2002), and four studies in the head‐to‐head comparison (Azizollahi 2013; Balercia 2005; Safarinejad 2009; Wong 2002).
Sperm concentration at nine months or more was reported by five studies in the antioxidants versus placebo or no treatment comparison (Balercia 2005; Balercia 2009; Ener 2016; Safarinejad 2009a; Safarinejad 2012), and one study in the head‐to‐head comparison (Balercia 2005).

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Table 1. Data for undefined or biochemical pregnancy

Undefined or biochemical pregnancy	Antioxidant		Control		Peto OR [CI]
Antioxidant(s) versus placebo or no treatment
Combined antioxidants	Events	Total	Events	Total
	35	234	32	194
Galatioto 2008	1	20	0	22	8.17 [0.16 to 413.39]
Gopinath 2013	13	92	2	46	2.72 [0.88 to 8.46]
Steiner 2020	18	85	26	86	0.62 [0.32 to 1.24]
Stenqvist 2018	3	37	4	40	0.80 [0.17 to 3.74]
Arginine
Pryor 1978	2	35	2	29	0.82 [0.11 to 6.16]
Carnitines	25	154	3	145
Sigman 2006	1	12	1	9	0.74 [0.04 to 13.02]
Peivandi 2010	3	15	0	15	8.57 [0.82 to 89.45]
Lenzi 2003	6	43	0	43	8.37 [1.61 to 43.58]
Lenzi 2004	4	30	0	26	7.20 [0.95 to 54.34]
Cavallini 2004	9	39	1	47	7.50 [2.01 to 27.98]
Coenzyme Q10	6	136	3	136
Safarinejad 2009a	0	106	0	106	Not estimable
Nadjarzadeh 2011	0	23	0	24	Not estimable
Vitamin C + Vitamin E
Rolf 1999	0	15	0	16	Not estimable
Vitamin E
Ener 2016	5	28	5	28	1.00 [0.26 to 3.88]
Head‐to‐head antioxidant(s)	Events	Total	Events	Total
L‐acetyl carnitine + L‐carnitine vs Vitamin E + Vitamin C
Li 2005	10	85	2	53	2.72 [0.81 to 9.14]
L‐carnitine + Vitamin E vs Vitamin E
Wang 2010	21	68	3	67	6.01 [2.49 to 14.47]
Vitamin E + amino acids vs Vitamin E
Zhou 2016	4	70	1	50	2.52 [0.41 to 15.35]

Data were extracted from 67 of the included studies. The 23 remaining studies either did not report any data or the number of patients in whom the outcome was assessed was not reported (Alahmar 2020; Biagiotti 2003; Eslamian 2013; Eslamian 2020; Exposito 2016; Galatioto 2008; Goswami 2015; Haje 2015; Huang 2020; Kumamoto 1988; Lenzi 2003; Lombardo 2002; Lu 2018; Martinez 2015; Micic 2019; Nozha 2001; Poveda 2013; Pryor 1978; Sivkov 2011; Sofikitis 2016; Vinogradov 2019; Wong 2002; Zalata 1998). In the current update, we calculated the mean and standard deviation from data presented as median and (interquartile) range from six studies included in previous versions of this review (Blomberg Jensen 2018; Cavallini 2004; Gamidov 2019; Micic 2019; Raigani 2014; Wong 2002). Another study reported data for a treatment duration of three to six months, but did not specify this any further and therefore data could not be used in the meta‐analysis (Haje 2015).

See Characteristics of included studies and the analyses 'data not usable for meta‐analysis'(Analysis 1.8; Analysis 1.10; Analysis 1.16; Analysis 1.20; Analysis 1.22). Table 2 also describes the outcomes and conclusions of all included studies. Attempts were made to contact all authors of the included studies for further details and clarification.

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Table 2. Outcomes and conclusions from all included studies

Study ID	Design, population	Outcomes described in methods section	Outcomes reported on in results	In meta‐analysis Y or N	Results	Conclusions + = positive effect ‐ = negative or no effect
Abbasi 2020	Parallel, placebo Men post‐varicocelectomy N = 60	Sperm parameters, DNA fragmentation	Sperm parameters, DNA fragmentation	Y ‐ sperm parameters Y ‐ DNA fragmentation	ALA improved sperm motility compared to baseline. No significant difference in sperm parameters between ALA and placebo.	‐ ALA does not improve semen quality compared to placebo after varicocelectomy
Akiyama 1999	Cross‐over, head‐to‐head Infertile men, high ROS levels N = 10	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Ethylcystein did not improve sperm density and motility but "sperm function" increased and ROS levels decreased, compared to vitamin E	+ Ethylcysteine shown to be effective for improvement of sperm parameters when compared to vitamin E
Alahmar 2019	Parallel, head‐to‐head Idiopathic OAT N = 65	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	CoQ10 200 and 400 mg improved sperm concentration and motility, greater improvement with 400 mg	+ CoQ10 improves sperm parameters, greater improvement with a 400 mg dose compared to 200 mg
Alahmar 2020	Parallel, head‐to‐head Idiopathic OAT N = 70	Sperm parameters	Sperm parameters	N ‐ number of drop‐outs unclear	CoQ10 and selenium each improved sperm concentration and motility, greater improvement with CoQ10	+ CoQ10 and selenium improve sperm parameters, greater improvement with CoQ10
Amini 2020	Parallel, placebo Infertile men N = 72	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Vitamin D did not improve sperm parameters	‐ Vitamin D does not improve sperm parameters
Ardestani 2019	Parallel, no treatment Men post‐varicocelectomy N = 64	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Co‐administration of folic acid, selenium and vitamin E improved sperm concentration and motility	+ A combination of folic acid + selenium + vitamin E improves sperm parameters after varicocelectomy
Attallah 2013	Parallel, no treatment Idiopathic asthenozospermia, IUI N = 30 Conference abstract	Sperm parameters, chemical and clinical pregnancy	Sperm parameters, chemical and clinical pregnancy	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical	NAC increased sperm concentration and motility. Clinical pregnancy was not significantly different between the groups	+ NAC improves semen quality and improves pregnancy rates prior to IUI, no improvement of pregnancy rate
Azizollahi 2013	Multiple arm trial Men post‐varicocelectomy N = 160	Sperm parameters	Sperm parameters	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical	Mild improvement in sperm parameters with the use of antioxidants zinc, folic acid or both	+ Co‐administration of zinc and folic acid improved sperm parameters and increased varicocelectomy outcomes, only zinc an improvement in pregnancy rate
Bahmyari 2021	Parallel, placebo Idiopathic OAT N = 70	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	No improvement of sperm parameters with the use of selenium, folic acid and vitamin E	‐ Co‐administration of selenium, folic acid and vitamin E were not effective to improve sperm parameters
Balercia 2005	Multiple arm, placebo Infertile men N = 60	Sperm parameters	Sperm parameters, pregnancy rate	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical Y ‐ live birth	Improvement in motility in LAC group.	+ Long‐term carnitine is effective in increasing sperm motility. No evidence of increased live birth or clinical pregnancy.
Balercia 2009	Parallel, placebo Infertile and unexplained N = 60	Sperm parameters	Sperm parameters, pregnancy rate	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical	Co enzyme Q10 increased sperm motility.	+ Q10 is effective in improving sperm kinetic features in asthenospermia. No evidence of increased live birth or clinical pregnancy.
Barekat 2016	Parallel, no treatment Subfertile men with varicocele N = 40	Sperm parameters, DNA fragmentation	Sperm parameters, DNA fragmentation, clinical spontaneous pregnancies	Y ‐ sperm parameters Y ‐ DNA fragmentation Y ‐ pregnancy rate, clinical (SEs converted to SDs)	Sperm parameters significantly improved after surgery compared to before surgery in both the NAC and control groups. NAC might have an additional value by improving sperm motility post‐varicocelectomy	+ The results of this study revealed that NAC improved chromatin integrity and pregnancy rate when administered as adjunct therapy post‐varicocelectomy
Biagiotti 2003	Multiple arm, no treatment Severe idiopathic oligoasthenospermia N = 42 Conference abstract	Sperm parameters	Sperm parameters	N ‐ no data available	A significant improvement in morphology concentration, motility in the carnitine group No side effects	+ Quality of semen is positively associated with fertilisation and implantation rates in assisted reproduction
Blomberg Jensen 2018	Parallel, placebo Infertile men with impaired semen quality N = 307	Sperm parameters, reproductive hormones, live birth rate	Sperm parameters, reproductive hormones, live birth rate	Y ‐ sperm parameters, concentration provided as median + IQR and converted to mean + SD Y ‐ live birth rate	Vitamin D was not associated with changes in semen parameters, although spontaneous pregnancies tended to be higher in couples in which the man was in the treatment group	± Vitamin D did not improve semen quality. The positive impact of vitamin D supplementation on live birth rate and serum inhibin B in oligozoospermic and vitamin D–deficient men may be of clinical importance and warrant verification by others.
Boonyarangkul 2015	Multiple arm, placebo, tamoxifen excluded Men with abnormal semen analysis N = 68	Sperm parameters, DNA damage (Comet assay)	Sperm parameters, DNA tail length	Y ‐ sperm parameters	Folate alone significantly decreased DNA tail length at 3‐months. Sperm motility was significantly increased after 3‐months Folate alone.	+ Our study indicated that folate in combination with Tamoxifen citrate could improve sperm quality including semen parameters and sperm DNA integrity
Busetto 2018	Parallel, placebo Infertile men with OAT, 50% included with varicocele N = 104	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical	Sperm concentration, total sperm count, progressive and total motility were significantly increased in supplemented (Proxeed Plus) patients. Increased pregnancy rate	+ Supplementation with metabolic and antioxidant compounds could be efficacious when included in strategies to improve fertility
Cavallini 2004	Multiple arm, placebo Idiopathic OAT men with varicocele N = 325	Sperm parameters, pregnancy rate, adverse events	Sperm parameters, pregnancy rate, adverse events	Y ‐ sperm parameters (median +IQR converted to mean + SD) N ‐ pregnancy rate, unclear if clinical Table 1 Y ‐ adverse events	Significant increase in sperm parameters for carnitines when compared to placebo. Carnitine groups had a significantly higher pregnancy rate than placebo group	+ The antioxidant plus anti‐inflammatory group was more effective in improving sperm parameters and pregnancy than those of carnitines alone or placebo however carnitines alone were more effective than placebo
Cheng 2018	Multiple arm, head‐to‐head Idiopathic OAT N = 312	Sperm parameters, DNA fragmentation, pregnancy rate	Sperm parameters, DNA fragmentation, pregnancy rate	Y/N ‐ sperm parameters, results not available for all groups and parameters Y ‐ DNA fragmentation Y ‐ pregnancy rate, clinical	Significant improvement of sperm parameters and DNA fragmentation in the L‐carnitine plus CoQ10 group compared to placebo. Combination and L‐carnitine groups had remarkably higher pregnancy rate than placebo group	+ Combination of LC and CoQ10 improve semen parameters and outcome of clinical pregnancy
Conquer 2000	Multiple arm, placebo Asthenozoospermic men N = 28	Sperm parameters	Sperm parameters	Y ‐ sperm parameters (SEs converted to SDs)	DHA showed no effect on sperm motility or concentration	± DHA supplementation increased DHA levels in the sperm but not motility or concentration
Cyrus 2015	Parallel, placebo Infertile men with varicocele N = 115	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Vitamin C was not effective on sperm count but improved sperm motility and morphology significantly	+ Ascorbic acid can play a role as adjuvant treatment after varicocelectomy in infertile men
Dawson 1990	Multiple arm, placebo Men with sperm agglutination N = 30	Sperm parameters	Sperm parameters	Y ‐ sperm parameters (SEs converted to SDs)	The group receiving 1000 mg of AA showed more improvement in parameters than the 200mg group and the placebo	+ Vitamin C can improve sperm parameters, especially dosage of 1000 mg.
Deng 2014	Head‐to‐head Men with idiopathic oligoasthenozoospermia N = 86	Sperm parameters, adverse reactions, pregnancy rate	Sperm parameters, adverse reactions, pregnancy rate	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical	Vitamin D is a safe option for the treatment of idiopathic oligoasthenozoospermia and can effectively improve the semen quality especially the progressive sperm motility	+ Vitamin D can improve forward movement sperm number and percentage, improve the woman's clinical pregnancy rate, and is well tolerated
Dimitriadis 2010	Multiple arm, no treatment, vardenafil/sildenafil arms excluded Men with oligoasthenospermia N = 75	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	An improvement in sperm concentration with carnitine versus no treatment	+ Enhancement of Leydig cell secretory function may increase sperm concentration and motility
Ener 2016	Parallel, no treatment Infertile men with varicocele N = 56	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	Y ‐ sperm parameters N ‐ pregnancy rate, unknown if clinical Table 1	The administration of vitamin E increased all of the parameters; however not statistically significant	‐ Vitamin E supplementation does not improve the sperm parameters after varicocelectomy
Eslamian 2013	Parallel, placebo Asthenoszoospermic men N = 50	Sperm parameters	Sperm parameters, sperm membrane and serum fatty acids	N ‐ sperm parameters, data not usable, no continuous data but categories from 'significantly improvement' to 'worsened'	Sperm parameters improved with DHA + vitamin E supplementation	+ Sperm parameters improve with DHA + vitamin E supplementation
Eslamian 2020	Multiple arm, placebo Asthenozoospermic men N = 180	Sperm parameters	Sperm parameters	N ‐ sperm parameters, only imputed data provided	Significant increase of sperm concentration in the DHA + vitamin E group compared to groups treated with DHA+placebo, vitamin E+placebo and placebo.	+ Combined DHA and vitamin E improve sperm parameters
Exposito 2016	Parallel, placebo Normozoospermic, oligozoospermic and asthenozoospermic men N = 113	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	N ‐ sperm parameters N ‐ pregnancy rate Both not included because data included normospermic men	50% of oligozoospermic men improved sperm concentration and sperm count to normozoospermic levels. This trend was also observed in asthenozoospermic men, but nog significantly	+ Vitamin E treatment by oral administration improves semen parameters
Galatioto 2008	Parallel, no treatment Men with persistent oligospermia after embolisation of varicocele N = 42	Sperm parameters, pregnancy rate, adverse events	Sperm parameters, pregnancy rate, adverse events	N ‐ sperm parameters, only medians given N ‐ pregnancy, unclear if clinical Table 1 N ‐ adverse events	Significant difference in sperm count in combined antioxidant group but not in motility. One pregnancy in the NAC group No significant adverse effects	± NAC does not improve pregnancy rate, no significant adverse events, but do significantly increase sperm count
Gamidov 2017	Multiple arm, no treatment Men with varicocele N = 114	Sperm parameters, DNA fragmentation, adverse events	Sperm parameters, DNA fragmentation, adverse events	Y ‐ sperm parameters (median+IQR converted to mean+SD) Y ‐ DNA fragmentation (median+IQR converted to mean+ SD) Y ‐ adverse events	SpermActine (SA) resulted in a 22.3% decrease in the level of sperm DNA fragmentation at 3 months. SA + vitamin complex resulted in a 27% increase in the sperm concentration at 3 months. There were no side effects of pharmacotherapy.	+ Antioxidant therapy leads to an improvement in the basic sperm parameters (sperm concentration and motility) and a decrease in the level of sperm DNA fragmentation in the short term. There were no side effects
Gamidov 2019	Parallel, placebo Infertile men with high oxidative stress and DNA fragmentation N = 80	Sperm parameters, DNA fragmentation, pregnancy rate, live birth	Sperm parameters, DNA fragmentation, pregnancy rate, live birth, adverse events	Y ‐ sperm parameters Y ‐ DNA fragmentation Y ‐ pregnancy rate, clinical Y ‐ live births Y ‐ adverse events	Spermactin Forte significantly improvement sperm motility and decreased oxidative stress. There were more pregnancies in the intervention group (13 versus 1)	+ The use of the SpermActin Forte antioxidant improves sperm analysis in most patients. SpermActin Forte is an effective and safe method of treating male infertility
Gonzalez‐Ravina 2018	Multiple arm, placebo Infertile men N = 60	Sperm parameters, DNA fragmentation	Sperm parameters, DNA fragmentation	N ‐ sperm parameters, outcomes provided as change + SD Analysis 1.15; Analysis 1.20 N ‐ DNA fragmentation, outcomes provided as change + SD Analysis 1.8	Significant increase of progressive sperm motility in the DHA 1g and 2g groups after 1 month and in the DHA 0.5 group after 3 months. Greater effect in asthenozoospermic men	+ DHA (0.5, 1 and 2g) had beneficial effects on sperm function without producing any adverse effects, obtaining more immediate results with higher doses
Gopinath 2013	Multiple arm, placebo Idiopathic OAT men N = 138	Sperm parameters, pregnancy rate, adverse events	Sperm parameters, pregnancy rate, adverse events	Y ‐ sperm parameters N ‐ pregnancy rate, not clinical Table 1 Y ‐ adverse events	Combined antioxidant significantly improved sperm count and total motility in both treatment arms (1 vs 2 tablets). Mild adverse events were reported, no severe.	+ Exogenous administration of fixed dose combination of antioxidants is safe and effective therapy in improving the male subfertility regarding sperm parameters. Only mild adverse events when using combined antioxidants
Goswami 2015	Multiple arm, placebo Arm treated with diet enriched in antioxidants not used Men with idiopathic infertility and high ROS N = 175 Conference abstract	Sperm parameters, DNA fragmentation	DNA fragmentation	N ‐ sperm parameters, not reported in results N ‐ DNA fragmentation, no results reported besides p‐value	No difference in DNA fragmentation between the study groups	+/‐ No conclusions on antioxidants versus placebo. A diet rich in antioxidants and lifestyle modifications can bring almost the same effect as antioxidant supplements
Greco 2005	Parallel, placebo Infertile males with high DNA fragmentation N = 64	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	No significant difference in concentration or motility however DNA fragmentation was significantly reduced in the vitamin C + E when compared to placebo	+ A short oral treatment of Vitamin C + E can reduce DNA fragmentation
Haghighian 2015	Parallel, placebo Men with idiopathic asthenozoospermia N = 48	Sperm parameters, adverse events	Sperm parameters, adverse events	Y ‐ sperm parameters N ‐ adverse events, reported "none", however not clear which side effects they aimed for	Sperm parameters were significantly higher in ALA group. No side effects due to the oral administration of ALA were observed in any participants.	+ Medical therapy of asthenoteratospermia with ALA supplement could improve quality of semen parameters
Haje 2015	Multiple arm, placebo, tamoxifen arms excluded Infertile men with idiopathic OAT N = 128	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	N ‐ sperm parameters, range of treatment 3 ‐ 6 months and not divided N ‐ pregnancy rate, unclear if pregnancy and no numbers but percentage	L‐carnitine did not improve sperm count or motility. Only tamoxifen or tamoxifen + L‐carnitine improved pregnancy rate, not significantly.	± Administration of tamoxifen or L‐carnitine can improve sperm parameters and ICSI outcomes. Combining those result in maximum therapeutic effect
Huang 2020	Parallel, placebo Oligozoospermic men N = 769	Sperm parameters, evaluation of MTHFR polymorphism, DNA fragmentation, pregnancy rate, live birth	Sperm parameters, evaluation of MTHFR polymorphism, DNA fragmentation, pregnancy rate, live birth	N ‐ sperm parameters N ‐ DNA fragmentation N ‐ pregnancy, clinical N ‐ live births All outcomes reported for MTHFR polymorphism groups only	Folic acid significantly increased sperm parameters, decreased oxidative stress and DNA fragmentation and lead to a higher pregnancy and live birth rate in the MTHFR 677 TT group. Effect not seen in other MTHFR groups.	+ Folic acid has a beneficial effect on oligozoospermia with MTHFR 677 TT genotype in terms of sperm parameters, DNA fragmentation and pregnancy outcomes
Joseph 2020	Parallel, no treatment Infertile men scheduled for ART N = 200	Sperm parameters, pregnancy rate, live birth, adverse events	Sperm parameters, pregnancy rate, live birth, adverse events	Y ‐ sperm parameters (median+IQR converted to mean+SD) Y ‐ pregnancy rate, clinical Y ‐ live births Y ‐ adverse events	No significant difference in clinical pregnancies or live births when combined vitamin C + vitamin E + zinc were compared to no treatment. No improvement of sperm parameters	‐ No difference in clinical pregnancy and live births. No improvement of sperm parameters
Kessopoulou 1995	Cross‐over, placebo Male infertility N = 30	Sperm parameters, adverse events, live birth	Sperm parameters, adverse effects, live birth	N ‐ sperm parameters, only medians given Analysis 1.10; Analysis 1.20 Y ‐ pregnancy rate, clinical Y ‐ live births Y ‐ adverse events	No differences in sperm outcomes were seen between the groups. 1 pregnancy in the vitamin E group and nil in the placebo (first phase data)	+ No difference in semen parameters. There is evidence of increased live birth and clinical pregnancy rate
Kizilay 2019	Parallel, no treatment Varicocele patients with oligozoospermia N = 93	Sperm parameters, clinical pregnancy, adverse events	Sperm parameters, clinical pregnancy, adverse events	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical Y ‐ adverse events	Significant improvement of sperm parameters and higher clinical pregnancy rate in combined antioxidant group compared to no treatment	+ Antioxidant treatment provides an important contribution to varicocelectomy outcomes and improves pregnancy rates
Kopets 2020	Parallel, placebo Idiopathic infertility N = 83	Sperm parameters, clinical pregnancy, adverse events	Sperm parameters, clinical pregnancy, adverse events	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical Y ‐ adverse events	The percentage of normal spermiograms was significantly higher in the combined antioxidant group. Higher spontaneous pregnancy rate in antioxidant group	+ Combined l‐carnitine/l‐acetyl‐carnitine, l‐arginine, glutathione, CoQ10, zinc, folic acid, cyanocobalamin, and selenium improves sperm quality and increases pregnancy rates
Korshunov 2018	Parallel, no treatment Obstructive azoospermia, TESA/ICSI candidates N = 46 Conference abstract	Clinical pregnancy, live births	Clinical pregnancy, live birth, embryo quality, early pregnancy loss	Y ‐ pregnancy rate, clinical Y ‐ live births N ‐ adverse events, miscarriage. No data provided by authors.	Clinical pregnancy and live birth rate were 62,5% vs 59,1% and 54,1% vs 40,9% in the antioxidant and no treatment group, respectively. Higher early pregnancy loss rate in control group	+ Antioxidant therapy may have a positive effect for patients with obstructive azoospermia. It might improve ART outcome and decrease pregnancy loss
Kumalic 2020	Parallel, placebo Infertile men with OAT N = 80	Sperm parameters, DNA fragmentation, adverse events	Sperm parameters, DNA fragmentation, adverse events, after contact with author: clinical pregnancy rate and live births after ICSI	Y ‐ sperm parameters Y ‐ DNA fragmentation Y ‐ adverse events Y ‐ pregnancy rate, clinical Y ‐ live births	No statistical differences in sperm parameters between astaxanthin + vitamin E group and placebo	‐ The oral intake of astaxanthin did not affect any semen parameters in patients with OAT
Kumamoto 1988	Multiple arm, placebo Men with abnormal sperm count or motility N = 396	Sperm parameters	Sperm parameters	N ‐ sperm parameters, only scales given	No statistical difference in sperm outcomes in vitamin B 12 groups or placebo	‐ No improvement in sperm parameters after use of vitamin B12
Lenzi 2003	Cross‐over, placebo Infertile men with OAT N = 100	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	Y ‐ sperm parameters N ‐ pregnancy rate, no definition of pregnancy given see Table 1	The patient groups showed no differences in sperm outcomes between therapy (carnitine) and placebo groups. Six pregnancies in the carnitine group and nil in the placebo (first phase)	+ The pregnancies obtained during the carnitine therapy period could suggest that carnitines may also lead to improvement in sperm function and fertilisation
Lenzi 2004	Parallel, placebo Infertile men with OAT N = 60	Sperm parameters, pregnancy rate, adverse events	Sperm parameters, pregnancy rate, adverse events	Y ‐ sperm parameters N ‐ pregnancy rate, no definition of pregnancy given Table 1 N ‐ adverse events	Four participants taking carnitine induced a pregnancy in their partner and nil in the placebo	+ No evidence of improved sperm parameters
Li 2005	Head‐to‐head Infertile men with OAT N = 150	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	Y ‐ sperm parameters N ‐ pregnancy rate, no definition given Table 1	L‐carnitine and acetyl carnitine more effective than vitamin E + vitamin C for pregnancy, sperm parameters and no evidence of adverse events	+ L‐carnitine and acetyl carnitine more effective than vitamin E + vitamin C for pregnancy, sperm parameters and no evidence of adverse events
Li 2005a	Head‐to‐head Infertile men with OAT N = 80	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Statistical significance for carnitines over vitamin E + C	+ Improvement of sperm parameters for carnitines compared to vitamin E + C
Lombardo 2002	Cross‐over Infertile men with OAT N = 100 Conference abstract	Sperm parameters	Sperm parameters	N ‐ sperm parameters, no data available	Sperm parameters (concentration, motility) carnitines versus placebo	+ Improvement of sperm parameters
Martinez 2015	Multiple arm, placebo, SG1002 arm excluded Men with idiopathic OAT N = 54	Sperm parameters	Sperm parameters	N ‐ sperm parameters, no SDs given	Resveratrol treatment did not significantly affect any of the parameters.	‐ Resveratrol treatment did not significantly affect any of the parameters. SG1002 may reverse oligoasthenozoospermia. It seems to be more potent antioxidant than resveratrol
Martinez‐Soto 2010	Parallel, placebo Infertile men N = 50 Conference abstract + manuscript from author	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	No differences were found in traditional sperm parameters or lipid composition of the sperm membrane after DHA treatment, only reduction in the percentage of spermatozoa with DNA damage	+ Positive effect only on DNA fragmentation
Mehni 2014	Multiple arm, placebo, pentoxifylline arms excluded Infertile men with OAT N = 235	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	L‐carnitine only improved sperm motility, combined with pentoxifylline it improves all sperm parameters.	+ Positive effect only on sperm motility
Micic 2019	Parallel, placebo Men with OAT N = 175	Sperm parameters, DNA fragmentation	Sperm parameters, DNA fragmentation	Y ‐ sperm parameters Y ‐ DNA fragmentation (median+IQR converted to mean + SD)	Proxeed Plus significantly improved sperm volume, motility and DNA fragmentation compared to baseline.	+ Beneficial effects of carnitine derivatives (Proxeed plus) on progressive motility, vitality and sperm DNA fragmentation
Morgante 2010	Parallel, no treatment Infertile men with idiopathic asthenospermia N = 180	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Significant improvement in sperm motility.	+ Improvement of sexual satisfaction Significant improvement in sperm motility
Nadjarzadeh 2011	Parallel, placebo Men with Idiopathic OAT N = 60	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Non‐significant changes in semen parameters of CoQ10 group.	‐ CoQ10 further evidence suggesting that supplementation is associated with alleviating oxidative stress, although it does not show any significant effects on sperm concentration, motility and morphology
Nouri 2019	Parallel, placebo Men with history of infertility N = 44	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Significant improvement of sperm concentration with lycopene compared to placebo. Increase of total motility in lycopene group compared to baseline.	+ Lycopene improves sperm parameters and oxidative stress biomarkers in infertile men
Nozha 2001	Head‐to‐head Men with OAT N = unclear, 20?	Sperm parameters	Sperm parameters	N ‐ sperm parameters, no data available	Vitamin E + selenium significantly improves sperm motility	+ Vitamin E + selenium associated with improved sperm motility when compared with vitamin B
Omu 1998	Parallel, no treatment Men with asthenozoopermia N = 100	Sperm parameters	Sperm parameters, pregnancy, live birth	N ‐ sperm parameters, only % increase or decrease, not usable Y ‐ pregnancy rate, clinical Y ‐ live birth	Significant improvement in sperm quality by zinc therapy	+ Zinc has a role in improving sperm parameters. Significant increase in pregnancy, not live birth
Omu 2008	Multiple arm, no treatment Men with asthenozoospermia N = 100	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Zinc therapy alone, in combination with vitamin E or with vitamin E+C were associated with comparably improved sperm parameters and less sperm DNA fragmentation	+ Zinc therapy reduces asthenozoospermia
Peivandi 2010	Cross‐over, placebo Infertile men N = 30	Sperm parameters	Sperm parameters, pregnancy rate	Y ‐ sperm parameters N ‐ pregnancy rate, not defined as clinical Table 1	Significant improvements in mean sperm concentration and progressive sperm motility upon two months of L‐carnitine intake but no significant changes were found in sperm volume or morphology.	+ Sperm outcomes and biochemical pregnancies. L‐carnitine intake effectively improved the mean sperm count and progressive sperm motility
Popova 2019	Parallel, no treatment Men planning ART treatment N = 80	Sperm parameters, clinical pregnancy, adverse events	Sperm parameters, clinical pregnancy, adverse events	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical Y ‐ adverse events	No significant change in sperm motility. A pregnancy rate in the combined antioxidants (Androdoz) group was 45% compared to 25% in the control group.	+/‐ Androdoz contributes to an increase in positive outcomes of ART program. "Androdoz improves the main criteria of sperm analysis and functional tests (HBA‐test)". This is based on the improvement of morphology
Pourmand 2014	Parallel, no treatment Men with male factor infertility and varicocele N = 100	Sperm parameters, DNA fragmentation, adverse events	Sperm parameters, DNA fragmentation, adverse events	N ‐ sperm parameters, no SD given N ‐ DNA fragmentation, no SD given Y ‐ adverse events	No statistical difference between the two groups (varicocelectomy with L‐carnitine or with no adjuvant therapy).	‐ Addition of 750 mg of L‐carnitine orally daily to standard inguinal varicocelectomy does not add any extra benefit in terms of improvement in semen analysis parameters or DNA damage
Poveda 2013	Multiple arm, placebo Infertile men N = 60 Conference abstract	Sperm parameters	Sperm parameters	N ‐ sperm parameters, data not available	L‐carnitine significantly improves sperm concentration, Spermotrend and Maca improve sperm motility.	+ Sperm concentration with L‐carnitine and motility with combined antioxidant Spermotrend
Pryor 1978	Cross‐over, placebo Men with severe oligozoospermia N = 64	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	N ‐ sperm parameters, bar graph of % patients showing an increase in motility and density N ‐ pregnancy rate, not clear if clinical. Included in biochemical analysis Table 1	Arginine was no more effective than placebo for sperm parameters and biochemical pregnancy rates	‐ There was no difference in the conception rates of the wives or changes in the quality of the semen during each period of treatment
Raigani 2014	Multiple arm, placebo Men with proven male factor infertility N = 83	Sperm parameters, DNA fragmentation	Sperm parameters, DNA fragmentation	Y ‐ sperm parameters ( median+IQR converted to mean+ SD) Y ‐ DNA fragmentation	Sperm concentration, DNA fragmentation not significantly improved in either group	‐ Zinc sulphate and folic acid supplementation did not ameliorate sperm quality in infertile men with severely compromised sperm parameters, OAT
Rolf 1999	Asthenospermia N = 33	Sperm parameters, pregnancy rates, adverse events	Sperm parameters, pregnancy rate, adverse events	Y ‐ sperm parameters N ‐ pregnancy rate, not stated as clinical pregnancy N ‐ adverse events, not clear which side effects aimed for	No adverse events or pregnancies in either group	‐ Overall no difference vitamin E + C versus placebo
Saeed Alkumait 2020	Multiple arm, placebo Infertile men N = 151	Sperm parameters	Sperm parameters	N ‐ sperm parameters, data provided as percentage improvement, Analysis 1.16; Analysis 1.22	Significantly higher percentage improvement of progressive sperm motility and concentration with glutathione or CoQ10 compared to placebo	+ Both glutathione and CoQ10 are effective treatment options for improving sperm motility, morphology and concentration
Safarinejad 2009	Multiple arm, placebo Men with idiopathic OAT N = 468	Sperm parameters, adverse events	Sperm parameters, adverse events	Y ‐ sperm parameters N ‐ adverse events, not specified which adverse events aimed for	All semen parameters significantly improved with selenium and N‐acetyl‐cysteine treatment. Administering selenium plus N‐acetyl‐cysteine resulted in additive beneficial effects. Zero adverse events	+ Supplemental selenium and N‐acetyl‐cysteine improve semen quality. Zero adverse events
Safarinejad 2009a	Parallel, placebo Men with idiopathic OAT N = 212	Sperm parameters, adverse events	Sperm parameters, adverse events	Y ‐ sperm parameters N ‐ adverse events, not specified which adverse events aimed for	Significant improvement in sperm density and motility after coenzyme Q10 therapy. Zero adverse events	+ Coenzyme Q10 supplementation resulted in a statistically significant improvement in certain sperm parameters. Zero adverse events
Safarinejad 2011b	Parallel, placebo Men with idiopathic OAT N = 238	Sperm parameters, adverse events	Sperm parameters, adverse events	Y ‐ sperm parameters N ‐ adverse events, not clear how many patients had gastrointestinal upsets in total	Significant improvement of sperm concentration and progressive motility after omega‐3 fatty acids therapy. Significantly more adverse events (gastrointestinal and pruritus) in the omega‐3 group	+ These findings suggest a protective effect of omega‐3 fatty acid intake in idiopathic infertile men. More adverse events in omega‐3 group
Safarinejad 2012	Parallel, placebo Infertile men N = 228	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Sperm parameters improved significantly after coenzyme Q10	+ Coenzyme Q10 was significantly effective in men with unexplained oligoasthenoteratozoospermia for improving sperm density, sperm motility and sperm morphology
Schisterman 2020	Parallel, placebo Male partner of couples planning infertility treatment. Data from subfertile men used. N = 2370	Sperm parameters, DNA fragmentation, clinical pregnancy, live births, adverse events	Sperm parameters, DNA fragmentation, clinical pregnancy, live births, adverse events	Y ‐ sperm parameters Y ‐ DNA fragmentation N ‐ pregnancy, clinical; N ‐ live births N ‐ adverse events Data not provided for male factor infertility subgroup	No significant difference in sperm parameters between folic acid + zinc and placebo. No results on clinical outcomes in male factor subgroup	‐ Folic acid and zinc did not significantly improve semen quality. The findings also were similar when restricted to men with known male factor infertility or poor semen quality at baseline
Scott 1998	Multiple arm, placebo Men with subfertility and low sperm motility N = 69	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	Y ‐ sperm parameters N ‐ pregnancy rate, not usable due to pooling of data in the two intervention groups Table 1	Sperm motility increased in both selenium‐treated groups, only significant if both treatment groups were combined. Sperm density unaffected	± Selenium supplementation in subfertile men with low selenium status can improve sperm motility and the chance of successful conception. However, not all patients responded; 56% showed a positive response to treatment
Sharifzadeh 2016	Parallel, placebo Idiopathic subfertile men N = 114	Sperm parameters, adverse events	Sperm parameters, adverse events	Y‐ sperm parameters Y ‐ adverse events	Significant increase in concentration in zinc group	+ Normal sperm percentage and total sperm concentration increased after zinc sulphate treatment
Sigman 2006	Parallel, placebo Infertile men with low sperm motility N = 26	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	Y ‐ sperm parameters N ‐ pregnancy rate, biochemical Table 1	No statistically significant or clinically significant increase in motility or total motile sperm counts between baseline, 12 weeks, or 24 weeks in the carnitine or placebo arms.	‐ Carnitine supplementation demonstrated no clinically or statistically significant effect on sperm motility or total motile sperm counts. No difference in pregnancy rate
Sivkov 2011	Parallel, placebo Men with chronic prostatitis and infertility N = 30	Sperm parameters	Sperm parameters	N ‐ sperm parameters, no SD given Analysis 1.10	One‐month course of therapy produced no side effects, had a positive effect on low fertility of ejaculate.	+ Selenium + zinc improve
Sofikitis 2016	Multiple arm, no treatment, Avanafil excluded Oligoasthenospermic infertile men N = 39 Abstract only	Sperm parameters	Sperm parameters	N ‐ sperm parameters, no data available	No significant difference in L‐carnitine group regarding sperm parameters	‐ No direct conclusion made about L‐carnitine. From result section concluded: no impact on sperm parameters after use of L‐carnitine
Steiner 2020	Parallel, placebo Men with one abnormal semen parameter N = 171	Sperm parameters, DNA fragmentation, clinical pregnancy, live birth	Sperm parameters, DNA fragmentation, clinical pregnancy, live birth	Y ‐ sperm parameters Y ‐ DNA fragmentation (data shared by authors after requested via e‐mail) Y ‐ pregnancy, clinical Y ‐ live birth	No difference in sperm motility, DNA fragmentation, pregnancy rate and live birth rate between combined antioxidants and placebo	‐ No improvement in semen parameters in infertile males. This study suggests that combination antioxidants does not improve pregnancy or live birth rates
Stenqvist 2018	Parallel, placebo Infertile men with DNA fragmentation ≥ 25% N = 79	Sperm parameters, DNA fragmentation, pregnancy rate, adverse events	Sperm parameters, DNA fragmentation, pregnancy rate, adverse events	Y ‐ sperm parameters Y ‐ DNA fragmentation N ‐ pregnancy rate, biochemical Table 1 Y ‐ adverse events	No statistically significant difference between the antioxidant and placebo group was seen for semen parameters including DNA fragmentation	‐ Six months treatment with combined antioxidants had no effect on sperm parameters including DNA fragmentation
Suleiman 1996	Parallel, placebo Asthenospermic men N = 110	Sperm parameters	Sperm parameters, pregnancy rate, live birth, miscarriage	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical Y ‐ live birth Y ‐ adverse events: miscarriage	Vitamin E significantly decreased the MDA concentration in spermatozoa and improved sperm motility. Significant increase pregnancy/live birth rate	+ Vitamin E increases sperm motility, pregnancy rate and live birth rate compared to placebo
Sun 2018	Parallel, head‐to‐head Infertile men with low acrosin activity N = 232	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Significant increase of progressive sperm otility in men treated with L‐carnitine compared to vitamin E	+ L‐carnitine can effectively elevate sperm acrosin activity in male infertility patients, particularly in those with asthenozoospermia
Tremellen 2007	Parallel, placebo Male factor infertility N = 60	Pregnancy rate, adverse events	Pregnancy rate, adverse events, live birth provided by author	Y ‐ pregnancy rate, clinical Y ‐ live birth Y ‐ adverse events	Antioxidant group recorded a statistically significant improvement in viable pregnancy rate. Side‐effects on the Menevit antioxidant were rare (8%) and mild in nature.	+ Menevit antioxidant appears to be a useful ancillary treatment that significantly improves pregnancy rates in couples undergoing IVF‐ICSI treatment. Side‐effects on the Menevit antioxidant were rare (8%) and mild in nature.
Tsounapi 2018	Multiple arm, head‐to‐head Profertil + avanafil and avafanil only groups not used Idiopathic OAT N = 217	Sperm parameters, DNA fragmentation, pregnancy rate	Sperm parameters, DNA fragmentation, pregnancy rate	N ‐ sperm parameters N ‐ DNA fragmentation Not reported in how many patients sperm outcomes were assessed Y ‐ pregnancy rate, clinical	Significantly higher total and progressive sperm motility in Profertil group compared to L‐carnitine and no treatment. No difference in pregnancy rate	+ Profertil or Profertil combined with avanafil or or avanafil alone improves sperm membrane permeability with an improvement in sperm motility
Vinogradov 2019	Parallel, placebo Infertile men with at least one abnormal sperm parameter N = 109	Sperm parameters, DNA fragmentation	Sperm parameters, DNA fragmentation	N ‐ sperm parameters N ‐ DNA fragmenation Only results after cryotolerance test provided	No statistical differences between results after Brudy plus (combined antioxidant) and placebo	+/‐ No conclusions on outcomes of interest. Brudy Plus increases cryotolerance, promotes the normal formation of the genetic material and reduces the frequency of ultrastructural sperm disorders.
Wang 2010	Head‐to‐head Infertile men with asthenozoospermia N = 135	Sperm parameters, pregnancy rate, adverse events	Sperm parameters, pregnancy rate, adverse events	Y ‐ sperm parameters N ‐ pregnancy rate, not clear if clinical Table 1 N ‐ adverse events, zero found, however not clear which they aimed for	Significant increase in L‐carnitine + vitamin E group for sperm motility, no difference for sperm density and morphology. Pregnancy rate significantly higher in L‐carnitine + vitamin E group	+ L‐carnitine (+vitamin E) significantly improves sperm motility and pregnancy rate
Wong 2002	Multiple arm, placebo Fertile and subfertile men N = 103	Sperm parameters	Sperm parameters	Y ‐ sperm parameters (median+IQR converted to mean+ SD)	Subfertile men demonstrated a significant 74% increase in total normal sperm count and a minor increase of 4% abnormal spermatozoa	+ Total normal sperm count increases after combined zinc sulphate and folic acid treatment in both subfertile and fertile men
Zalata 1998	Head‐to‐head, pilot Men attending andrology clinic N = 22 Conference abstract	Sperm parameters	Sperm parameters	N ‐ sperm parameters, only before and after median data given	No significant difference in sperm parameters after treatment (acetyl‐cysteine or DHA). DNA damage measured by 8‐OHdG (fmol/ug) was significantly decreased after supplementation	‐ No improvement of sperm parameters
Zavaczki 2003	Parallel, placebo Men with idiopathic infertility N = 20	Sperm parameters, clinical pregnancy, adverse events	Sperm parameters, clinical pregnancy, adverse events	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical Y ‐ adverse events	No significant changes in sperm characteristics were detected	‐ Magnesium neither leads to a significant improvement of sperm variables nor does it increase the pregnancy rates
Zhou 2016	Parallel, head‐to‐head Idiopathic asthenozoospermia N = 120	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	Y ‐ sperm parameters N ‐ pregnancy rate, definition unclear Table 1 Y ‐ adverse events	Significant increase of total and progressive sperm motility in vitamin E and vitamin E + compound amino acids group. Greater increase in compound amino acids group. 5.7% pregnancy in combined group, 2% in vitamin E group. No adverse events	+ Compound amino acid combined with vitamin E can safely and effectively improve sperm motility in idiopathic asthenospermia patients.

DHA: docosahexaenoic acid; IUI: intrauterine insemination; NAC: N‐acetylcysteine; OAT:oligoasthenoteratozoospermia; ROS: reactive oxygen species

Excluded studies

We retrieved the full text of studies that were identified as potentially eligible (see Figure 1). In this update we excluded nine studies, in total we excluded 67 studies. Previously excluded study Raigani 2010, excluded based on ineligible outcome (MTHFR polymorphisms), was included as a sub‐study of the primary included study Raigani 2014. The most common reasons for exclusions were ineligible due to use of a different intervention, study design or population. See details in Characteristics of excluded studies.

In summary:

22/67 ineligible based on different intervention such as an added sperm wash or herbal extract; also pentoxifylline studies were excluded;
15/67 ineligible based on different study design; they were not randomised;
20/67 ineligible based on different population, either women, normospermic men or used the exact same population as other already included studies; in the search of this update; two of the studies was already included in the 2018 update;
2/67 ineligible based on different outcome;
6/67 ineligible based on different control group, fertile men without treatment or control group was not treated with placebo, no treatment or another antioxidant;
2 previously 'ongoing studies' were placed in excluded studies because they were terminated due to insufficient recruiting (NCT01075334; NCT01520584).

Ongoing studies

Twelve studies were “ongoing studies” in the 2018 update. In the current update, six of the 12 previously ongoing studies were included (Amini 2020; Bahmyari 2021; Eslamian 2020; Joseph 2020; Kumalic 2020; Steiner 2020). NCT03337360 continued as an ongoing study with the status of still recruiting. The former ongoing study NCT03104998 was excluded based on withdrawal on the trial registry website. The former ongoing study NCT01407432 was placed in Studies awaiting classification after a message from the author that the manuscript had been submitted but not yet published. Former ongoing studies NCT00975117 and NCT01828710 were also placed in Studies awaiting classification with the status of “completed” on the trial registry website. The recruitment for former ongoing study DRKS00011616 had stopped and was therefore placed in Studies awaiting classification as well. Authors were contacted for (unpublished) results, with no reply.

Awaiting classification

One study was “awaiting classification” in the 2018 update of this review (Goswami 2015). We included the study after confirmation by the authors that this was a randomised controlled trial.

Four formerly ongoing studies were placed in Studies awaiting classification (DRKS00011616; NCT00975117; NCT01407432; NCT01828710). The authors from NCT01407432 replied that the manuscript was under submission. The authors from the other three studies did not reply when contacted for further information.

One study from the updated 2021 search was placed in Studies awaiting classification (Kuzmenko 2018). It was not clear whether the study population was infertile men with abnormal semen parameters. The full report of this study was requested from the authors, with no reply.

Risk of bias in included studies

See Figure 2 for a summary of risk of bias in individual studies, and Figure 3 for a summary of each risk of bias item across all included studies.

Allocation

Sequence generation

All 90 included studies were randomised, six of these were cross‐over studies (Akiyama 1999; Kessopoulou 1995; Lenzi 2003; Lombardo 2002; Peivandi 2010; Pryor 1978), and the remaining studies were parallel design studies.

Only 47 studies described their methods of sequence generation and were rated as low risk in this domain (Abbasi 2020; Amini 2020; Ardestani 2019; Azizollahi 2013; Bahmyari 2021; Balercia 2005; Barekat 2016; Biagiotti 2003; Blomberg Jensen 2018; Busetto 2018; Cavallini 2004; Cheng 2018; Cyrus 2015; Eslamian 2013; Eslamian 2020; Exposito 2016; Galatioto 2008; Gamidov 2017; Gamidov 2019; Gonzalez‐Ravina 2018; Gopinath 2013; Haghighian 2015; Huang 2020; Joseph 2020; Kessopoulou 1995; Kizilay 2019; Kopets 2020; Kumalic 2020; Lu 2018; Martinez‐Soto 2010; Micic 2019; Nadjarzadeh 2011; Popova 2019; Rolf 1999; Safarinejad 2009; Safarinejad 2009a; Safarinejad 2011b; Safarinejad 2012; Schisterman 2020; Scott 1998; Sharifzadeh 2016; Sigman 2006; Steiner 2020; Stenqvist 2018; Tremellen 2007; Wong 2002; Zhou 2016) (see Figure 2 and Figure 3).

Figure 2

Methodological risk of bias summary: review authors' judgements about each methodological bias item for each included study.

Figure 3

Methodological risk of bias graph: review authors' judgements about each methodological bias item presented as percentages across all included studies.

One study was rated as high risk in this domain, because the authors reported that "a placebo‐controlled group was maintained in parallel" (Goswami 2015). The review team suspected that the placebo group in this study had not been randomised. Authors were contacted, with no reply to date.

The remaining 42 studies were rated as unclear risk (Alahmar 2019; Alahmar 2020; Akiyama 1999; Attallah 2013; Balercia 2009; Boonyarangkul 2015; Conquer 2000; Dawson 1990; Deng 2014; Dimitriadis 2010; Ener 2016; Greco 2005; Haje 2015; Korshunov 2018; Kumamoto 1988; Lenzi 2003; Lenzi 2004; Li 2005; Li 2005a; Lombardo 2002; Martinez 2015; Mehni 2014; Morgante 2010; Nouri 2019; Nozha 2001; Omu 1998; Omu 2008; Peivandi 2010; Pourmand 2014; Poveda 2013; Pryor 1978; Raigani 2014; Saeed Alkumait 2020; Sivkov 2011; Sofikitis 2016; Suleiman 1996; Sun 2018; Tsounapi 2018; Vinogradov 2019; Wang 2010; Zalata 1998; Zavaczki 2003).

The predominant method of randomisation was by computer‐generated blocks. Tremellen 2007 reported a 2:1 ratio randomisation schedule, Cyrus 2015 reported a 3:2 randomisation schedule, Li 2005 appeared to have a blocked 3:2 allocation, Kizilay 2019 appeared to have a 2:1 ratio, Gamidov 2019; Popova 2019; Sun 2018 appeared to have a 3:1 ratio, Micic 2019 appeared to have a 5:2 ratio and Zhou 2016 appeared to have a 7:5 ratio.

Allocation concealment

The methods of allocation concealment were generally quite poorly described in the included studies. Thirty‐two studies described both their methods of randomisation and allocation concealment and were rated as low risk in this domain (Abbasi 2020; Amini 2020; Azizollahi 2013; Balercia 2005; Blomberg Jensen 2018; Busetto 2018; Cavallini 2004; Cyrus 2015; Eslamian 2013; Eslamian 2020; Exposito 2016; Galatioto 2008; Gonzalez‐Ravina 2018; Gopinath 2013; Haghighian 2015; Huang 2020; Joseph 2020; Kopets 2020; Kumalic 2020; Lu 2018; Martinez‐Soto 2010; Nadjarzadeh 2011; Peivandi 2010; Popova 2019; Safarinejad 2009; Safarinejad 2012; Schisterman 2020; Sharifzadeh 2016; Sigman 2006; Stenqvist 2018; Tremellen 2007; Wong 2002).

There were three studies with a high risk of allocation concealment: one due to the use of a randomisation table by the doctor (Barekat 2016); one due to great baseline imbalance for sperm parameters between the intervention and control group (Boonyarangkul 2015); and one due to the use of an open randomisation list, showing what the next randomisation would be (Ardestani 2019).

The remaining 55 studies were rated as unclear risk (Akiyama 1999; Alahmar 2019; Alahmar 2020; Attallah 2013; Bahmyari 2021; Balercia 2009; Biagiotti 2003; Cheng 2018; Conquer 2000; Dawson 1990; Deng 2014; Dimitriadis 2010; Ener 2016; Gamidov 2017; Gamidov 2019; Goswami 2015; Greco 2005; Haje 2015; Kessopoulou 1995; Kizilay 2019; Korshunov 2018; Kumamoto 1988; Lenzi 2003; Lenzi 2004; Li 2005; Li 2005a; Lombardo 2002; Martinez 2015; Mehni 2014; Micic 2019; Morgante 2010; Nozha 2001; Nouri 2019; Omu 1998; Omu 2008; Pourmand 2014; Poveda 2013; Pryor 1978; Raigani 2014; Rolf 1999; Saeed Alkumait 2020; Safarinejad 2009a; Safarinejad 2011b; Scott 1998; Sivkov 2011; Sofikitis 2016; Steiner 2020; Suleiman 1996; Sun 2018; Tsounapi 2018; Vinogradov 2019; Wang 2010; Zalata 1998; Zavaczki 2003; Zhou 2016). The methods of allocation concealment included anonymous coloured boxes, sealed opaque envelopes, and numbered bottles.

Blinding

Performance bias

Forty‐three studies were described as randomised, double‐blind controlled trials in which clinicians and participants were blinded (Azizollahi 2013; Balercia 2005; Balercia 2009; Blomberg Jensen 2018; Boonyarangkul 2015; Busetto 2018; Cavallini 2004; Cyrus 2015; Dawson 1990; Eslamian 2020; Exposito 2016; Gonzalez‐Ravina 2018; Gopinath 2013; Greco 2005; Huang 2020; Kessopoulou 1995; Kopets 2020; Kumalic 2020; Kumamoto 1988; Lenzi 2003; Lenzi 2004; Lombardo 2002; Martinez 2015; Martinez‐Soto 2010; Mehni 2014; Micic 2019; Nadjarzadeh 2011; Nouri 2019; Poveda 2013; Pryor 1978; Raigani 2014; Rolf 1999; Safarinejad 2009; Safarinejad 2009a; Safarinejad 2011b; Safarinejad 2012; Scott 1998; Sharifzadeh 2016; Sigman 2006; Steiner 2020; Tremellen 2007; Vinogradov 2019; Wong 2002). In seven studies investigators, clinicians and participants were blinded (Abbasi 2020; Amini 2020; Eslamian 2013; Gamidov 2019; Haghighian 2015; Schisterman 2020; Stenqvist 2018). A total of fifty studies were rated as low risk (see Figure 3 and Figure 2). In one of the low risk studies (Dawson 1990), it was stated that a placebo was used as the control but only the participants were blinded.

Twenty‐three other studies were rated high risk (Alahmar 2019; Alahmar 2020; Ardestani 2019; Attallah 2013; Barekat 2016; Biagiotti 2003; Deng 2014; Dimitriadis 2010; Ener 2016; Galatioto 2008; Gamidov 2017; Joseph 2020; Kizilay 2019; Korshunov 2018; Morgante 2010; Nozha 2001; Omu 1998; Omu 2008; Popova 2019; Pourmand 2014; Sofikitis 2016; Suleiman 1996; Tsounapi 2018). Of these high‐risk studies, 18 studies used 'no treatment' as their comparator. Two studies were head‐to‐head trials and open‐labelled (Alahmar 2019; Alahmar 2020; Deng 2014; Nozha 2001). The double‐blinded trial Suleiman 1996 used a placebo, however they reported that if a couple became pregnant then "the treatment was stopped; otherwise it was continued for 6 months. The placebo was given for 6 months." This does appear that they did not stop the placebo. This could suggest that the investigators had knowledge of whether the participants were in the placebo or antioxidant group, therefore this study was rated as high risk.

Sixteen studies did not give a statement regarding blinding and were rated as unclear risk of bias (Akiyama 1999; Bahmyari 2021; Cheng 2018; Conquer 2000; Goswami 2015; Haje 2015; Li 2005; Li 2005a; Lu 2018; Saeed Alkumait 2020; Sivkov 2011; Sun 2018; Wang 2010; Zalata 1998; Zavaczki 2003; Zhou 2016). Seven of these studies used a placebo as the control but did not discuss blinding (Bahmyari 2021; Conquer 2000; Goswami 2015; Lu 2018; Saeed Alkumait 2020; Sivkov 2011; Zavaczki 2003).

As nutritional supplements with antioxidant properties are freely available, this could have introduced bias in the included studies. None of the included studies monitored or reported use of additional supplements other than the intervention during the study. However, most included studies reported the use of other nutritional supplement as an exclusion criterion and instructed participants to withhold from such supplement use during the study.

Detection bias

The methods of blinding outcome assessment were generally poorly described in the included studies. Only 26 studies reported this aspect of blinding and were therefore classified as low risk (Abbasi 2020; Amini 2020; Ardestani 2019; Azizollahi 2013; Balercia 2005; Barekat 2016; Blomberg Jensen 2018; Busetto 2018; Cavallini 2004; Cyrus 2015; Eslamian 2013; Galatioto 2008; Gamidov 2017; Gamidov 2019; Gopinath 2013; Haghighian 2015; Martinez 2015; Micic 2019; Nadjarzadeh 2011; Peivandi 2010; Popova 2019; Raigani 2014; Safarinejad 2009a; Safarinejad 2012; Schisterman 2020; Stenqvist 2018).

The other 64 studies were rated as unclear risk due to the lack of information (Akiyama 1999; Alahmar 2019; Alahmar 2020; Attallah 2013; Bahmyari 2021; Balercia 2009; Biagiotti 2003; Boonyarangkul 2015; Cheng 2018; Conquer 2000; Dawson 1990; Deng 2014; Dimitriadis 2010; Ener 2016; Eslamian 2020; Exposito 2016; Gonzalez‐Ravina 2018; Greco 2005; Goswami 2015; Haje 2015; Huang 2020; Joseph 2020; Kessopoulou 1995; Kizilay 2019; Kopets 2020; Korshunov 2018; Kumalic 2020; Kumamoto 1988; Lenzi 2003; Lenzi 2004; Li 2005; Li 2005a; Lombardo 2002; Lu 2018; Martinez‐Soto 2010; Mehni 2014; Morgante 2010; Nouri 2019; Nozha 2001; Omu 1998; Omu 2008; Pourmand 2014; Poveda 2013; Pryor 1978; Rolf 1999; Saeed Alkumait 2020; Safarinejad 2009; Safarinejad 2011b; Scott 1998; Sharifzadeh 2016; Sigman 2006; Sivkov 2011; Sofikitis 2016; Steiner 2020; Suleiman 1996; Sun 2018; Tremellen 2007; Tsounapi 2018; Vinogradov 2019; Wang 2010; Wong 2002; Zalata 1998; Zavaczki 2003; Zhou 2016).

Incomplete outcome data

Fifty‐one studies were rated as low risk for incomplete outcome data (Akiyama 1999; Alahmar 2019; Amini 2020; Ardestani 2019; Azizollahi 2013; Bahmyari 2021; Balercia 2005; Balercia 2009; Blomberg Jensen 2018; Busetto 2018; Conquer 2000; Cyrus 2015; Dawson 1990; Eslamian 2013; Eslamian 2020; Exposito 2016; Gopinath 2013; Galatioto 2008; Gamidov 2017; Gamidov 2019; Gonzalez‐Ravina 2018; Greco 2005; Haghighian 2015; Kizilay 2019; Kopets 2020; Korshunov 2018; Kumalic 2020; Lenzi 2003; Lenzi 2004; Li 2005; Martinez 2015; Micic 2019; Nadjarzadeh 2011; Nouri 2019; Omu 2008; Popova 2019; Pourmand 2014; Rolf 1999; Safarinejad 2009; Safarinejad 2009a; Safarinejad 2011b; Safarinejad 2012; Schisterman 2020; Scott 1998; Sharifzadeh 2016; Sigman 2006; Tremellen 2007; Vinogradov 2019; Wang 2010; Zavaczki 2003; Zhou 2016).

Thirty‐two studies were rated as unclear, most of them did report the number of dropouts, but did not provide the reasons (Alahmar 2020; Attallah 2013; Biagiotti 2003; Boonyarangkul 2015; Deng 2014; Dimitriadis 2010; Ener 2016; Goswami 2015; Haje 2015; Huang 2020; Kessopoulou 1995; Kumamoto 1988; Li 2005a; Lombardo 2002; Lu 2018; Martinez‐Soto 2010; Mehni 2014; Morgante 2010; Nozha 2001; Omu 1998; Peivandi 2010; Poveda 2013; Pryor 1978; Raigani 2014;Saeed Alkumait 2020; Sivkov 2011; Sofikitis 2016; Stenqvist 2018; Sun 2018; Tsounapi 2018; Wong 2002; Zalata 1998).

Six studies were rated as high risk of attrition bias due to lack of compliance directly related to treatment and high dropout rates (16% to 42%) (Abbasi 2020; Barekat 2016; Cavallini 2004; Cheng 2018; Joseph 2020; Suleiman 1996). One study was rated as high risk of attrition bias despite the fact that high dropout rates were accounted for, because the results tables appeared to have additional missing data without clarification (Steiner 2020).

None of the included studies reported on "missing not at random", which could be introduced by participants not returning for a subsequent semen analysis if a pregnancy occurred before that date.

Only 10 studies (Balercia 2009; Blomberg Jensen 2018; Busetto 2018; Eslamian 2020; Galatioto 2008; Joseph 2020; Pryor 1978; Safarinejad 2011b; Schisterman 2020; Steiner 2020) actually stated that they used intention‐to‐treat (ITT) in their analysis. However, Pryor 1978 stated they had used ITT, but the data were not presented. Most of the other included studies accounted for the participants that withdrew from their studies and then analysed the groups using a per protocol approach.

Five studies (Azizollahi 2013; Barekat 2016; Cheng 2018; Kizilay 2019; Wang 2010) did not use ITT, however the numbers of dropouts were given for each intervention and control group, and therefore we were able to use ITT in the data analysis by making the assumption of no event for the binary outcomes. No imputation was carried out on the continuous outcome data; these were analysed as they were reported in the studies.

Nine studies had over 20% withdrawal from their studies. Cavallini 2004 had a 30% dropout rate and reasons were provided for only 53 out of the 55 dropouts; these reasons included refusal due to the chance of taking a placebo and preference for assisted reproduction techniques (ARTs). There also remained some confusion in this study on the total numbers randomised and analysed. Abbasi 2020 and Joseph 2020 both had a dropout rate of around 32%; Azizollahi 2013 had a 30% dropout rate; Li 2005a; Steiner 2020; Suleiman 1996, Nadjarzadeh 2011, and Barekat 2016 had slightly over 20% withdrawal from their studies.

One study (Suleiman 1996) had a large imbalance in numbers. There were found to be 52 in the treatment group and 35 in the placebo group once the code had been broken at the end of the study. There was no indication of how the randomisation was performed. The reasons given for dropout were only accounted for broadly: many couples had left the region and some simply failed to continue; no numbers were given for individual dropout reasons (see Figure 3 and Figure 2). The numbers of participants that were initially randomised to each group were not available, so ITT for the dichotomous outcomes was not possible.

Selective reporting

Study protocols were only available for 18 out of the 90 included studies (Amini 2020; Ardestani 2019; Azizollahi 2013; Bahmyari 2021; Blomberg Jensen 2018; Cyrus 2015; Eslamian 2020; Exposito 2016; Gonzalez‐Ravina 2018; Joseph 2020; Kopets 2020; Kumalic 2020; Nouri 2019; Raigani 2014; Schisterman 2020; Sharifzadeh 2016; Steiner 2020; Stenqvist 2018). The study protocol of Alahmar 2019 was published after completion of the study and was therefore rated as unclear risk.

Thirteen studies were rated at high risk of reporting bias. Kumamoto 1988 performed subgroup analysis post‐treatment and Safarinejad 2012 did not pre‐specify outcomes. Two of these 13 studies were rated at high risk of reporting bias because outcomes defined in the study protocol were not reported in the full text of the study (Kopets 2020; Kumalic 2020). Nine of these 13 studies were rated at high risk of reporting bias because outcomes defined in the methods section of the articles were not reported in the outcomes section, or the results of certain subgroups of the study population were omitted (Huang 2020; Joseph 2020; Kizilay 2019; Micic 2019; Popova 2019; Saeed Alkumait 2020; Schisterman 2020; Steiner 2020; Vinogradov 2019).

Seven studies were rated as unclear risk as they were conference abstracts (Attallah 2013; Biagiotti 2003; Goswami 2015; Korshunov 2018; Lombardo 2002; Sofikitis 2016; Zalata 1998), and two studies were rated as unclear as it was possible that these were two publications of the same study that were reporting on different intervention arms (Li 2005; Li 2005a). Obtaining help with Chinese translation did not clarify this and attempts to contact the authors were unsuccessful. The remaining 52 studies were rated as unclear risk in this domain because there were no published study protocols available.

Other potential sources of bias

There were no other sources of bias in the included studies.

In summary, none of the included studies was rated as low risk of bias in all domains. More than half of the included studies (52 of the 90 included studies) was rated as unclear risk of bias in at least one domain. Thirty‐eight included studies were rated as high risk of bias in at least one domain (Figure 2).

In the comparison of antioxidant versus placebo or no treatment with the outcome of live birth, half of the studies was rated as unclear risk of bias in at least one domain. The other half of the studies in this comparison was rated as high risk of bias in at least one domain (Figure 4).

Figure 4

Forest plot of comparison: 1 Antioxidant(s) versus placebo or no treatment, outcome: 1.1 Live birth; type of antioxidant.

Effects of interventions

See: Summary of findings 1 Antioxidants compared to placebo or no treatment for patients with male subfertility

1 Antioxidants versus placebo or no treatment (natural conception and undergoing fertility treatment)

1.1 Live birth; type of antioxidant

See Analysis 1.1 and Figure 4, Figure 5.

Figure 5

Funnel plot of comparison: 1 Antioxidant(s) versus placebo or no treatment, outcome: 1.1 Live birth; type of antioxidant.

Only 12 studies reported on live birth; seven of these had methodological inadequacies as they did not describe their methods of randomisation or allocation concealment. Three studies reported that all clinical pregnancies led to a live birth (Balercia 2005; Balercia 2009; Kessopoulou 1995). The meta‐analysis of the 12 studies showed that antioxidants were associated with increased live birth rate compared with placebo or no treatment (Peto odds ratio (OR) 1.43, 95% confidence interval (CI) 1.07 to 1.91, 1283 men, 12 RCTs, P = 0.02, I² = 44%, very low‐certainty evidence). This means that, for subfertile men with a baseline expected live birth rate of 16%, use of an antioxidant could increase this rate to between 17% and 27% (summary of findings Table 1).

1.1.1 One study reported on this outcome comparing astaxanthin plus vitamin E versus placebo (Kumalic 2020). There was no evidence of increased live birth rate (Peto OR 1.63, 95% CI 0.34 to 7.69, 36 men, P = 0.54, I² = not applicable).

1.1.2 One study reported on this outcome comparing carnitines versus placebo (Balercia 2005). There was no evidence of increased live birth rate (Peto OR 1.00, 95% CI 0.24 to 4.25; 60 men, P = 1.00, I² = not applicable).

1.1.3 One study reported on this outcome comparing coenzyme Q10 versus placebo (Balercia 2009). There was no evidence of increased live birth rate (Peto OR 2.16, 95% CI 0.53 to 8.82; 60 men, P = 0.28, I² = not applicable).

1.1.4 One study reported on this outcome comparing vitamin D plus calcium versus placebo (Blomberg Jensen 2018). There was no evidence of increased live birth rate (Peto OR 1.03, 95% CI 0.59 to 1.80, 330 men, P = 0.93, I² = not applicable).

1.1.5 Two studies reported on this outcome comparing vitamin E versus placebo (Kessopoulou 1995; Suleiman 1996). There appeared to be evidence of increased live birth rate (Peto OR 8.51, 95% CI 2.36 to 30.70, 140 men, 2 RCTs, P = 0.001, I² = 0%).

1.1.6 One study reported on this outcome comparing zinc versus no treatment (Omu 1998). There was no evidence of increased live birth rate (Peto OR 3.74, 95% CI 1.02 to 13.74, 100 men, P = 0.05, I² = not applicable).

1.1.7 Five studies reported on this outcome comparing combined antioxidants versus placebo or no treatment (Gamidov 2019; Joseph 2020; Korshunov 2018; Steiner 2020; Tremellen 2007). There was no evidence of increased live birth rate (Peto OR 1.28, 95% CI 0.86 to 1.91, 557 men, P = 0.23, I² = 63%). The results from Tremellen 2007 also included three sets of twins in the combined antioxidant group and nil in the placebo group. Each twin birth was counted as one event as stated in the methods section in the review protocol.

There was no evidence that different antioxidants had differing effects (test for subgroup differences Chi² = 11.76, P = 0.07).

A sensitivity analysis was carried out using as‐treated data, which did not show a different result compared with the intention‐to‐treat data (Peto OR 1.49, 95% CI 1.10 to 2.00, 1090 men, 12 RCTs, P = 0.009, I² = 28%).

Sensitivity analysis for studies with no treatment as control

Three studies (Joseph 2020; Korshunov 2018; Omu 1998) used 'no treatment' as the control group instead of placebo. When these studies were removed from the analysis, no evidence of increased live birth remained when compared with placebo only (Peto OR 1.39, 95% CI 0.98 to 1.97, 937 men, 9 RCTs, P = 0.06, I² = 52%).

There was no evidence that different antioxidants had differing effects (test for subgroup differences: Chi² = 9.50, P = 0.09).

Sensitivity analysis for studies reporting live birth and clinical pregnancy

The 12 studies that reported live birth had an OR for live birth of 1.43, and in these same studies the OR for clinical pregnancy was 1.62. When we pooled all 20 studies reporting the clinical pregnancy rate there was a higher OR 1.89. This suggests that there is no overestimation of live birth. However, the true effect is unknown unless all studies reporting on clinical pregnancy rate also reported on live birth rate.

Sensitivity analysis for studies rated as high risk of bias

When the four studies (Joseph 2020; Korshunov 2018; Omu 1998; Suleiman 1996) rated with a high risk of bias were removed from the analysis, there was no evidence of association between antioxidants and an increased live birth rate when compared with placebo (Peto OR 1.22, 95% CI 0.85 to 1.75, 827 men, 8 RCTs, P = 0.27, I² = 32%).

1.2 Live birth; in vitro fertilisation (IVF)/intracytoplasmic sperm injection (ICSI)

See Analysis 1.2.

There were only five studies in women undergoing IVF/ICSI which reported on live birth (Joseph 2020; Kessopoulou 1995; Korshunov 2018; Kumalic 2020; Tremellen 2007). There appeared to be evidence of increased live birth rate, in those couples undergoing IVF/ICSI, with antioxidant use when compared with placebo (Peto OR 1.63, 95% CI 1.01 to 2.16, 5 RCTs, 372 men, P = 0.04, I² = 0%).

1.3 Clinical pregnancy; type of antioxidant

See Analysis 1.3 and Figure 6 and Figure 7.

Figure 6

Forest plot of comparison: 1 Antioxidant(s) versus placebo or no treatment, outcome: 1.3 Clinical pregnancy; type of antioxidant.

Figure 7

Funnel plot of comparison: 1 Antioxidant(s) versus placebo or no treatment, outcome: 1.5 Clinical pregnancy; type of antioxidant.

Only 20 studies (with 25 intervention arms) reported on clinical pregnancy rate; six of these had methodological inadequacies with high risk of bias for methods of randomisation, allocation concealment or blinding. The meta‐analysis of these studies showed that antioxidants were associated with an increased clinical pregnancy rate when compared with placebo or no treatment (Peto OR 1.89, 95% CI 1.45 to 2.47, 1706 men, 20 RCTs, 25 intervention arms, P < 0.00001, I² = 3%, low‐certainty evidence). This means that, for subfertile men with a baseline expected clinical pregnancy rate of 15%, use of an antioxidant could increase this rate to between 20% and 30% (summary of findings Table 1).

1.3.1 One study reported on this outcome comparing astaxanthin plus vitamin E versus placebo (Kumalic 2020). There was no evidence of increased clinical pregnancy rate (Peto OR 1.32, 95% CI 0.35 to 4.96, 36 men, P = 0.68, I² = not applicable).

1.3.2 Two studies reported on this outcome comparing carnitines versus placebo (Balercia 2005; Tsounapi 2018). There was no evidence of increased clinical pregnancy rate (Peto OR 1.17, 95% CI 0.30 to 4.59, 125 men, 2 RCTs, P = 0.82, I² = 0%). In Tsounapi 2018, the one and only event in the control group was used in the "Combined antioxidants" subgroup (1.5.11), as all results for clinical pregnancies were pooled.

1.3.3 One study reported on this outcome comparing coenzyme Q10 versus placebo (Balercia 2009). There was no evidence of increased clinical pregnancy rate (Peto OR 2.16, 95% CI 0.53 to 8.82, 60 men, 1 RCT, P = 0.28, I² = not applicable).

1.3.4 One study reported on this outcome comparing folic acid versus placebo (Azizollahi 2013). There was no OR estimable due to the occurrence of zero pregnancies in both groups.

1.3.5 One study reported on this outcome comparing magnesium versus placebo (Zavaczki 2003). There was no evidence of increased clinical pregnancy rate (Peto OR 8.73, 95% CI 0.17 to 445.08, 1 RCT, 26 men, P = 0.28, I² = not applicable).

1.3.6 Two studies reported on this outcome comparing N‐acetylcysteine versus placebo or no treatment (Attallah 2013; Barekat 2016). There was no evidence of increased clinical pregnancy rate (Peto OR 2.00, 95% CI 0.71 to 5.63, 100 men, 2 RCTs, P = 0.19, I² = 0%).

1.3.7 Two studies reported on this outcome comparing vitamin E versus placebo (Kessopoulou 1995; Suleiman 1996). There appeared to be an increased clinical pregnancy rate (Peto OR 6.71, 95% CI 1.98 to 22.69, 2 RCTs, 117 men, P = 0.002, I² = 0%).

1.3.8 Two studies reported on this outcome comparing zinc versus placebo or no treatment (Azizollahi 2013; Omu 1998). There appeared to be an increased clinical pregnancy rate (Peto OR 4.43, 95% CI 1.39 to 14.14, 2 RCTs, 153 men, P = 0.01, I² = 0%).

1.3.9 One study reported on this outcome comparing zinc with folic acid versus placebo (Azizollahi 2013). There was no evidence of increased clinical pregnancy rate (Peto OR 3.86, 95% CI 0.15 to 99.84, 53 men, 1 RCT, P = 0.42, I² = not applicable).

1.3.10 Ten studies reported on this outcome comparing combined antioxidants versus placebo or no treatment (Busetto 2018; Gamidov 2019; Joseph 2020; Kizilay 2019; Kopets 2020; Korshunov 2018; Popova 2019; Steiner 2020; Tremellen 2007; Tsounapi 2018). There appeared to be an increased clinical pregnancy rate (Peto OR 1.67, 95% CI 1.22 to 2.28, 983 men, 10 RCTs, P = 0.001, I² = 36%).

There was no evidence that different antioxidants had differing effects (test for subgroup differences: Chi² = 8.41, P = 0.39).

Sensitivity analysis for studies with no treatment as control

Seven studies used 'no treatment' as control group instead of placebo (Attallah 2013; Joseph 2020; Kizilay 2019; Korshunov 2018; Omu 1998; Popova 2019; Tsounapi 2018). When these studies were removed from the analysis, the association between antioxidant use and increased clinical pregnancy rate remained (Peto OR 1.96, 95% CI 1.36 to 2.83, 996 men, 13 RCTs, 17 intervention arms, P = 0.0003, I² = 25%).

There was no evidence that different antioxidants had differing effects (test for subgroup differences: Chi² = 6.43, P = 0.60).

Sensitivity analysis for studies rated as high risk of bias

When the seven studies rated with a high risk of bias were removed from the analysis, there remained an association between antioxidants and an increased clinical pregnancy rate (Peto OR 1.78, 95% CI 1.26 to 2.51, 1042 men, 13 RCTs, P = 0.001, I² = 8%) (Attallah 2013; Barekat 2016; Joseph 2020; Korshunov 2018; Omu 1998; Suleiman 1996; Tsounapi 2018).

Sensitivity analysis for studies enrolling men with varicocele

When the four studies that enrolled men with varicocele or after varicocelectomy were removed from the analysis, the use of antioxidants remained associated with increased clinical pregnancy rate when compared with placebo or no treatment (Peto OR 1.78, 95% CI 1.34 to 2.38, 1179 men, 15 RCTs, P < 0.0001, I² = 23%) (Azizollahi 2013; Barekat 2016; Busetto 2018; Kizilay 2019).

Sensitivity analysis for studies enrolling men in couples undergoing intrauterine insemination (IUI)

Two studies reported on men in couples undergoing IUI (Attallah 2013; Steiner 2020). When these studies were removed from the analysis there remained an association between the use of antioxidants and increased clinical pregnancy rate when compared with placebo or no treatment (Peto OR 2.25, 95% CI 1.69 to 3.00, 1245 men, 17 RCTs, P < 0.0001, I² = 0%).

1.4 Clinical pregnancy; IVF/ICSI

See Analysis 1.4.

There were six studies in women undergoing IVF/ICSI which reported on clinical pregnancy rate (Joseph 2020; Kessopoulou 1995; Korshunov 2018; Kumalic 2020; Popova 2019; Tremellen 2007). The meta‐analysis of these studies showed an increase in clinical pregnancy in those couples undergoing IVF/ICSI, when antioxidant use was compared with placebo or no treatment (Peto OR 1.73, 95% CI 1.15 to 2.61, 452 men, 6 RCTs, P = 0.009, I² = 0%).

1.5 Adverse events

See Analysis 1.5 and Figure 8.

Figure 8

Forest plot of comparison: 1 Antioxidant(s) versus placebo or no treatment, outcome: 1.5 Adverse events.

The adverse events reported in the studies were miscarriage, ectopic pregnancy, stillbirth, gastrointestinal disorders, euphoria, headache, upper respiratory infection, and nasopharyngitis.

1.5.1 Miscarriage. Only six studies reported on miscarriage and the event rate was very low (28 miscarriages from 618 couples) (Joseph 2020; Korshunov 2018; Omu 1998; Steiner 2020; Suleiman 1996; Tremellen 2007). The analysis of these six studies showed no evidence of increased miscarriage between the use of antioxidants when compared with placebo or no treatment (Peto OR 1.46, 95% CI0.75 to 2.83, 6 RCTs, 664 men, P = 0.27, I² = 3%, very low‐certainty evidence). This means that, for subfertile men with a baseline expected miscarriage rate of 5%, the chances of having a miscarriage could lie between 4% and 13% with the use of an antioxidant (summary of findings Table 1).

1.5.2 Ectopic pregnancy. Only two studies (Joseph 2020; Tremellen 2007) reported on this adverse event and there was no evidence of increase of ectopic pregnancy when antioxidants were compared with placebo or no treatment (Peto OR 1.59, 95% CI 0.16 to 16.01, 2 RCTs, 260 men, P = 0.69, I² = 0%).

1.5.3 Stillbirth. Only one study (Joseph 2020) reported on this adverse event and there was no evidence of increase of stillbirth when antioxidants were compared with no treatment (Peto OR 0.14, 95% CI 0.00 to 6.82, 1 RCT, 200 men, P = 0.32, I² = not applicable).

1.5.4 Gastrointestinal. The analysis of 16 studies showed an association between the use of antioxidants and an increase in gastrointestinal discomfort when compared with placebo or no treatment (Peto OR 2.70, 95% CI 1.46 to 4.99, 1355 men, 16 RCTs, P = 0.002, I² = 40%, low‐certainty evidence) (Busetto 2018; Cavallini 2004; Gamidov 2017; Gamidov 2019; Gopinath 2013; Kessopoulou 1995; Kizilay 2019; Kopets 2020; Kumalic 2020; Pourmand 2014; Safarinejad 2009a; Sharifzadeh 2016; Sigman 2006; Stenqvist 2018; Tremellen 2007; Zavaczki 2003). However, the event rate was very low, so we could not be sure of these results. Six of these 16 studies reported no events, therefore a funnel plot was not created.

1.5.5 Euphoria. Only one study (Cavallini 2004) reported on this adverse event and there was no evidence of increased occurrence of euphoria when antioxidants were compared with placebo (Peto OR 1.21, 95% CI 0.16 to 9.01, 1 RCT, 86 men, P = 0.85, I² = not applicable).

1.5.6 Headache. Only one study (Steiner 2020) reported on this adverse event and there was no evidence of increased occurrence of headache when antioxidants were compared with placebo (Peto OR 2.32, 95% CI 0.95 to 5.67, 1 RCT, 171 men, P = 0.06, I² = not applicable).

1.5.7 Upper respiratory infection. Only one study (Steiner 2020) reported on this adverse event and there was no evidence of increased occurrence of upper respiratory infection when antioxidants were compared with placebo (Peto OR 1.01, 95% CI 0.25 to 4.17, 1 RCT, 171 men, P = 0.99, I² = not applicable).

1.5.8 Nasopharyngitis. Only one study (Steiner 2020) reported on this adverse event and there was no evidence of increased occurrence of nasopharyngitis when antioxidants were compared with placebo (Peto OR 0.57, 95% CI 0.17 to 1.92, 1 RCT, 171 men, P = 0.36, I² = not applicable).

It was unlikely that the adverse events ectopic pregnancy, stillbirth, euphoria, headache, upper respiratory infection, and nasopharyngitis were related to intake of antioxidants especially with the reported extreme low event rate. Therefore, these outcomes were not included in the 'Summary of findings' table.

1.6 Sperm DNA fragmentation at three months or less; type of antioxidant

See Analysis 1.6, Figure 9.

Figure 9

Forest plot of comparison: 1 Antioxidant(s) versus placebo or no treatment, outcome: 1.6 Sperm DNA fragmentation; type of antioxidant.

We analysed this outcome using a fixed‐effect model and used subtotals as pooling was not possible.

1.6.1 Astaxanthin plus vitamin E did not show evidence of decreased sperm DNA fragmentation when compared with placebo (Kumalic 2020) mean difference(MD) 1.40, 95% CI ‐6.64 to 9.44, 72 men, 1 RCT, P = 0.73, I² = not applicable).

1.6.2 Folic acid did not show evidence of decreased sperm DNA fragmentation when compared with placebo (Raigani 2014) (MD ‐5.80, 95% CI ‐13.40 to 1.80, 38 men, 1 RCT, P = 0.13, I² = not applicable).

1.6.3 Folic acid plus zinc did not show evidence of decreased sperm DNA fragmentation when compared with placebo (Raigani 2014) (MD ‐1.20, 95% CI ‐9.36 to 6.96, 39 men, 1 RCT, P = 0.77, I² = not applicable).

1.6.4 N‐acetylcysteine (NAC) did not show evidence of decreased sperm DNA fragmentation when compared with no treatment (Barekat 2016) (MD 3.90, 95% CI ‐0.42 to 8.22, 35 men, 1 RCT, P = 0.08, I² = not applicable).

1.6.5 Three studies (six intervention arms) compared polyunsaturated fatty acids (PUFAs) with placebo. Gonzalez‐Ravina 2018 did not report SDs, we assumed the outcome to have an SD equal to the highest SD from other studies within this analysis. As heterogeneity was high (51%), we have not reported the pooled analysis; individually their results were:

Abbasi 2020 (two intervention arms) did not show evidence of decreased sperm DNA fragmentation when alpha‐lipoic acid (ALA) was compared with placebo (MD 0.53, 95% CI ‐2.65 to 3.72, 41 men, P = 0.74, I² = 20%);
Gonzalez‐Ravina 2018 (three intervention arms) did not show evidence of decreased sperm DNA fragmentation when docosahexaenoic acid (DHA) was compared with placebo (MD ‐1.97, 95% CI ‐10.55 to 6.62, 60 men, P = 0.65, I² = 0%);
Martinez‐Soto 2010 did show evidence of decreased sperm DNA fragmentation when Brudy Plus (DHA plus eicosapentaenoic acid (EPA)) was compared with placebo (MD ‐14.10, 95% CI ‐23.22 to ‐4.98, 36 men, P = 0.002, I²= not applicable).

1.6.6 Vitamin C plus vitamin E appeared to be associated with decreased sperm DNA fragmentation when compared with placebo (Greco 2005) (MD ‐13.80, 95% CI ‐17.50 to ‐10.10, 64 men, 1 RCT, P < 0.00001, I² = not applicable).

1.6.7 Zinc did not show evidence of decreased sperm DNA fragmentation when compared with placebo (Raigani 2014) (MD 1.30, 95% ‐8.62 to 11.22, 42 men, 1 RCT, P = 0.80, I² = not applicable).

1.6.8 Five studies (six intervention arms) compared combined antioxidants with placebo or no treatment. As heterogeneity was high (85%), we have not reported the pooled analysis; individually their results were:

Gamidov 2017 (two intervention arms) did show evidence of increased sperm DNA fragmentation when combined antioxidants were compared with no treatment (MD 6.09, 95% CI 3.37 to 8.81, 114 men, P < 0.0001, I²= 0%);
Gamidov 2019 did show evidence of decreased sperm DNA fragmentation when combined antioxidants were compared with placebo (MD ‐5.00, 95% CI ‐8.41 to ‐1.59, 80 men, P = 0.004, I²= not applicable);
Micic 2019 did show evidence of decreased sperm DNA fragmentation when combined antioxidants were compared with placebo (MD ‐3.00, 95% CI ‐5.73 to ‐0.27, 165 men, P = 0.03, I²= not applicable);
Steiner 2020 did not show evidence of decreased sperm DNA fragmentation when combined antioxidants were compared with placebo (MD ‐1.90, 95% CI ‐5.89 to 2.09, 135 men, P = 0.35, I²= not applicable);
Stenqvist 2018 did not show evidence of decreased sperm DNA fragmentation when combined antioxidants were compared with placebo (MD ‐2.90, 95% CI ‐8.10 to 2.30, 75 men, P = 0.27, I²= not applicable).

We performed a post‐hoc sensitivity analysis of the combined antioxidants subgroup for studies enrolling men with varicocele. In the literature it is reported that men with varicocele have higher levels of sperm DNA fragmentation. One study in this subgroup reported on men with varicocele (Gamidov 2017). When this study was removed from the analysis, heterogeneity was low and there appeared to be an association between the use of combined antioxidants and decreased sperm DNA fragmentation (MD ‐3.31, 95% CI ‐5.08 to ‐1.54, 455 men, 4 RCTs, P = 0.0002, I² = 0%).

There was evidence that different antioxidants had differing effects (test for subgroup differences: Chi² = 52.10, P < 0.00001).

1.7 Sperm DNA fragmentation at six months; type of antioxidant

See Analysis 1.7.

We analysed this outcome using a fixed‐effect model and used subtotals as pooling was not possible.

1.7.1 Three studies compared combined antioxidants with placebo. As heterogeneity was high (74%), we have not reported the pooled analysis; individually their results were:

Gamidov 2019 did show evidence of decreased sperm DNA fragmentation (MD ‐7.10, 95% CI ‐10.79 to ‐3.41, 80 men, P = 0.0002, I² = not applicable);
Micic 2019 did show evidence of decreased sperm DNA fragmentation (MD ‐4.70, 95% CI ‐7.12 to ‐2.28, 165 men, P = 0.0001, I² = not applicable);
Stenqvist 2018 did not show evidence of decreased sperm DNA fragmentation (MD 2.90, 95% CI ‐3.11 to 8.91, 75 men, P = 0.34, I² = not applicable).

1.7.2 Zinc plus folic acid did not show evidence of decreased sperm DNA fragmentation when compared with placebo (Schisterman 2020) (MD 3.00, 95% CI 0.02 to 5.98, 853 men, P = 0.05, I² = not applicable).

There was evidence that different antioxidants had differing effects (test for subgroup differences: Chi² = 17.51, P < 0.0001).

1.8 Data not usable for meta‐analysis

See Analysis 1.8.

One study reported on DNA fragmentation, but could not be included in the forest plots of the meta‐analysis. Boonyarangkul 2015 reported the tail length in micrometer measured with the Comet assay instead of a percentage. They reported no statistically significant difference in tail length when folic acid was compared with placebo after 3 months and 6 months.

1.9 Total sperm motility at three months or less; type of antioxidant

See Analysis 1.9 and Figure 10

Figure 10

Forest plot of comparison: 1 Antioxidant(s) versus placebo or no treatment, outcome: 1.9 Total sperm motility at 3 months or less; type of antioxidant.

We analysed this outcome using a fixed‐effect‐model and used subtotals as pooling was not possible.

1.9.1 Astaxanthin plus vitamin E did not show evidence of an increase in total sperm motility compared with placebo (Kumalic 2020) (MD ‐5.20, 95% CI ‐11.56 to 1.16, 72 men, 1 RCT, P = 0.11, I² = not applicable).

1.9.2 Five studies (seven intervention arms) comparing carnitines with placebo or no treatment did show an increase in total sperm motility (Balercia 2005; Dimitriadis 2010; Lenzi 2003; Peivandi 2010; Sigman 2006) (MD 31.28, 95% CI 31.19 to 31.37, 244 men, 5 RCTs, 7 intervention arms, P < 0.00001, I² = 97%). One study (Lenzi 2003) did not report standard deviations (SDs); we assumed the outcome to have an SD equal to the highest SD from other studies within this analysis. The heterogeneity was extremely high due to the fact that one study (Peivandi 2010) had very small SDs when compared with data in the other studies. However, the authors confirmed, when contacted, that they are indeed SDs and not standard errors (SEs). When these two studies were removed from the analysis, carnitines appeared to be associated with an increase in total sperm motility when compared with placebo or no treatment, with low heterogeneity (MD 11.83, 95% CI 7.78 to 15.87, 128 men, 3 RCTs, 5 intervention arms, P < 0.00001, I² = 0%).

1.9.3 Carotenoids did not show evidence of an increase in total sperm motility compared with placebo (Nouri 2019) (MD 3.50, 95% CI ‐6.95 to 13.95, 36 men, 1 RCT, P = 0.51, I² = not applicable).

1.9.4 Coenzyme Q10 did not show evidence of an increase in total sperm motility compared with placebo (Nadjarzadeh 2011) (MD 3.61, 95% CI ‐6.13 to 13.35, 47 men, 1 RCT, P = 0.47, I² = not applicable).

1.9.5 Two studies compared folic acid with placebo and did not show evidence of an increase in total sperm motility (Azizollahi 2013; Raigani 2014) (MD 4.56, 95% CI ‐5.63 to 14.74, 89 men, 2 RCTs, P = 0.38, I² = 0%).

1.9.6 Magnesium did not show evidence of an increase in total sperm motility compared with placebo (Zavaczki 2003) (MD 14.50, 95% CI ‐6.01 to 35.01, 20 men, 1 RCT, P = 0.17, I² = not applicable).

1.9.7 N‐acetylcysteine (NAC) did not show evidence of an increase in total sperm motility compared with placebo (Barekat 2016) (MD 14.60, 95% CI 0.32 to 28.88, 35 men, P = 0.05, I² = not applicable).

1.9.8 Three studies (four intervention arms) compared polyunsaturated fatty acids (PUFAs) with placebo and did not show evidence of an increase in total sperm motility (Abbasi 2020; Conquer 2000; Martinez‐Soto 2010) (MD ‐2.40, 95% CI ‐9.89 to 5.09, 105 men, 3 RCTs, 4 intervention arms, P = 0.53, I² = 48%).

1.9.9 Selenium appeared to be associated with an increase in total sperm motility compared with placebo (Scott 1998) (MD 14.90, 95% CI 1.14 to 28.66, 34 men, 1 RCT, P = 0.03, I² = not applicable).

1.9.10 Vitamin C plus vitamin E did not show evidence of an increase in total sperm motility compared with placebo (Greco 2005) (MD 2.90, 95% CI ‐7.76 to 13.56, 64 men, 1 RCT, P = 0.59, I² = not applicable).

1.9.11 Vitamin E appeared to be associated with an increase in total sperm motility compared with no treatment (Ener 2016) (MD 18.90, 95% CI 4.90 to 32.90, 45 men, 1 RCT, P = 0.008, I² = not applicable).

1.9.12 Three studies compared zinc with placebo or no treatment (Azizollahi 2013; Omu 2008; Raigani 2014). As the heterogeneity was high (78%), we have not reported the pooled analysis; individually their results were:

Azizollahi 2013 did not show evidence of an increase in total sperm motility at three months when compared with placebo (MD 4.00, 95% CI ‐12.11 to 20.11, 57 men, P = 0.63);
Omu 2008 did show an increase in total sperm motility at three months when compared with no treatment (MD 25.00, 95% CI 14.07 to 35.93, 19 men, P < 0.00001);
Raigani 2014 did not show evidence of an increase in total sperm motility at 16 weeks when compared with placebo (MD 1.20, 95% CI ‐11.92 to 14.32, 42 men, P = 0.86).

1.9.13 Two studies compared zinc plus folic acid with placebo and did not show evidence of an increase in total sperm motility (Azizollahi 2013; Raigani 2014) (MD 5.26, 95% CI ‐3.64 to 14.16, 93 men, 2 RCTs, P = 0.25, I² = 0%).

1.9.14 Zinc plus vitamin E appeared to be associated with an increase in total sperm motility compared with no treatment (Omu 2008) (MD 26.00, 95% CI 12.85 to 39.15, 20 men, 1 RCT, P = 0.0001, I² = not applicable)

1.9.15 Zinc plus vitamin E plus vitamin C appeared to be associated with an increase in total sperm motility compared with no treatment (Omu 2008) (MD 26.00, 95% CI 12.62 to 39.38, 22 men, 1 RCT, P = 0.0001, I² = not applicable).

1.9.16 Seven studies (eight intervention arms) compared combined antioxidants with placebo or no treatment (Bahmyari 2021; Gopinath 2013; Morgante 2010; Scott 1998; Sivkov 2011; Steiner 2020; Stenqvist 2018). As heterogeneity was high (88%), we have not reported the pooled analysis; individually their results were:

Bahmyari 2021 did not show evidence of increased total sperm motility when compared with placebo (MD ‐6.40, 95% CI ‐15.52 to 2.72, 62 men, P = 0.17);
Gopinath 2013 did show an increase in total sperm motility when compared with placebo (MD 8.72, 95% CI 4.44 to 13.01, 125 men, P < 0.0001);
Morgante 2010 did show an increase in total sperm motility when compared with no treatment (MD 15.20, 95% CI 13.62 to 16.78, 180 men, P < 0.00001);
Scott 1998 did show an increase in total sperm motility when compared with placebo (MD 11.70, 95% CI 0.87 to 22.53, 48 men, P = 0.003);
Sivkov 2011 did show an increase in total sperm motility when compared with placebo (MD 20.30, 95% CI 6.77 to 33.83, 30 men, P = 0.003);
Steiner 2020 did not show evidence of increased total sperm motility when compared with placebo (MD 0.60, 95% CI ‐4.37 to 5.57, 164 men, P = 0.81);
Stenqvist 2018 did not show evidence of increased total sperm motility when compared with placebo (MD 2.90, 95% CI ‐7.31 to 13.12, 75 men, P = 0.58).

There was evidence that different antioxidants had differing effects (test for subgroup differences: Chi² = 1086.87, P < 0.00001).

1.10 Data not usable for meta‐analysis

Analysis 1.10

Data from two studies could not be used in the forest plot. Galatioto 2008 reported percentage of WHO class A motile sperm instead of class A plus B, and Kessopoulou 1995 reported median differences. Both studies found no difference between intervention and placebo or no treatment for this outcome.

1.11 Total sperm motility at six months or less; type of antioxidant

See Analysis 1.11.

We analysed this outcome using a fixed‐effect model and used subtotals as pooling was not possible.

1.11.1 Three studies (five intervention arms) compared carnitines with placebo (Balercia 2005; Lenzi 2004; Sigman 2006). As the heterogeneity was high (78%), we have not reported the pooled analysis for these studies; individually their results were:

Balercia 2005 (three arms) did show an increased total sperm motility at six months when compared with placebo (MD 18.63, 95% CI 12.92 to 24.35, 59 men, P < 0.00001);
Lenzi 2004 did not show evidence of increased total sperm motility at six months when compared with placebo (MD 1.50, 95% CI‐4.56 to 7.56, 56 men, P = 0.63);
Sigman 2006 did not show evidence of increased total sperm motility at six months when compared with placebo (MD ‐7.70, 95% CI ‐33.24 to 17.84, 21 men, P = 0.55).

1.11.2 Three studies compared coenzyme Q10 with placebo (Balercia 2009; Safarinejad 2009a; Safarinejad 2012). As the heterogeneity was extremely high (99%), we have not reported the pooled analysis; individually their results were:

Balercia 2009 did show an increased total sperm motility when compared with placebo (MD 4.50, 95% 0.74 to 8.26, 60 men, P = 0.02);
Safarinejad 2009a did show an increased total sperm motility when compared with placebo (MD 4.50, 95% CI 3.89 to 5.11, 194 men, P < 0.000001);
Safarinejad 2012 did show an increased total sperm motility when compared with placebo (MD 10.40, 95% CI 9.77 to 11.03, 225 men, P < 0.000001).

1.11.3 Two studies compared folic acid with placebo (Azizollahi 2013; Wong 2002) and did not show evidence of increased total sperm motility (MD 0.16, 95% CI ‐6.96 to 7.29, 98 men, 2 RCTs, P = 0.96, I² = 0).

1.11.4 N‐acetylcysteine (NAC) appeared to be associated with an increased total sperm motility when compared with placebo (MD 1.90, 95% CI 1.20 to 2.60, 211 men, P < 0.00001, I² = not applicable) (Safarinejad 2009).

1.11.5 Selenium appeared to be associated with an increased total sperm motility when compared with placebo (MD 3.20, 95% CI 2.50 to 3.90, 211 men, P < 0.00001, I² = not applicable) (Safarinejad 2009).

1.11.6 Selenium plus N‐acetylcysteine appeared to be associated with increased total sperm motility when compared with placebo (Safarinejad 2009) (MD 6.30, 95% CI 5.60 to 7.00, 210 men, P < 0.00001, I² = not applicable).

1.11.7 Vitamin D plus calcium did not show evidence of increased total sperm motility when compared with placebo (Blomberg Jensen 2018) (MD ‐4.00, 95% CI ‐9.57 to 1.57, 260 men, P = 0.16, I² = not applicable).

1.11.8 Two studies compared vitamin E with placebo or no treatment (Ener 2016; Suleiman 1996). There appeared to be an association between vitamin E and an increased total sperm motility (MD 11.60, 95% CI 6.18 to 17.02, 132 men, 2 RCTs, P < 0.0001, I² = 16%).

1.11.9 Two studies compared zinc with placebo (Azizollahi 2013; Wong 2002) and did not show evidence of increased total sperm motility (MD 0.00, 95% CI ‐6.95 to 6.95, 105 men, P = 1.00, I² = 0%).

1.11.10 Three studies compared zinc plus folic acid to placebo (Azizollahi 2013; Schisterman 2020; Wong 2002) and did not show evidence of increased total sperm motility (MD 0.24, 95% CI ‐2.54 to 3.02, 956 men, P = 0.87, I² = 0%).

1.11.11 Four studies compared combined antioxidants with placebo or no treatment (Busetto 2018; Gopinath 2013; Kizilay 2019; Stenqvist 2018) and did not show evidence of increased total sperm motility. As the heterogeneity was high (69%), we have not reported the pooled analysis; individually their results were:

Busetto 2018 did show increased total sperm motility when compared with placebo (MD 4.40, 95% CI 1.49 to 7.31, 104 men, P = 0.003);
Gopinath 2013 (three arms), did show increased total sperm motility when compared with placebo (MD 12.44, 95% CI 8.29 to 16.59, 125 men, P < 0.00001);
Kizilay 2019 did show increased total sperm motility compared with no treatment (MD 7.60, 95% CI 3.58 to 11.62, 90 men, P = 0.0002);
Stenqvist 2018 did not show evidence of increased total sperm motility compared with placebo (MD ‐0.80, 95% CI ‐9.36 to 7.76, 75 men, P = 0.85).

There was evidence that different antioxidants had differing effects (test for subgroup differences: Chi² = 254.81, P < 0.00001).

1.12 Total sperm motility at nine months or more; type of antioxidant

See Analysis 1.12.

We analysed this outcome using a fixed‐effect model and used subtotals as pooling was not possible.

1.12.1 One study reported on different types of carnitines. Carnitines appeared to be associated with an increased total sperm motility when compared with placebo (Balercia 2005) (MD 8.54, 95% CI 3.01 to 14.07, 59 men, P = 0.002, I² = 0%).

1.12.2 Three studies reported on coenzyme Q10 (Balercia 2009; Safarinejad 2009a; Safarinejad 2012). As the heterogeneity was extremely high (98%), we have not reported the pooled analysis; individually their results were:

Balercia 2009 did not show evidence of increased total sperm motility when compared with placebo (MD ‐2.30, 95% CI ‐5.94 to 1.34, 60 men, P = 0.22);
Safarinejad 2009a did show increased total sperm motility when compared with placebo (MD 1.40, 95% CI 0.79 to 2.01, 194 men, P < 0.00001);
Safarinejad 2012 did show increased total sperm motility when compared with placebo (MD 5.40, 95% CI 4.80 to 6.00, 225 men, P < 0.00001).

1.12.3 Vitamin E did not show evidence of increased total sperm motility when compared with no treatment (Ener 2016) (MD 2.20, 95% CI ‐8.48 to 12.88, 45 men, 1 RCT, P = 0.69, I² = not applicable).

There was no evidence that different antioxidants had differing effects (test for subgroup differences: Chi² = 3.42, P = 0.18).

1.13 Total sperm motility over time

See Analysis 1.13

This analysis was only useful in directly comparing the same studies reporting at the three time points and not in comparing results of meta‐analyses that included different subsets of studies.

1.13.1 Total sperm motility at three months or less. We analysed this outcome using a fixed‐effect model (MD 31.17, 95% CI 31.07 to 31.26, 1638 men, 25 RCTs, 36 intervention arms, P < 0.00001, I² = 97%) and used subtotals (Abbasi 2020; Attallah 2013; Azizollahi 2013; Bahmyari 2021; Balercia 2005; Barekat 2016; Conquer 2000; Dimitriadis 2010; Ener 2016; Gopinath 2013; Greco 2005; Kumalic 2020; Lenzi 2003; Martinez‐Soto 2010; Morgante 2010; Nadjarzadeh 2011; Nouri 2019; Omu 2008; Peivandi 2010; Raigani 2014; Scott 1998; Sigman 2006; Steiner 2020; Stenqvist 2018; Zavaczki 2003).

1.13.2 Total sperm motility at six months. We analysed this outcome using a fixed‐effect model (MD 5.77, 95% CI 5.45 to 6.10, 2880 men,17 RCTs, 26 intervention arms, P < 0.00001, I² = 94%) and used subtotals (Azizollahi 2013; Balercia 2005; Balercia 2009; Blomberg Jensen 2018; Busetto 2018; Ener 2016; Gopinath 2013; Kizilay 2019; Lenzi 2004; Safarinejad 2009; Safarinejad 2009a; Safarinejad 2012; Schisterman 2020; Sigman 2006; Stenqvist 2018; Suleiman 1996; Wong 2002).

1.13.3 Total sperm motility at nine months or more. We analysed this outcome using a fixed‐effect model (MD 3.36, 95% CI 2.94 to 3.78, 583 men, 5 RCTs, 7 intervention arms, P < 0.00001, I² = 94%) and used subtotals (Balercia 2005; Balercia 2009; Ener 2016; Safarinejad 2009a; Safarinejad 2012).

Two of the studies included in the analysis of the semen parameter outcomes (Safarinejad 2009; Safarinejad 2009a) had consistently reported SDs very much smaller than those reported by most of the other included studies. The review authors considered that these were potentially erroneous, but an attempt to check with the study authors was unsuccessful. One other study (Peivandi 2010), also had very small SDs when compared with data in the other studies, but the authors confirmed, when contacted, that they are indeed SDs and not SEs. We tried to manage these analyses in two different ways: firstly we assumed the outcome to have a SD equal to the highest SD from other studies within the same analysis and secondly by treating the data as SEs and converting back to SDs, however heterogeneity remained high in both situations so for the final analyses we reverted to the SDs as reported in the studies. The low SDs may have been due to the strict inclusion and exclusion criteria indicating that the study was homogenous in nature, however we were unable to carry out a sensitivity analysis on these studies as pooling was not possible due to high heterogeneity.

1.14 Progressive sperm motility at three months or less; type of antioxidant

See Analysis 1.14 and Figure 11.

Figure 11

Forest plot of comparison: 1 Antioxidant(s) versus placebo or no treatment, outcome: 1.14 Progressive sperm motility at 3 months or less; type of antioxidant.

We analysed this outcome using a fixed‐effect model and used subtotals as pooling was not possible.

1.14.1 Astaxanthin plus vitamin E did not show evidence of increased progressive sperm motility when compared with placebo (Kumalic 2020) (MD ‐5.10, 95% CI ‐11.46 to 1.26, 72 men, 1 RCT, P = 0.12, I² = not applicable).

1.14.2 Four studies with carnitines reported an increase in progressive sperm motility when compared with placebo (Balercia 2005; Cavallini 2004; Mehni 2014; Peivandi 2010). As the heterogeneity was high (87%), we have not reported the pooled analysis; individually their results were:

Balercia 2005 (three arms) did show an increase in progressive sperm motility when compared with placebo (MD 13.72, 95% CI 9.08 to 18.35, 59 men, P < 0.00001);
Cavallini 2004 did show an increase in progressive sperm motility when compared with placebo (MD 9.80, 95% CI 5.62 to 13.98, 86 men, P < 0.00001);
Mehni 2014 did show an increase in progressive sperm motility when compared with placebo (MD 21.30, 95% CI 20.50 to 22.10, 110 men, P < 0.00001);
Peivandi 2010 did show an increase in progressive sperm motility when compared with placebo (MD 21.00, 95% CI 20.53 to 21.47, 30 men, P < 0.00001).

1.14.3 Carotenoids did not show evidence of increased progressive sperm motility when compared with placebo (Nouri 2019) (MD ‐0.20, 95% CI ‐7.27 to 6.87, 36 men, 1 RCT, P = 0.96, I² = not applicable).

1.14.4 Coenzyme Q10 did not show evidence of increased progressive sperm motility when compared with placebo (Nadjarzadeh 2011) (MD 4.60, 95% CI ‐3.54 to 12.74, 47 men, 1 RCT, P = 0.27, I² = not applicable).

1.14.5 Two studies compared folic acid to placebo and did not show evidence of increased progressive sperm motility (Azizollahi 2013; Boonyarangkul 2015) (MD 5.08, 95% CI ‐4.00 to 14.16, 81 men, 2 RCTs, P = 0.27, I² = 18%).

1.14.6 N‐acetylcysteine (NAC) did not show evidence of increased progressive sperm motility when compared with no treatment (Attallah 2013) (MD 3.80, 95% CI ‐1.03 to 8.63, 60 men, 1 RCT, P = 0.12, I² = not applicable).

1.14.7 Four studies (six intervention arms) compared PUFAs with placebo (Abbasi 2020; Gonzalez‐Ravina 2018; Haghighian 2015; Martinez‐Soto 2010). Gonzalez‐Ravina 2018 did not report SDs; we assumed the outcome to have an SD equal to the highest SD from other studies within this analysis. The heterogeneity was extremely high (95%), which may be due to the relatively small SDs reported in Haghighian 2015 and Martinez‐Soto 2010. We tried to manage these small SDs by imputing SDs from studies of a similar size and by considering the SDs to be SEs and converting them to SDs. Despite these efforts, heterogeneity remained high, and we reverted the SDs as reported in the studies. We have not reported the pooled analysis; individually their results were:

Abbasi 2020 did not show evidence of increased progressive sperm motility when compared with placebo (MD 8.99, 95% CI ‐1.84 to 19.82, 41 men, P = 0.10);
Gonzalez‐Ravina 2018 (three intervention arms) did not show evidence of increased progressive sperm motility when compared with placebo (MD 7.23, 95% CI ‐3.21 to 17.67, 60 men, P = 0.17);
Haghighian 2015 did show an increase in progressive sperm motility when compared with placebo (MD 6.40, 95% CI 4.83 to 7.97, 44 men, P < 0.00001);
Martinez‐Soto 2010 did show a decrease in progressive sperm motility when compared with placebo (MD ‐6.60, 95% CI ‐8.57 to ‐4.63, 36 men, P < 0.00001).

1.14.8 Two studies (three intervention arms) compared vitamin C with placebo and did show an increase in progressive sperm motility (Cyrus 2015; Dawson 1990). As the heterogeneity was high (64%), we have not reported the pooled analysis; individually their results were:

Cyrus 2015 did show an increase in progressive sperm motility when compared with placebo (MD 9.60, 95% CI 2.29 to 16.91, 115 men, P = 0.01);
Dawson 1990 did not show evidence of increased progressive sperm motility when vitamin C 200 mg was compared with placebo (MD 2.00, 95% CI ‐24.07 to 28.07, 15 men, P = 0.88);
Dawson 1990 did show an increase in progressive sperm motility when vitamin C 1000 mg was compared with placebo (MD 45.00, 95% CI 15.25 to 74.75, 15 men, P = 0.03).

1.14.9 Vitamin C plus vitamin E did not show evidence of increased progressive sperm motility when compared with placebo (Rolf 1999) (MD 0.20, 95% CI ‐9.77 to 10.17, 31 men, 1 RCT, P = 0.97, I² = not applicable).

1.14.10 Vitamin D did not show evidence of increased progressive sperm motility when compared with placebo (Amini 2020) (MD ‐0.84, 95% CI ‐7.65 to 5.97, 62 men, 1 RCT, P = 0.81, I² = not applicable).

1.14.11 Two studies with zinc did not show evidence of increased progressive sperm motility when compared with placebo (Azizollahi 2013; Sharifzadeh 2016) (MD 1.14, 95% CI ‐3.37 to 5.64, 157 men, 2 RCTs, P = 0.62, I² = 0%).

1.14.12 Zinc plus folic acid did not show evidence of increased progressive sperm motility when compared with placebo (Azizollahi 2013) (MD 3.80, 95% CI ‐13.66 to 21.26, 54 men, 1 RCT, P = 0.67, I² = not applicable).

1.14.13 Nine studies (10 intervention arms) compared antioxidants with placebo or no treatment (Bahmyari 2021; Gamidov 2017; Gamidov 2019; Joseph 2020; Kopets 2020; Micic 2019; Morgante 2010; Popova 2019; Stenqvist 2018). As the heterogeneity was very high (91%), we have not reported the pooled analysis; individually their results were:

Bahmyari 2021 did not show evidence of increased progressive sperm motility when compared with placebo (MD ‐3.30, 95% CI ‐12.08 to 5.48, 62 men, P = 0.46);
Gamidov 2017 (two arms) did not show evidence of increased progressive sperm motility when compared with placebo (MD ‐0.42, 95% CI ‐5.53 to 4.69, 57 men, P = 0.87);
Gamidov 2019 did not show evidence of increased progressive sperm motility when compared with placebo (MD 0.20, 95% CI ‐11.43 to 11.83, 80 men, P = 0.97);
Joseph 2020 did not show evidence of increased progressive sperm motility when compared with placebo (MD 1.70, 95% CI ‐4.51 to 7.91, 154 men, P = 0.59);
Kopets 2020 did show an in increase in progressive sperm motility when compared with placebo (MD 10.10, 95% CI 5.41 to 14.79, 83 men, P < 0.0001);
Micic 2019 did not show evidence of increased progressive sperm motility when compared with placebo (MD2.80, 95% CI ‐1.41 to 7.01, 165 men, P = 0.19);
Morgante 2010 did show an increase in progressive sperm motility when compared with placebo (MD 15.20, 95% CI 13.62 to 16.78, 180 men, 1 RCT, P < 0.00001, I² = not applicable).
Popova 2019 did show an increase in progressive sperm motility when compared with placebo (MD 18.00, 95% CI 11.75 to 24.25, 80 men, P < 0.00001);
Stenqvist 2018 did not show evidence of increased progressive sperm motility when compared with placebo (MD 0.00, 95% CI ‐12.24 to 12.24, 75 men, P = 1.00).

There was evidence that different antioxidants had differing effects (test for subgroup differences: Chi² = 1258.83, P < 0.00001).

1.15 Progressive sperm motility at six months; type of antioxidant

See Analysis 1.15.

We analysed this outcome using a fixed‐effect model and used subtotals as pooling was not possible.

1.15.1 Two studies (four intervention arms) compared carnitines with placebo (Balercia 2005; Cavallini 2004) and did show increased progressive sperm motility (MD 11.66, 95% CI 8.68 to 14.64, 145 men, 2 RCTs, 4 intervention arms, P < 0.00001, I² = 49%).

1.15.2 Coenzyme Q10 appeared to be associated with increased progressive sperm motility when compared with placebo (Balercia 2009) (MD 5.00, 95% CI 2.13 to 7.87, 60 men, 1 RCT, P = 0.0006, I² = not applicable).

1.15.3 Two studies with folic acid did not show evidence of increased progressive sperm motility when compared with placebo (Azizollahi 2013; Boonyarangkul 2015) (MD ‐1.77, 95% CI ‐10.21 to 6.67, 81 men, 2 RCTs, P = 0.68, I² = 0%).

1.15.4 PUFAs appeared to be associated with increased progressive sperm motility when compared with placebo (Safarinejad 2011b) (MD 8.80, 95% CI 8.11 to 9.49, 227 men, 1 RCT, P < 0.00001, I² = not applicable).

1.15.5 Vitamin D plus calcium did not show evidence of increased progressive sperm motility when compared with placebo (Blomberg Jensen 2018) (MD ‐4.00, 95% CI ‐9.59 to 1.59, 260 men, P = 0.16, I² = not applicable).

1.15.6 Zinc did not show evidence of increased progressive sperm motility when compared with placebo (Azizollahi 2013) (MD 2.00, 95% CI ‐13.56 to 17.56, 57 men, 1 RCT, P = 0.80, I² = not applicable).

1.15.7 Zinc plus folic acid did not show evidence of increased progressive sperm motility when compared with placebo (Azizollahi 2013) (MD 2.70, 95% CI ‐14.58 to 19.98, 54 men, 1 RCT, P = 0.76, I² = not applicable).

1.15.8 Five studies compared antioxidants with placebo or no treatment (Ardestani 2019; Gamidov 2019; Kizilay 2019; Micic 2019; Stenqvist 2018). As heterogeneity was high (65%), we have not reported the pooled analysis; individually their results were:

Ardestani 2019 did not show evidence of increased progressive sperm motility when compared with no treatment (MD 3.90, 95% CI ‐4.10 to 11.90, 60 men, P < 0.34);
Gamidov 2019 did show an increase in progressive sperm motility when compared with placebo (MD 13.20, 95% CI 4.46 to 21.94, 80 men, P =0.003);
Kizilay 2019 did not show evidence of increased progressive sperm motility when compared with placebo (MD 1.90, 95% CI ‐0.85 to 4.65, 90 men, P = 0.18);
Micic 2019 did show an increase in progressive sperm motility when compared with placebo (MD 6.70, 95% CI 3.36 to 10.04, 180 men, P < 0.0001);
Stenqvist 2018 did not show evidence of increased progressive sperm motility when compared with placebo (MD ‐3.40, 95% CI ‐12.89 to 6.09, 75 men, P = 0.48).

There was evidence that different antioxidants had differing effects (test for subgroup differences: Chi² = 54.94, P < 0.00001).

1.16 Data not suitable for meta analysis

See Analysis 1.16.

One study provided data as percentage improvement and therefore could not be used in the forest plot (Saeed Alkumait 2020). The percentage improvement in the intervention groups was higher compared with placebo (P = 0.01).

1.17 Progressive sperm motility at nine months or more; type of antioxidant

See Analysis 1.17.

We analysed this outcome using a fixed‐effect model and used subtotals as pooling was not possible.

1.17.1 Carnitines appeared to be associated with an increase in progressive sperm motility when compared with placebo (Balercia 2005, three intervention arms) (MD 7.77, 95% CI 2.68 to 12.87, 59 men, 1 RCT, 3 intervention arms, P = 0.003, I² = 0%).

1.17.2 Coenzyme Q10 did not show evidence of increased progressive sperm motility when compared with placebo (Balercia 2009) (MD ‐0.90, 95% CI ‐2.68 to 0.88, 60 men, 1 RCT, P = 0.32, I² = not applicable).

There was evidence that different antioxidants had differing effects (test for subgroup differences: Chi² = 9.93, P = 0.002).

1.18 Progressive sperm motility over time

See Analysis 1.18.

This analysis was only useful in directly comparing the same studies reporting at the three time points and not in comparing results of meta‐analyses that included different subsets of studies.

1.18.1 Progressive sperm motility at three months or less. We analysed this outcome using a fixed‐effect model (MD 17.98, 95% CI 17.62 to 18.34, 2054 men, 27 RCTs, 35 intervention arms, P < 0.00001, I² = 98%) and used subtotals (Abbasi 2020; Amini 2020; Attallah 2013; Azizollahi 2013; Bahmyari 2021; Balercia 2005; Boonyarangkul 2015; Cavallini 2004; Cyrus 2015; Dawson 1990; Gamidov 2017; Gamidov 2019; Gonzalez‐Ravina 2018; Haghighian 2015; Joseph 2020; Kopets 2020; Kumalic 2020; Martinez‐Soto 2010; Mehni 2014; Micic 2019; Morgante 2010; Nadjarzadeh 2011; Nouri 2019; Peivandi 2010; Popova 2019; Rolf 1999; Stenqvist 2018).

1.18.2 Progressive sperm motility at six months. We analysed this outcome using a fixed‐effect model (MD 8.05, 95% CI 7.43 to 8.66, 1304 men, 12 RCTs, 16 intervention arms, P < 0.00001, I² = 79%) and used subtotals (Ardestani 2019; Azizollahi 2013; Balercia 2005; Balercia 2009; Blomberg Jensen 2018; Boonyarangkul 2015; Cavallini 2004; Gamidov 2019; Kizilay 2019; Micic 2019; Safarinejad 2011b; Stenqvist 2018).

1.18.3 Progressive sperm motility at nine months or more. We analysed this outcome using a fixed‐effect model (MD 0.04, 95% CI ‐1.64 to 1.72, 119 men, 2 RCTs, 4 intervention arms, P = 0.96, I² = 72%) and used subtotals (Balercia 2005; Balercia 2009).

1.19 Sperm concentration at three months or less; type of antioxidant

See Analysis 1.19 and Figure 12.

Figure 12

Forest plot of comparison: 1 Antioxidant(s) versus placebo or no treatment, outcome: 1.19 Sperm concentration at 3 months or less; type of antioxidant.

We analysed this outcome using a fixed‐effect model. We used only subtotals in this analysis.

1.19.1 Astaxanthin plus vitamin E did not show evidence of increased sperm concentration when compared with placebo (Kumalic 2020) (MD ‐1.00, 95% CI ‐6.79 to 4.79, 72 men, 1 RCT, P = 0.74, I² = not applicable).

1.19.2 Five studies (7 intervention arms) compared carnitines with placebo or no treatment and showed an increase in sperm concentration (Balercia 2005; Cavallini 2004; Dimitriadis 2010; Mehni 2014; Peivandi 2010). As the heterogeneity was extremely high (95%) we have not reported the pooled analysis; individually their results were:

Balercia 2005 did not show evidence of increased sperm concentration when compared with placebo (MD 7.76, 95% CI ‐0.73 to 16.25, 59 men, P = 0.07, I² = 0%);
Cavallini 2004 did show an increase in sperm concentration when compared with placebo (MD 7.90, 95% CI 4.89 to 10.91, 86 men, P < 0.00001, I² = not applicable);
Dimitriadis 2010 did not show evidence of increased sperm concentration when compared with no treatment (MD ‐0.90, 95% CI ‐4.80 to 3.00, 48 men, P = 0.65, I² = not applicable);
Mehni 2014 did show an increase in sperm concentration when compared with placebo (MD 8.50, 95% CI 7.85 to 9.15, 110 men, P < 0.00001, I² = not applicable);
Peivandi 2010 did show an increase in sperm concentration when compared with placebo (MD 29.50, 95% CI 25.39 to 33.61, 30 men, P < 0.00001, I² = not applicable).

1.19.3 Carotenoids appeared to be associated with an increase in sperm concentration when compared with placebo (Nouri 2019) (MD 6.30, 95% CI 0.62 to 11.98, 36 men, 1 RCT, P = 0.03, I² = not applicable).

1.19.4 Coenzyme Q10 did not show evidence of increased sperm concentration when compared with placebo (Nadjarzadeh 2011) (MD ‐0.10, 95% CI ‐12.37 to 12.17, 47 men, 1 RCT, P = 0.99, I² = not applicable).

1.19.5 Three studies compared folic acid with placebo and did not show evidence of increased sperm concentration (Azizollahi 2013; Boonyarangkul 2015; Raigani 2014). As the heterogeneity was high (61%) we have not reported the pooled analysis; individually their results were:

Azizollahi 2013 did show an increase in sperm concentration when compared with placebo (MD 22.20, 95% CI 3.80 to 40.60, 51 men, P = 0.02);
Boonyarangkul 2015 did not show evidence of increased sperm concentration when compared with placebo (MD ‐9.60, 95% CI ‐39.36 to 20.16, 30 men, P = 0.53). However, in this study there was great baseline imbalance for sperm parameters between the intervention and control group;
Raigani 2014 did not show evidence of increased sperm concentration when compared with placebo at 16 weeks (MD 0.60, 95% CI ‐8.28 to 9.48, 38 men, P =0.89).

1.19.6 Magnesium did not show evidence of increased sperm concentration when compared with placebo (Zavaczki 2003) (MD 5.20, 95% CI ‐2.61 to 13.01, 20 men, 1 RCT, P = 0.19, I² = not applicable).

1.19.7 Two studies compared N‐acetylcysteine (NAC) with placebo or no treatment (Attallah 2013; Barekat 2016) and did not show evidence of increased sperm concentration (MD 4.59, 95% CI ‐0.27 to 9.46, 95 men, 2 RCTs, P = 0.06, I² = 0%).

1.19.8 Five studies (eight intervention arms) compared PUFAs with placebo or no treatment (Abbasi 2020; Conquer 2000; Gonzalez‐Ravina 2018; Haghighian 2015; Martinez‐Soto 2010) and did show an increase in sperm concentration (MD 3.42, 95% CI 1.69 to 5.15, 209 men, 5 RCTs, 8 intervention arms, P = 0.0001, I² = 0%). Haghighian 2015 reported remarkably small SDs compared with the other included studies. A sensitivity analysis was performed, showing no evidence of increased sperm concentration (MD ‐1.07, 95% CI ‐14.37 to 12.24, 165 men, 4 RCTs, 7 intervention arms, P = 0.88, I² = 0%).

1.19.9 Selenium did not show evidence of increased sperm concentration when compared with placebo (Scott 1998) (MD 21.20, 95% CI ‐4.90 to 47.30, 34 men, 1 RCT, P = 0.11, I² = not applicable).

1.19.10 Vitamin C did not show evidence of increased sperm concentration when compared with placebo (Cyrus 2015) (MD 9.70, 95% CI 0.09 to 19.31, 115 men, 1 RCT, P = 0.05, I² = not applicable).

1.19.11 Two studies compared vitamin C plus vitamin E with placebo (Greco 2005; Rolf 1999). As the heterogeneity was high (52%), we have not reported the pooled analysis; individually their results were:

Greco 2005 did not show evidence of increased sperm concentration when compared with placebo (MD 7.20, 95% CI ‐4.05 to 18.45, 64 men, P = 0.21);
Rolf 1999 did not show evidence of increased sperm concentration when compared with placebo (MD ‐4.40, 95% CI ‐15.48 to 6.68, 31 men, P = 0.44).

1.19.12 Vitamin D did not show evidence of increased sperm concentration compared with placebo (Amini 2020) (MD ‐2.12, 95% CI ‐8.85 to 4.61, 62 men, P = 0.54, I² = not applicable).

1.19.13 Vitamin E appeared to be associated with an increase in sperm concentration when compared with no treatment (Ener 2016) (MD 18.90, 95% CI 3.92 to 33.88, 45 men, 1 RCT, P = 0.01, I² = not applicable).

1.19.14 Three studies compared zinc with placebo (Azizollahi 2013; Raigani 2014; Sharifzadeh 2016). There appeared to be an association between zinc and increased sperm concentration (MD 6.74 95% CI 2.81 to 10.68, 199 men, 3 RCTs, P = 0.0008, I² = 41%).

1.19.15 Two studies compared folic acid plus zinc with placebo (Azizollahi 2013; Raigani 2014). As heterogeneity was high (80%), we have not reported the pooled analysis; individually their results were:

Azizollahi 2013 did show an increase in sperm concentration when compared with placebo (MD 18.00, 95% CI 1.11 to 34.89, 54 men, P = 0.04);
Raigani 2014 did not show evidence of increased sperm concentration when compared with placebo (MD ‐3.50, 95% CI ‐11.55 to 4.55, 39 men, P = 0.39).

1.19.16 Eleven studies (13 intervention arms) compared combined antioxidants with placebo or no treatment (Bahmyari 2021; Gamidov 2017; Gamidov 2019; Gopinath 2013; Joseph 2020; Kopets 2020; Morgante 2010; Popova 2019; Scott 1998; Steiner 2020; Stenqvist 2018). As the heterogeneity was very high (86%), we have not reported the pooled analysis; individually their results were:

Bahmyari 2021 did not show evidence of increased sperm concentration when compared with placebo (MD ‐1.10, 95% CI ‐19.48 to 17.28, 62men, P = 0.91);
Gamidov 2017 (two arms) did not show evidence of increased sperm concentration when compared with placebo (MD 3.55, 95% CI ‐3.14 to 10.24, 114 men, P = 0.30);
Gamidov 2019 did not show evidence of increased sperm concentration when compared with placebo (MD ‐3.10, 95% CI ‐18.89 to 12.69, 80 men, P = 0.70);
Gopinath 2013 did show an increase in sperm concentration when compared with placebo (MD 10.69, 95% CI 8.15 to 13.22, 125 men, P < 0.00001);
Joseph 2020 did not show evidence of increased sperm concentration when compared with placebo (MD ‐5.60, 95% CI ‐14.50 to 3.30, 154 men, P = 0.22);
Kopets 2020 did show an increase in sperm concentration when compared with placebo (MD 18.40, 95% CI 6.04 to 30.76, 83 men, P < 0.004);
Morgante 2010 did not show evidence of an increased sperm concentration when compared with no treatment (MD ‐0.90, 95% CI ‐1.85 to 0.05, 180 men, P = 0.06);
Popova 2019 did not show evidence of an increased sperm concentration when compared with placebo (MD ‐4.40, 95% CI ‐16.74 to 7.94, 80 men, P = 0.48);
Scott 1998 did not show evidence of increased sperm concentration when compared with placebo (MD 6.50, 95% CI ‐16.66 to 29.66, 39 men, P = 0.58);
Steiner 2020 did not show evidence of increased sperm concentration when compared with placebo (MD ‐7.30, 95% CI ‐20.25 to 5.65, 164 men, P = 0.27);
Stenqvist 2018 did not show evidence of increased sperm concentration when compared with placebo (MD ‐11.50, 95% CI ‐33.04 to 10.04, 75 men, P = 0.30).

There was evidence that different antioxidants did not have differing effects (test for subgroup differences: Chi² = 252.54, P < 0.00001).

1.20 Data not usable for meta‐analysis

See Analysis 1.20.

One study (Kessopoulou 1995) provided data as median differences and range and therefore could not be used in the forest plot. This study might indicate some improvement in sperm concentration in the intervention group when measured at three months, however these data were not rigorous and no conclusions could be made. One study (Lenzi 2003) provided data as the mean with no SD and did not report the number of patients in whom the outcome was assessed. The P value in Lenzi 2003 was 0.03, indicating that there may have been an association between L‐carnitine and improved sperm concentration at three months.

1.21 Sperm concentration at six months; type of antioxidant

See Analysis 1.21.

We analysed this outcome using a fixed‐effect model. We used only subtotals in this analysis.

1.21.1 Three studies (five intervention arms) compared carnitines with placebo (Balercia 2005; Cavallini 2004; Lenzi 2004). There appeared to be an association between carnitines and increased sperm concentration (MD 7.42, 95% CI 4.97 to 9.87, 201 men, 3 RCTs, 5 intervention arms, P < 0.00001, I² = 23%).

1.21.2 Three studies compared coenzyme Q10 with placebo (Balercia 2009; Safarinejad 2009a; Safarinejad 2012). As the heterogeneity was extremely high (96%) we have not reported the pooled analysis; individually their results were:

Balercia 2009 did not show evidence of increased sperm concentration when compared with placebo (MD ‐1.50, 95% CI ‐11.39 to 8.39, 60 men, P = 0.77);
Safarinejad 2009a did show an increase in sperm concentration when compared with placebo (MD 5.60, 95% CI 4.38 to 6.82, 194 men, P < 0.00001);
Safarinejad 2012 did show an increase in sperm concentration when compared with placebo (MD 11.90, 95% CI 10.72 to 13.08, 225 men, P < 0.00001).

1.21.3 Three studies compared folic acid with placebo (Azizollahi 2013; Boonyarangkul 2015; Wong 2002). As the heterogeneity was high (58%) we have not reported the pooled analysis; individually their results were:

Azizollahi 2013 did show an increase in sperm concentration when compared with placebo (MD 19.20, 95% CI 12.24 to 26.16, 51 men, P < 0.00001);
Boonyarangkul 2015 did not show evidence of increased sperm concentration when compared with placebo (MD ‐22.80, 95% CI ‐60.44 to 14.84, 30 men, P = 0.24). However, in this study there was great baseline imbalance for sperm parameters between the intervention and control group;
Wong 2002 did not show evidence of increased sperm concentration when compared with placebo (MD 15.00, 95% CI ‐1.19 to 31.19, 47 men, P = 0.07).

1.21.4 N‐acetylcysteine (NAC) appeared to be associated with an increase in sperm concentration when compared with placebo (Safarinejad 2009) (MD 3.30, 95% CI 1.80 to 4.80, 211 men, 1 RCT, P < 0.0001, I² = not applicable).

1.21.5 PUFAs appeared to be associated with an increase in sperm concentration when compared with placebo (Safarinejad 2011b) (MD 12.50, 95% CI 11.39 to 13.61, 227 men, 1 RCT, P < 0.00001, I² = not applicable).

1.21.6 Selenium appeared to be associated with an increase in sperm concentration when compared with placebo (Safarinejad 2009) (MD 4.10, 95% CI 2.45 to 5.75, 211 men, 1 RCT, P < 0.00001, I² = not applicable).

1.21.7 Selenium plus N‐acetylcysteine (NAC) appeared to be associated with an increase in sperm concentration when compared with placebo (Safarinejad 2009) (MD 8.60, 95% CI 6.89 to 10.31, 210 men, 1 RCT, P < 0.00001 I² = not applicable).

1.21.8 Vitamin D plus calcium did not show evidence of increased sperm concentration when compared with placebo (Blomberg Jensen 2018) (MD ‐2.50, 95% CI ‐8.18 to 3.18, 269 men, 1 RCT, P = 0.39, I² = not applicable).

1.21.9 Vitamin E did not show evidence of increased sperm concentration when compared with no treatment (Ener 2016) (MD 5.90, 95% CI ‐10.83 to 22.63, 45 men, 1 RCT, P = 0.49, I² = not applicable).

1.21.10 Two studies compared zinc with placebo (Azizollahi 2013; Wong 2002) and did not show evidence of increased sperm concentration (MD 5.51, 95% CI ‐4.00 to 15.01, 105 men, 2 RCTs, P = 0.26, I² = 0%).

1.21.11 Three studies compared zinc plus folic acid with placebo (Azizollahi 2013; Schisterman 2020; Wong 2002). As heterogeneity was high (84%), we have not reported the pooled analysis; individually their results were:

Azizollahi 2013 did not show evidence of increased sperm concentration when compared with placebo (MD 17.70, 95% CI ‐1.88 to 37.28, 54 men, P = 0.08);
Schisterman 2020 did not show evidence of increased sperm concentration when compared with placebo (MD ‐9.00, 95% CI ‐19.00 to 1.00, 853 men, P = 0.08);
Wong 2002 did show an increase in sperm concentration when compared with placebo (MD 26.40, 95% CI 6.33 to 46.47, 49 men, P = 0.01).

1.21.12 Six studies (7 intervention arms) compared combined antioxidants to placebo or no treatment (Ardestani 2019; Busetto 2018; Gamidov 2019; Gopinath 2013; Kizilay 2019; Stenqvist 2018). As the heterogeneity was very high (91%), we have not reported the pooled analysis; individually their results were:

Ardestani 2019 did not show evidence of increased sperm concentration when compared with placebo (MD 5.50, 95% CI ‐6.57 to 17.57, 60 men, P = 0.37);
Busetto 2018 did show an increase in sperm concentration when compared with placebo (MD 7.70, 95% CI 2.41 to 12.99, 104 men, P = 0.004);
Gamidov 2019 did not show evidence of increased sperm concentration when compared with placebo (MD 4.50, 95% CI ‐12.17 to 21.17, 80 men, P = 0.60);
Gopinath 2013 did show an increase in sperm concentration when compared with placebo (MD 16.48, 95% CI 13.08 to 19.87, 125 men, P < 0.00001);
Kizilay 2019 did show an increase in sperm concentration when compared with placebo (MD 2.00, 95% CI 1.06 to 2.94, 90 men, P < 0.0001);
Stenqvist 2018 did not show evidence of increased sperm concentration when compared with placebo (MD ‐2.60, 95% CI ‐25.30 to 20.10, 75 men, P = 0.82).

There was evidence that different antioxidants had differing effects (test for subgroup differences: Chi² = 246.11, P < 0.00001).

1.22 Data not usable for meta‐analysis

Analysis 1.22

One study (Saeed Alkumait 2020) provided data as percentage improvement and therefore could not be used in the forest plot. The percentage improvement was higher in the two intervention groups than in the placebo group (P = 0.01).

1.23 Sperm concentration at nine months; type of antioxidant

See Analysis 1.23.

We analysed this outcome using a fixed‐effect model. We used only subtotals in this analysis.

1.23.1 Carnitines (three intervention arms) did not show evidence of increased sperm concentration when compared with placebo (Balercia 2005) (MD 4.17, 95% CI ‐1.71 to 10.06, 59 men, 1 RCT, 3 intervention arms, P = 0.16, I² = not applicable).

1.23.2 Three studies compared coenzyme Q10 with placebo (Balercia 2009; Safarinejad 2009a; Safarinejad 2012). As the heterogeneity was extremely high (95%), we have not reported the pooled analysis; individually their results were:

Balercia 2009 did not show evidence of increased sperm concentration when compared with placebo (MD ‐5.40, 95% CI ‐15.75 to 4.95, 60 men, P = 0.31);
Safarinejad 2009a did show an increase in sperm concentration when compared with placebo (MD 1.60, 95% CI 0.53 to 2.67, 194 men, P = 0.003);
Safarinejad 2012 did show an increase in sperm concentration when compared with placebo (MD 6.20, 95% CI 5.17 to 7.23, 225 men, P < 0.00001).

1.23.3 Vitamin E did not show evidence of increased sperm concentration when compared with no treatment (Ener 2016) (MD 11.40, 95% CI ‐2.56 to 25.36, 45 men, 1 RCT, P = 0.11, I² = not applicable).

There was no evidence that different antioxidants had differing effects (test for subgroup differences: Chi² = 1.10, P = 0.58).

1.24 Sperm concentration over time

See Analysis 1.24.

This analysis was only useful in directly comparing the same studies reporting at the three time points and not in comparing results of meta‐analyses that included different subsets of studies.

1.24.1 Total sperm concentration at three months or less. We analysed this outcome using a fixed‐effect model (MD 5.49, 95% CI 5.02 to 5.96, 2535 men, 35 RCTs, 47 intervention arms, P < 0.00001, I² = 91%) and used subtotals (Abbasi 2020; Amini 2020; Attallah 2013; Azizollahi 2013; Bahmyari 2021; Balercia 2005; Barekat 2016; Boonyarangkul 2015; Cavallini 2004; Conquer 2000; Cyrus 2015; Dimitriadis 2010; Ener 2016; Gamidov 2017; Gamidov 2019; Gonzalez‐Ravina 2018; Gopinath 2013; Greco 2005; Haghighian 2015; Joseph 2020; Kopets 2020; Kumalic 2020; Martinez‐Soto 2010; Mehni 2014; Morgante 2010; Nadjarzadeh 2011; Nouri 2019; Peivandi 2010; Popova 2019; Raigani 2014; Rolf 1999; Scott 1998; Steiner 2020; Stenqvist 2018; Zavaczki 2003).

1.24.2 Total sperm concentration at six months. We analysed this outcome using a fixed‐effect model (MD 7.21, 95% CI 6.73 to 7.70, 2995 men, 19 RCTs, 28 intervention arms, P < 0.00001, I² = 92%) and used subtotals (Ardestani 2019; Azizollahi 2013; Balercia 2005; Balercia 2009; Boonyarangkul 2015; Busetto 2018; Cavallini 2004; Ener 2016; Gamidov 2019; Gopinath 2013; Kizilay 2019; Lenzi 2004; Safarinejad 2009; Safarinejad 2009a; Safarinejad 2011b; Safarinejad 2012; Schisterman 2020; Stenqvist 2018; Wong 2002).

1.24.3 Total sperm concentration at nine months or more. We analysed this outcome using a fixed‐effect model (MD 3.95, 95% CI 3.22 to 4.69, 583 men, 5 RCTs, seven intervention arms, P < 0.00001, I² = 86%) and used subtotals (Balercia 2005; Balercia 2009; Ener 2016; Safarinejad 2009a; Safarinejad 2012).

2. Head‐to‐head antioxidants (natural conception and undergoing fertility treatment)

The studies included in this comparison did not report on adverse events.

2.1 Live birth; type of antioxidant

See Analysis 2.1.

2.1.1 L‐carnitine versus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased live birth rate when compared with L‐acetyl carnitine (Balercia 2005) (Peto OR 1.00, 95% CI 0.13 to 7.92, 30 men, 1 RCT, P = 1.00).

2.1.2 L‐carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased live birth rate when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (Peto OR 0.34, 95% CI 0.06 to 1.79, 30 men, 1 RCT, P = 0.20).

2.1.3 L‐acetyl carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐acetyl carnitine and increased live birth rate when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (Peto OR 0.34, 95% CI 0.06 to 1.79, 30 men, 1 RCT, P = 0.20).

There was no evidence that different antioxidants had differing effects (test for subgroup differences: Chi² = 0.79, P = 0.67)

2.2 Clinical pregnancy; type of antioxidant

See Analysis 2.2.

2.2.1. L‐carnitine versus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased clinical pregnancy rate when compared with L‐acetyl carnitine (Balercia 2005) (Peto OR 1.00, 95% CI 0.13 to 7.92, 30 men, 1 RCT, P = 1.00).

2.2.2 L‐carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased clinical pregnancy rate when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (Peto OR 0.34, 95% CI 0.06 to 1.79, 30 men,1 RCT, P = 0.20).

2.2.3 L‐acetyl carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐acetyl carnitine and increased clinical pregnancy rate when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (Peto OR 0.34, 95% CI 0.06 to 1.79, 30 men,1 RCT, P = 0.20).

2.2.4 L‐carnitine versus coenzyme Q10. There was no evidence of the use of L‐carnitine and increased clinical pregnancy rate when compared with coenzyme Q10 (Cheng 2018) (Peto OR 1.48, 95% CI 0.54 to 4.05, 156 men, 1 RCT, P = 0.44).

2.2.5 L‐carnitine versus L‐carnitine plus coenzyme Q10. There was no evidence of the use of L‐carnitine and increased clinical pregnancy rate when compared with L‐carnitine plus coenzyme Q10 (Cheng 2018) (Peto OR 0.62, 95% CI 0.27 to 1.46, 156 men, 1 RCT, P = 0.28).

2.2.6 Coenzyme Q10 versus L‐carnitine plus coenzyme Q10. There was no evidence of the use of coenzyme Q10 and increased clinical pregnancy rate when compared with L‐carnitine plus coenzyme Q10 (Cheng 2018) (Peto OR 0.43, 95% CI 0.18 to 1.06, 156 men, 1 RCT, P = 0.07).

2.2.7 Vitamin D plus calcium versus vitamin E plus vitamin C. There appeared to be an association between the use of vitamin D plus calcium and increased clinical pregnancy rate when compared with vitamin E plus vitamin C (Deng 2014) (Peto OR 5.13, 95% CI 1.21 to 21.79, 86 men, P = 0.03).

2.2.8 Combined antioxidants versus L‐carnitine. There was no evidence of the use of combined antioxidants and increased clinical pregnancy rate when compared with L‐carnitine (Tsounapi 2018) (Peto OR 1.93, 95% CI 0.20 to 19.08, 89 men, P = 0.57).

There was no evidence that different antioxidants had differing effects (test for subgroup differences: Chi² = 12.59, P = 0.08).

2.3 Sperm DNA fragmentation; type of antioxidant

See Analysis 2.3.

2.3.1 L‐carnitine versus coenzyme Q10. There was no evidence of the use of L‐carnitine and decreased DNA fragmentation when compared with coenzyme Q10 (Cheng 2018) (MD ‐0.80, 95% CI ‐2.22 to 0.62, 125 men, P = 0.27).

2.3.2 L‐carnitine versus L‐carnitine plus coenzyme Q10. There was no evidence of the use of L‐carnitine and decreased DNA fragmentation when compared with L‐carnitine plus coenzyme Q10 (Cheng 2018) (MD 0.40, 95% CI ‐1.14 to 1.94, 125 men, P = 0.61).

2.3.3 Coenzyme Q10 verus L‐carnitine plus coenzyme Q10. There was no evidence of the use of coenzyme Q10 and decreased DNA fragmentation when compared with L‐carnitine plus coenzyme Q10 (Cheng 2018) (MD 1.20, 95% CI ‐0.25 to 2.65, 126 men, P = 0.11).

2.3.4 L‐carnitine versus vitamin B1. There was no evidence of the use of L‐carnitine and decreased DNA fragmentation when compared with vitamin B1 (Cheng 2018) (MD ‐1.50, 95% CI ‐3.22 to 0.22, 136 men, P = 0.09).

2.3.5 Coenzyme Q10 versus vitamin B1. There was no evidence of the use of coenzyme Q10 and decreased DNA fragmentation when compared with vitamin B1 (Cheng 2018) (MD ‐0.70, 95% CI ‐2.34 to 0.94, 137 men, P = 0.40).

2.3.6 Vitamin B1 versus L‐carnitine plus coenzyme Q10. There appeared to be an association between the use of vitamin B1 and increased DNA fragmentation when compared with L‐carnitine plus coenzyme Q10 (Cheng 2018) (MD 1.90, 95% CI 0.16 to 3.64, 137 men, P = 0.03).

2.4 Total sperm motility at three months or less; type of antioxidant

See Analysis 2.4.

2.4.1 Coenzyme Q10 200 mg versus coenzyme Q10 400 mg. There was no evidence of the use of coenzyme Q10 200 mg/day and increased sperm motility when compared with coenzyme Q10 400 mg/day (Alahmar 2019) (MD ‐4.86, 95% CI ‐10.60 to 0.88, 65 men, P = 0.10).

2.4.2 Docosahexaenoic acid (DHA) 400 mg versus DHA 800 mg. There was no evidence of the use of DHA 400 g/day and increased sperm motility when compared with DHA 800 mg/day (Conquer 2000) (MD 7.40, 95% CI ‐11.35 to 26.15, 19 men, P = 0.44).

2.4.3 DHA versus DHA plus vitamin E. There appeared to be an association between the use of DHA and decreased sperm motility when compared with DHA combined with vitamin E (Eslamian 2020) (MD ‐3.77, 95% CI ‐5.42 to ‐2.12, 90 men, P < 0.00001).

2.4.4 DHA versus vitamin E. There was no evidence of the use of DHA and increased sperm motility when compared with vitamin E (Eslamian 2020) (MD ‐1.60, 95% CI ‐3.30 to 0.10, 90 men, P = 0.07).

2.4.5 DHA plus vitamin E versus vitamin E. There appeared to be an association between the use of DHA plus vitamin E and increased sperm motility when compared with vitamin E alone (Eslamian 2020) (MD 2.17, 95% CI 0.54 to 3.80, 90 men, P = 0.009).

2.4.6 Ethylcysteine versus vitamin E. There was no evidence of the use of ethyl cysteine and increased sperm motility when compared with vitamin E (Akiyama 1999) (MD ‐1.90, 95% CI ‐41.97 to 38.17, 10 men, P = 0.93).

2.4.7 L‐acetyl carnitine plus L carnitine versus vitamin E plus vitamin C. There appeared to be an association between the use of L acetyl carnitine + L carnitine and increased sperm motility when compared with vitamin E + vitamin C (Li 2005) (MD 23.10, 95% CI 20.14 to 26.06, 138 men, P < 0.00001).

2.4.8 L‐carnitine versus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased sperm motility when compared with L‐acetyl carnitine (Balercia 2005) (MD 3.40, 95% CI ‐3.73 to 10.53, 30 men, P = 0.35).

2.4.9 L‐carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased sperm motility when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (MD 4.80, 95% CI ‐1.76 to 11.36, 30 men, P = 0.15).

2.4.10 L‐acetyl carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐acetyl carnitine and increased sperm motility when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (MD 1.40, 95% CI ‐6.42 to 9.22, 30 men, P = 0.73).

2.4.11 Selenium versus combined antioxidants. There was no evidence of the use of selenium and increased sperm motility when compared with combined antioxidants (Scott 1998) (MD 3.20, 95% CI ‐10.13 to 16.53, 46 men, P = 0.64).

2.4.12 Vitamin C 200 mg versus vitamin C 1000 mg. There appeared to be an association between the use of ascorbic acid 200 mg/day and decreased sperm motility when compared with ascorbic acid 1000 mg/day (Dawson 1990) (MD ‐43.00, 95% CI ‐67.10 to ‐18.90, 20 men, P = 0.0005).

2.4.13 Vitamin E plus "compound amino acids" versus vitamin E. There appeared to be an association between the use of vitamin E plus "compound amino acids" and increased sperm motility when compared with vitamin E only (Zhou 2016) (MD 11.90, 95% CI 8.71 to 15.09, 120 men, P < 0.00001). The authors of the study did not define the "compound amino acids" in more detail.

2.4.14 Zinc versus folic acid. Two studies compared zinc with folic acid and did not show evidence of an increased sperm motility (Azizollahi 2013; Raigani 2014) (MD ‐3.01, 95% CI ‐11.38 to 5.35, 124 men, P = 0.48, I² = 0%).

2.4.15 Zinc versus zinc plus folic acid. Two studies compared zinc with zinc plus folic acid and did not show evidence of an increased sperm motility (Azizollahi 2013; Raigani 2014) (MD ‐2.91, 95% CI ‐10.92 to 5.10, 125 men, P = 0.48, I² = 0%).

2.4.16 Zinc plus folic acid versus folic acid. Two studies compared zinc plus folic acid with folic acid only and did not show evidence of an increased sperm motility (Azizollahi 2013; Raigani 2014) (MD 0.24, 95% CI‐6.17 to 6.66, 121 men, P = 0.94, I² = 0%).

2.4.17 Zinc versus zinc plus vitamin E. There was no evidence of the use of zinc and increased sperm motility when compared with zinc plus vitamin E (Omu 2008) (MD ‐1.00, 95% CI ‐15.00 to 13.00, 18 men, P = 0.89).

2.4.18 Zinc versus zinc plus vitamin E plus vitamin C. There was no evidence of the use of zinc and increased sperm motility when compared with zinc plus vitamin E plus vitamin C (Omu 2008) (MD ‐1.00, 95% CI ‐19.66 to 17.66, 12 men, P = 0.92).

2.4.19 Zinc plus vitamin E versus zinc plus vitamin E plus vitamin C. There was no evidence of the use of zinc plus vitamin E and increased sperm motility when compared with zinc plus vitamin E plus vitamin C (Omu 2008) (MD ‐0.00, 95% CI ‐18.97 to 18.97, 18 men, P = 1.00).

2.5 Total sperm motility at six months or less; type of antioxidant

See Analysis 2.5.

2.5.1 L‐carnitine versus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased sperm motility when compared with L‐acetyl carnitine (Balercia 2005) (MD 4.10, 95% CI ‐2.70 to 10.90, 30 men, P = 0.24).

2.5.2 L‐carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased sperm motility when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (MD 3.40, 95% CI ‐2.87 to 9.67, 30 men, P = 0.29).

2.5.3 L‐acetyl carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐acetyl carnitine and increased sperm motility when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (MD ‐0.70, 95% CI ‐7.73 to 6.33, 30 men, P = 0.85).

2.5.4 N‐acetylcysteine versus selenium plus NAC. There appeared to be an association between the use of NAC and decreased sperm motility when compared with selenium plus NAC (Safarinejad 2009) (MD ‐4.40, 95% CI ‐5.14 to ‐3.66, 234 men, P < 0.00001).

2.5.5 Selenium versus N‐acetylcysteine (NAC). There appeared to be an association between the use of selenium and increased sperm motility when compared with NAC (Safarinejad 2009) (MD 1.30, 95% CI 0.56 to 2.04, 234 men, P = 0.0006).

2.5.6 Selenium versus selenium plus N‐acetylcysteine (NAC). There appeared to be an association between the use of selenium and decreased sperm motility when compared with selenium plus NAC (Safarinejad 2009) (MD ‐3.10, 95% CI ‐3.85 to ‐2.35, 232 men, P < 0.00001).

2.5.7 Zinc versus folic acid. Two studies compared zinc with folic acid (Azizollahi 2013; Wong 2002) and did not show evidence of the use of zinc and increased sperm motility when compared with folic acid (MD ‐1.03, 95% CI ‐5.18 to 3.13, 125 men, P = 0.63, I² = 0%).

2.5.8 Zinc versus zinc plus folic acid. Two studies compared zinc with zinc plus folic acid (Azizollahi 2013; Wong 2002) and did not show evidence of the use of zinc and increased sperm motility when compared with zinc plus folic acid (MD ‐1.69, 95% CI ‐6.95 to 3.58, 127 men, P = 0.53, I² = 0%).

2.5.9 Zinc plus folic acid versus folic acid. Two studies compared zinc plus folic acid with folic acid (Azizollahi 2013; Wong 2002) and did not show evidence of the use of zinc plus folic acid and increased sperm motility when compared with folic acid only (MD 1.03, 95% CI ‐4.23 to 6.29, 126 men, P = 0.70, I² = 0%).

2.6 Total sperm motility at nine months or more; type of antioxidant

See Analysis 2.6.

2.6.1 L‐carnitine versus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased sperm motility when compared with L‐acetyl carnitine (Balercia 2005) (MD 3.70, 95% CI ‐1.69 to 9.09, 30 men, P = 0.18).

2.6.2 L‐carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased sperm motility when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (MD 5.30, 95% CI ‐0.73 to 11.33,30 men, P = 0.08).

2.6.3 L‐acetyl carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐acetyl carnitine and increased sperm motility when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (MD 1.60, 95% CI ‐3.29 to 6.49, 30 men, P = 0.52).

2.7 Progressive sperm motility at three months or less; type of antioxidant

See Analysis 2.7.

2.7.1 Coenzyme Q10 200 mg versus coenzyme Q10 400 mg. There was no evidence of the use of coenzyme 200 mg/day and increased progressive sperm motility when compared with coenzyme Q10 400 mg/day (Alahmar 2019) (MD ‐3.52, 95% CI ‐9.71 to 2.67, 65 men, P = 0.26).

2.7.2 Docosahexaenoic acid (DHA) versus DHA plus vitamin E. There appeared to be an association between the use of DHA and decreased progressive sperm motility when compared with DHA combined with vitamin E (Eslamian 2020) (MD ‐2.22, 95% CI ‐3.50 to 0.94, 90 men, P = 0.0007).

2.7.3 DHA versus vitamin E. There was no evidence of the use of DHA and increased progressive sperm motility when compared with vitamin E (Eslamian 2020) (MD ‐0.39, 95% CI ‐1.67 to 0.89, 90 men P = 0.55).

2.7.4 DHA plus vitamin E versus vitamin E. There appeared to be an association between the use of DHA plus vitamin E and increased progressive sperm motility when compared with vitamin E alone (Eslamian 2020) (MD 1.83, 95% CI 0.68 to 2.98, 90 men, P = 0.002).

2.7.5 L‐carnitine versus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased progressive sperm motility when compared with L‐acetyl carnitine (Balercia 2005) (MD 4.00, 95% CI ‐1.88 to 9.88, 30 men, P = 0.18).

2.7.6 L‐carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased progressive sperm motility when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (MD 5.00, 95% CI ‐0.68 to 10.68, 29 men, P = 0.08)

2.7.7 L‐acetyl carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐acetyl carnitine and increased progressive sperm motility when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (MD 1.00, 95% CI ‐5.41 to 7.41, 29 men, P = 0.76).

2.7.8 L‐carnitine versus vitamin B1. There was no evidence of the use of L‐carnitine and increased progressive sperm motility when compared with vitamin B1 (Cheng 2018) (MD 1.70, 95% CI ‐1.54 to 4.94, 136 men, P = 0.30).

2.7.9 L‐carnitine versus coenzyme Q10. There was no evidence of the use of L‐carnitine and increased progressive sperm motility when compared with coenzyme Q10 (Cheng 2018) (MD 1.30, 95% CI ‐1.70 to 4.30, 125 men, P = 0.40).

2.7.10 L‐carnitine versus L‐carnitine plus coenzyme Q10. There appeared to be an association between the use of L‐carnitine and decreased progressive sperm motility when compared with L‐carnitine plus coenzyme Q10 (Cheng 2018) (MD ‐8.20, 95% CI ‐12.31 to ‐4.09, 125 men, P < 0.0001).

2.7.11 Coenzyme Q10 versus L‐carnitine plus coenzyme Q10. There appeared to be an association between the use of coenzyme Q10 and decreased progressive sperm motility when compared with L‐carnitine plus coenzyme Q10 (Cheng 2018) (MD ‐9.50, 95% CI ‐13.54 to ‐5.46, 126 men, P < 0.00001).

2.7.12 Coenzyme Q10 versus vitamin B1. There was no evidence of the use of coenzyme Q10 and increased progressive sperm motility when compared with vitamin B1 (Cheng 2018) (MD 0.40, 95% CI ‐2.75 to 3.55, 137 men, P = 0.80).

2.7.13 Vitamin B1 versus L‐carnitine plus coenzyme Q10. There appeared to be an association between the use of vitamin B1 and decreased progressive sperm motility when compared with L‐carnitine plus coenzyme Q10 (Cheng 2018) (MD‐9.90, 95% CI ‐14.12 to ‐5.68, 137 men, P < 0.00001).

2.7.14 L‐acetyl carnitine versus L‐carnitine plus vitamin E plus vitamin C. There appeared to be an association between the use of L‐acetyl carnitine and increased progressive sperm motility when compared with L‐carnitine plus vitamin E plus vitamin C (Li 2005) (MD 13.30, 95% CI 11.21 to 15.39, 138 men, P < 0.00001).

2.7.15 L‐carnitine versus vitamin E plus vitamin C. There appeared to be an association between the use of L‐carnitine and increased progressive sperm motility when compared with vitamin E plus vitamin C (Li 2005a) (MD 30.50, 95% CI 27.70 to 33.30, 63 men, P < 0.00001).

2.7.16 L‐carnitine versus vitamin E. There appeared to be an association between the use of L‐carnitine and increased progressive sperm motility when compared with vitamin E (Sun 2018) (MD 1.90, 95% CI 1.31 to 2.49, 212 men, P < 0.00001).

2.7.17 L‐carnitine plus vitamin E versus vitamin E. There appeared to be an association between the use of L‐carnitine plus vitamin E and increased progressive sperm motility when compared with vitamin E (Wang 2010) (MD 14.10, 95% CI 10.11 to 18.09, 113 men, P < 0.00001).

2.7.18 Vitamin D plus calcium versus vitamin E plus vitamin C. There appeared to be an association between the use of vitamin D plus calcium and increased progressive sperm motility when compared with vitamin E plus vitamin C (Deng 2014) (MD 6.90, 95% CI 5.38 to 8.42, 86 men, P < 0.000001).

2.7.19 Vitamin E plus "compound amino acids" versus vitamin E. There appeared to be an association between the use of vitamin E plus "compound amino acids" and increased progressive sperm motility when compared with vitamin E only (Zhou 2016) (MD 6.10, 95% CI 3.87 to 8.33, 120 men, P < 0.00001). The authors of the study did not define the "compound amino acids" in more detail.

2.8 Progressive sperm motility at six months; type of antioxidant

See Analysis 2.8.

2.8.1 L‐carnitine versus L‐acetyl carnitine. There appeared to be an association between the use of L‐carnitine and increased progressive sperm motility when compared with L‐acetyl carnitine (Balercia 2005) (MD 6.30, 95% CI 0.42 to 12.18, 30 men, P = 0.04).

2.8.2 L‐carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of a difference in progressive sperm motility when L‐carnitine was compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (MD 5.70, 95% CI 0.10 to 11.30, 29 men, P = 0.05).

2.8.3 L‐acetyl carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of a difference in progressive sperm motility when L‐acetyl carnitine was compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (MD ‐0.60, 95% CI ‐6.93 to 5.73, 29 men, P = 0.85).

2.9 Progressive motility at six months (data not suitable for meta‐analysis)

Analysis 2.9

One study (Saeed Alkumait 2020) compared coenzyme Q10 versus glutathione and provided data as percentage improvement and therefore could not be used in the forest plot. The authors did not provide a P value of the head‐to‐head comparison.

2.10 Progressive sperm motility at nine months or more; type of antioxidant

See Analysis 2.10.

2.10.1 L‐carnitine versus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased progressive sperm motility when compared with L‐acetyl carnitine (Balercia 2005) (MD 3.80, 95% CI ‐1.50 to 9.10, 30 men, P = 0.16).

2.10.2 L‐carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased progressive sperm motility when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (MD 5.50, 95% CI ‐0.11 to 11.11,29 men, P = 0.05).

2.10.3 L‐acetyl carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐acetyl carnitine and increased progressive sperm motility when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (MD 1.70, 95% CI ‐4.17 to 7.57, 29 men, P = 0.57).

2.11 Sperm concentration at three months or less; type of antioxidant

See Analysis 2.11.

2.11.1 Coenzyme Q10 200 mg versus coenzyme Q10 400 mg. There was no evidence of the use of the use of coenzyme Q10 200 mg/day and increased sperm concentration when compared with coenzyme Q10 400 mg/day (Alahmar 2019) (MD 0.20, 95% CI ‐3.26 to 3.66, 65 men, P = 0.91).

2.11.2 Docosahexaenoic acid (DHA) 400 mg versus DHA 800 mg. There was no evidence of the use of DHA 400 mg/day and increased sperm concentration when compared with DHA 800 mg/day (Conquer 2000) (MD ‐6.80, 95% CI ‐41.87 to 28.27, 19 men, P = 0.70).

2.11.3 DHA versus DHA + vitamin E. There appeared to be an association between the use of DHA and decreased sperm concentration when compared with DHA combined with vitamin E (Eslamian 2020) (MD ‐1.45, 95% CI ‐2.47 to ‐0.43, 90 men, P = 0.005).

2.11.4 DHA versus vitamin E. There was no evidence of the use of DHA and increased sperm concentration when compared with vitamin E (Eslamian 2020) (MD ‐0.24, 95% CI ‐1.26 to 0.78, 90 men, P = 0.64).

2.11.5 DHA plus vitamin E versus vitamin E. There appeared to be an association between the use of DHA plus vitamin E and increased sperm concentration when compared with vitamin E alone (Eslamian 2020) (MD 1.21, 95% CI 0.28 to 2.14, 90 men, P = 0.01).

2.11.6 Ethyl cysteine versus vitamin E. There was no evidence of the use of ethyl cysteine and increased sperm concentration when compared with vitamin E (Akiyama 1999) (MD 2.20, 95% CI ‐16.65 to 21.05, 10 men, P = 0.82).

2.11.7 L‐carnitine versus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased sperm concentration when compared with L‐acetyl carnitine (Balercia 2005) (MD 1.70, 95% CI ‐10.97 to 14.37, 30 men, P = 0.79).

2.11.8 L‐carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased sperm concentration when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (MD 4.10, 95% CI ‐9.17 to 17.37, 30 men, P = 0.54).

2.11.9 L‐acetyl carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐acetyl carnitine and increased sperm concentration when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (MD 2.40, 95% CI ‐11.14 to 15.94, 30 men, P = 0.73).

2.11.10 L‐carnitine versus vitamin E plus vitamin C. There appeared to be an association between the use of L‐carnitine and increased sperm concentration when compared with vitamin E plus vitamin C (Li 2005a) (MD 15.50, 95% CI 12.49 to 18.51, 63 men, P < 0.00001).

2.11.11 L‐carnitine versus vitamin E. There was no evidence of the use of L‐carnitine and increased sperm concentration when compared with vitamin E (Sun 2018) (MD 0.70, 95% CI ‐0.34 to 1.74, 212 men, P = 0.19).

2.11.12 L‐carnitine plus vitamin E versus vitamin E. There was no evidence of the use of L‐carnitine plus vitamin E and increased sperm concentration when compared with vitamin E (Wang 2010) (MD 1.90, 95% CI ‐10.52 to 14.32, 113 men, P = 0.76).

2.11.13 Selenium versus combined antioxidants. There was no evidence of the use of selenium and increased sperm concentration when compared with combined antioxidants (Scott 1998) (MD 14.70, 95% CI ‐6.51 to 35.91, 46 men, P = 0.17).

2.11.14 Zinc versus folic acid. Two studies compared zinc with folic acid (Azizollahi 2013; Raigani 2014) and did not show evidence of the use of zinc and increased sperm concentration when compared with folic acid (MD ‐1.30, 95% CI ‐8.65 to 6.06, 124 men, P = 0.73, I² = 0%).

2.11.15 Zinc plus folic acid versus folic acid. Two studies compared zinc plus folic acid with folic acid (Azizollahi 2013; Raigani 2014) and did not show evidence of the use of zinc plus folic acid and increased sperm concentration when compared with folic acid only (MD 2.93, 95% CI ‐3.67 to 9.54, 125 men, P = 0.38, I² = 0%).

2.11.16 Zinc versus zinc plus folic acid. Two studies compared zinc with zinc plus folic acid (Azizollahi 2013; Raigani 2014) and did not show evidence of the use of zinc and increased sperm concentration when compared with zinc plus folic acid (MD ‐4.11, 95% CI ‐9.79 to 1.57, 121 men, P = 0.16, I² = 0%).

2.12 Sperm concentration at six months or less; type of antioxidant

See Analysis 2.12.

2.12.1 L‐carnitine versus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased sperm concentration when compared with L‐acetyl carnitine (Balercia 2005) (MD 5.90, 95% CI ‐8.92 to 20.72, 30 men, P = 0.44).

2.12.2 L‐carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased sperm concentration when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (MD 8.10, 95% CI ‐5.54 to 21.74, 30 men, P = 0.24).

2.12.3 L‐acetyl carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐acetyl carnitine and increased sperm concentration when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (MD 2.20, 95% CI ‐10.89 to 15.29, 30 men, P = 0.74).

2.12.4 N‐acetylcysteine (NAC) versus selenium plus NAC. There appeared to be an association between the use of NAC and decreased sperm concentration when compared with selenium plus NAC (Safarinejad 2009) (MD ‐5.30, 95% CI ‐6.86 to ‐3.74, 234 men, P < 0.00001).

2.12.5 Selenium versus N‐acetylcysteine (NAC). There was no evidence of the use of selenium and increased sperm concentration when compared with NAC (Safarinejad 2009) (MD 0.80, 95% CI ‐0.71 to 2.31, 234 men, P = 0.30).

2.12.6 Selenium versus selenium plus N‐acetylcysteine (NAC). There appeared to be an association between the use of selenium and decreased sperm concentration when compared with selenium plus NAC (Safarinejad 2009) (MD ‐4.50, 95% CI ‐6.20 to ‐2.80, 232 men, P < 0.00001).

2.12.7 Zinc versus folic acid. Two studies compared zinc with folic acid (Azizollahi 2013; Wong 2002) and did show an association between the use of zinc and decreased sperm concentration when compared with folic acid (MD ‐10.10, 95% CI ‐19.12 to ‐1.08, 125 men, P = 0.03, I² = 0%).

2.12.8 Zinc plus folic acid versus folic acid. Two studies compared zinc plus folic acid with folic acid (Azizollahi 2013; Wong 2002) and did not show evidence of the use of zinc plus folic acid and increased sperm concentration when compared with folic acid only (MD ‐13.58, 95% CI ‐25.99 to ‐1.17, 127 men, P = 0.03, I² = 23%).

2.12.9 Zinc versus zinc plus folic acid. Two studies compared zinc with zinc plus folic acid (Azizollahi 2013; Wong 2002) and did not show evidence of the use of zinc and increased sperm concentration when compared with zinc plus folic acid (MD 1.78, 95% CI ‐9.93 to 13.49, 126 men, P = 0.77, I² = 0%).

2.13 Sperm concentration at six months (data not suitable for meta‐analysis)

One study (Saeed Alkumait 2020) compared coenzyme Q10 with glutathione and provided data as percentage improvement, and therefore could not be used in the forest plot. The authors did not provide a P value for this head‐to‐head comparison.

2.14 Sperm concentration at nine months or more; type of antioxidant

See Analysis 2.14.

Pooling was not possible in this analysis as only one study reported on two subgroups.

2.14.1 L‐carnitine versus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased sperm concentration when compared with L‐acetyl carnitine (Balercia 2005) (MD 8.20, 95% CI ‐0.07 to 16.47, 30 men, P = 0.05).

2.14.2 L‐carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐carnitine and increased sperm concentration when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (MD 6.10, 95% CI ‐3.74 to 15.94, 30 men, P = 0.22).

2.14.3 L‐acetyl carnitine versus L‐carnitine plus L‐acetyl carnitine. There was no evidence of the use of L‐acetyl carnitine and increased sperm concentration when compared with L‐carnitine plus L‐acetyl carnitine (Balercia 2005) (MD ‐2.10, 95% CI ‐10.24 to 6.04, 30 men, P = 0.61).

Funnel plot

We assessed publication bias by using a funnel plot. The outcomes live birth and clinical pregnancy included 12 and 20 studies, respectively.

For the outcome of live birth, there was suspected publication bias (Figure 5). The funnel plot shows a remarkable lack of studies in the left lower section. This could be due to the fact that relatively small studies that do not show an increase of live birth with antioxidants, were not published. For the outcome of clinical pregnancy, there was no clear evidence of publication bias (Figure 7). We did not have enough studies to look at each of the subgroups for publication bias.

The studies reporting on the primary outcome of live birth did not all have study characteristics in common. They differed in terms of sample size, type and age of studied population, treatment period, and intervention and control. The results of the semen parameters in these studies were similar to those from the other included studies; the great majority did not show a significant improvement in semen parameters.

Discussion

Summary of main results

Effectiveness of antioxidants versus placebo or no treatment

Live birth

Evidence of low quality suggests that for subfertile men, the use of antioxidants may be effective in increasing a couple's chances of having a live birth when compared to placebo or no treatment. It was found within the studies that contributed to the analysis of live birth rate, that for subfertile men with a baseline live birth rate of 16%, with the use of an antioxidant this rate could increase to between 17% and 27%. However, there were only 12 studies with a total of 1283 couples reporting on live birth and the certainty of this evidence was considered to be very low (summary of findings Table 1). The methods were not well explained in three out of 12 of these studies (Korshunov 2018; Omu 1998; Suleiman 1996), two studies had a significant number of participants who dropped out of the study (Joseph 2020; Suleiman 1996), and Joseph 2020, Korshunov 2018, and Omu 1998 used 'no treatment' as control which introduced a degree of performance bias. When these four high‐risk studies were removed from the analysis, there was no evidence of association between the use of antioxidants and increased live birth.

The apparent benefit from antioxidants did not persist when analyses were restricted to placebo‐controlled studies. There was no evidence of increased live birth with the use of antioxidants in studies enrolling men undergoing assisted reproductive techniques (ART) (in vitro fertilisation (IVF)/intracytoplasmic sperm injection(ICSI)).

Clinical pregnancy

The findings of this review also suggest that for subfertile men the use of antioxidants may be effective in increasing a couple's chances of clinical pregnancy rate when compared to placebo or no treatment. It was found that within the studies that contributed to the analysis of clinical pregnancy, the population of subfertile men had a baseline or expected clinical pregnancy rate of 15%,and with the use of antioxidants this would increase to between 20% and 30%. However, there were only 20 studies with a total of 1706 men reporting on clinical pregnancy and the certainty of this evidence was considered to be low (summary of findings Table 1). The methods were not well explained in six of the 20 studies, with three of these studies having a significant number of participants who dropped out of the study (Barekat 2016; Joseph 2020; Suleiman 1996). Furthermore, five of the 25 analyses (one trial had three arms) crossed the line of no effect with wide confidence intervals.

The apparent benefit from antioxidants persisted when analyses were restricted to studies at lower risk of bias, placebo‐controlled studies, studies of men not undergoing IUI, studies of men undergoing ART (IVF/ICSI), and studies of men post‐varicocelectomy.

Adverse events

There is no evidence that antioxidants used by the subfertile male lead to an increased miscarriage risk when compared to placebo or no treatment. It was found that within this population of subfertile men with an expected miscarriage rate of 5%, the use of an antioxidant would increase the chances of having a miscarriage to between 4% and 13%. However, there were only six studies with a total of 664 men reporting on miscarriage and the certainty of this evidence was very low (summary of findings Table 1). The event rate in this analysis was very low with only 39 miscarriages reported in six studies, furthermore there was a high risk of bias within these studies.

The use of antioxidants by subfertile men may increase the occurrence of mild gastrointestinal complaints when compared to placebo or no treatment. It was found that within this population of subfertile men with an expected gastrointestinal event rate of 2%, the use of an antioxidant would increase the chances of having gastrointestinal complaints to between 2% and 7%. However, there were only 16 studies with a total of 1355 men reporting on gastrointestinal complaints and the certainty of this evidence was low (summary of findings Table 1). The event rate in this analysis was low with only 46 events reported; furthermore there was a high risk of bias within these studies.

There was no evidence that the risk of other adverse events, such as stillbirth and ectopic pregnancy, differed between antioxidant or control group.

Sperm DNA fragmentation

Thirteen studies (1813 men) reported on DNA fragmentation with suitable data for meta‐analysis. Pooled analysis of these 13 studies was not possible due to high heterogeneity. Pooling of the results from the subgroups was not possible either because of heterogenic data. One study reported substantially higher DNA fragmentation rates (> 80%) compared to other included studies, which could be explained by enrolment of participants post‐varicocelectomy (Barekat 2016).

Sperm parameters

The pooled results for total sperm motility, progressive sperm motility and concentration at three, six and nine months were unreliable as heterogeneity was extremely high in each analysis. Studies could be pooled in some antioxidant subgroups, with differing results per type of antioxidant and duration of treatment.

Effectiveness of antioxidants versus antioxidants (head‐to‐head)

In the head‐to‐head studies only four studies reported on live birth and/or clinical pregnancy; one study with different types of carnitines in multiple arms (versus placebo), one study comparing L‐carnitine with coenzyme Q10, and a combination of these two, one study comparing vitamin D plus calcium with vitamin E plus vitamin C, and one study comparing combined antioxidants with L‐carnitine (versus no treatment). Only vitamin D plus calcium showed an association. However, due to the small study size no direct conclusions can be drawn. The head‐to‐head studies did not report adverse events.

Overall completeness and applicability of evidence

Of the 90 studies included in this review, only 14 reported on the primary outcome of live birth, and only 22 reported on clinical pregnancy rate. Live birth and clinical pregnancy rate are the outcomes of most interest to subfertile couples and until these are robustly reported by all subfertility studies we will not be able to draw clear conclusions for the use of antioxidants for subfertile men. We believe that the lower baseline rate for clinical pregnancy than the baseline rate for live birth could be due to the difference in included populations. In the clinical pregnancy analysis (20 studies) there were four studies including men with varicocele; those studies did not report live birth and were therefore not included in the live birth rate analysis (12 studies). Adverse events such as miscarriage, ectopic pregnancy, stillbirth, gastrointestinal side effects, euphoria, headache, upper respiratory infection, and nasopharyngitis appear to be poorly reported, and the evidence is of very low certainty. The high heterogeneity may be an artefact caused by some studies reporting very small and potentially erroneous standard deviations (SDs). This undermines the credibility of the data.

Three of the trials included in the analysis of the semen parameter outcomes (Haghighian 2015; Safarinejad 2009; Safarinejad 2009a) had consistently reported SDs very much smaller than those reported by most of the other included trials. The review authors considered that these were potentially erroneous, but an attempt to check with the study authors was unsuccessful. One other trial (Peivandi 2010), also had very small SDs when compared to data in the other trials, but the authors confirmed, when contacted, that they are indeed SDs and not standard errors (SEs). We tried to manage these analyses in two different ways: firstly by imputing SDs from studies of a similar size and secondly by treating the data as SEs and converting back to SDs, however heterogeneity remained high in both situations so for the final analyses we reverted to the SDs as reported in the studies. The low SDs may have been due to the strict inclusion and exclusion criteria indicating that the trial was homogenous in nature, however we were unable to carry out a sensitivity analysis on these trials as pooling was not possible due to high heterogeneity. For the analysis of sperm concentration at three months, the heterogeneity was low despite the small SDs reported in Haghighian 2015. After sensitivity analysis the heterogeneity remained low, however this resulted in a confidence interval crossing the line of no effect.

Eighteen of the 90 included trials were very small in size (randomising less than 50 men), 39 of 90 included trials were small in size (randomising between 50 and 100 men) and only 33 of 90 included trials included more than 100 men. The estimates of the intervention effect tend to be more beneficial in smaller studies. Smaller studies also may not be as rigorous as the larger studies in their methodology (Higgins 2011).

We tried to assess which type of antioxidant might have a beneficial effect on the outcomes of interest in this review, however only three studies at the most could be pooled in any antioxidant subgrouping. Eighteen studies (Ardestani 2019; Bahmyari 2021; Busetto 2018; Gamidov 2017; Gamidov 2019; Gopinath 2013; Joseph 2020; Kizilay 2019; Kopets 2020; Korshunov 2018; Micic 2019; Morgante 2010; Popova 2019; Scott 1998; Steiner 2020; Stenqvist 2018; Tsounapi 2018; Tremellen 2007) used combined antioxidants versus placebo or no treatment and were used in the meta‐analysis. Ten of these studies reported on clinical pregnancy rate, showing an association between the use of combined antioxidants and increased clinical pregnancy rate. Only five of these studies reported on live births, showing no evidence of an increased live birth with the use of combined antioxidants. When the analysis of clinical pregnancy rate was restricted to these five studies reporting live birth, there was no evidence of increased clinical pregnancy rate with the use of combined antioxidants.

The head‐to‐head comparison does not provide constructive information as we could not pool direct comparisons. Subgrouping of antioxidants could be performed in 11 comparisons, each comparison pooling two studies. These were all studies comparing zinc with folic acid or a combination of the two.

There were 29 studies that contained data that were unusable in the analysis, with either some or all of their data (Alahmar 2020; Biagiotti 2003; Boonyarangkul 2015; Cheng 2018; Eslamian 2013; Eslamian 2020; Exposito 2016; Galatioto 2008; Haje 2015; Huang 2020; Kessopoulou 1995; Kumamoto 1988; Lenzi 2003; Lombardo 2002; Lu 2018; Martinez 2015; Nozha 2001; Omu 1998; Pourmand 2014; Poveda 2013; Pryor 1978; Saeed Alkumait 2020; Schisterman 2020; Sivkov 2011; Sofikitis 2016; Steiner 2020; Tsounapi 2018; Vinogradov 2019; Zalata 1998). The reasons for this were baseline imbalance, no report of the number of patients in whom outcome was assessed, and presentation of percentages or mean differences (Analysis 1.8; Analysis 1.10; Analysis 1.16; Analysis 1.20; Analysis 1.22). Attempts were made to contact these authors regarding the data. There was no clear evidence of publication bias.

Quality of the evidence

The evidence was graded as low to very low certainty. The main limitation was that out of the 67 included studies in the meta‐analysis only 20 studies reported clinical pregnancy, and of those 12 reported on live birth. Other limitations included poor reporting of study methods, imprecision, the number of small studies, reporting bias, and lack of data about adverse events. Publication bias was suspected for the outcome of live birth.

Figure 3 shows the review authors' judgements about the risk of bias of the studies included in this review. All included studies were described as randomised, however only just over 50% gave information on how the randomisation was achieved. Allocation concealment was described in only 36% of the studies. Blinding was better described with over 57% of the studies being double‐blinded or occasionally single‐blinded; 7% of studies stated that there was no blinding, and 20% of included studies used no treatment as a control. Dropout rates were high in some studies and dropout rates tended to be higher in the control groups, which created a potential for differential follow‐up with better reporting of clinical pregnancies in the intervention groups. Reporting bias was unclear in 68% of studies.

Potential biases in the review process

There may have been some potential for bias in the review process, as there were some changes in previous updates of the review compared to the protocol. These included additions and deletions to exclusion criteria such as the removal of pentoxifylline, and adding the new outcome progressive sperm motility. Some bias in the review process may have arisen due to the inclusion of studies that have had a dropout of participants of > 20%, with subsequent imbalances in the number of participants between the treatment and control groups.

Agreements and disagreements with other studies or reviews

The results of our review are in agreement with those of other published systematic reviews. Two other reviews described the effects of L‐carnitine and L‐acetylcarnitine on subfertile men. The systematic review and meta‐analysis by Zhou and colleagues (Zhou 2007) compared L‐carnitine and L‐acetylcarnitine therapy versus placebo treatment and found improvements in pregnancy rate and total sperm motility. Zhang 2020 also found improved total sperm motility, progressive sperm motility, and pregnancy rates with the use of L‐carnitine and L‐acetyl carnitine. Our review was unable to pool the results of the carnitine studies due to inconsistencies between the studies and excluded biochemical and undefined pregnancy from the meta‐analysis. The descriptive review by Patel and Sigman (Patel 2008) discusses the improvement in pregnancy rates with oral intake of antioxidants, however Patel states that randomised controlled trials (RCTs) have not shown an effect on sperm motility and that there is a need for more RCTs in men with oxidative stress. Furthermore, Garg 2016 discusses in a review the effect of antioxidants in men with varicocele. They conclude that antioxidant therapy is a potential option as primary treatment or adjunct after surgical repair of varicocele. Wang 2019 discussed antioxidant therapy in men with varicocele as well, and found no evidence of increased pregnancy rate.

Agarwal and colleagues discussed in both an overview of the literature (Agarwal 2004) and systematic review (Majzoub 2018), the effectiveness of antioxidants. In the 2004 overview Agarwal notes that vitamin E and a combination of vitamin E with other antioxidants such as N‐acetylcysteine, vitamin A and fatty acids appear to improve pregnancy rates in men with asthenozoospermia. This is in agreement with our review. However, their conclusion that carnitines also appear to have an effect on pregnancy rates could not be confirmed. In the systematic review Majzoub 2018 included 29 studies, of which there were 19 RCTs and 10 prospective studies. In 26 studies they found a significant positive effect on basic semen parameters, advanced sperm function tests, ART outcomes or live birth rate. Specifically, a positive effect was seen on live birth rate and fertilisation rate when using vitamin E, vitamin C, carnitines, coenzyme Q10 and zinc. A difference between differing antioxidants was not seen in our review.

Another review (Ross 2010) showed improvement in pregnancy rate and sperm quality after antioxidant therapy. This is in agreement with our review, although we are uncertain of the sperm parameter outcomes due to the extreme heterogeneity. A systematic review (Lafuente 2013) looking at the effect of coenzyme Q10 and male subfertility found an association between this antioxidant and improved pregnancy rate, sperm concentration and motility. We did agree on the effect of coenzyme Q10 on sperm motility and concentration at six months, however we could not draw clear conclusions due to the heterogeneity in these analyses. A more recent systematic review with meta‐analysis studied the effectiveness of folate and folate plus zinc on sperm parameters in subfertile men (Irani 2017). They concluded that folate alone was only effective on sperm concentration, and folate plus zinc only on sperm concentration and morphology. Both interventions did not have any effect on sperm motility. This effect of zinc plus folate or folate alone could be confirmed with our review. The review by Zhou and colleagues (Zhou 2021) focused on N‐acetyl‐cysteine (NAC) and men with idiopathic infertility and found an increased sperm concentration and total motility after use of NAC. We found a similar effect in the six months comparisons, however due to inclusion of other studies, we found no evidence of increased sperm parameters in the other comparisons including NAC.

A review on nutritional and medical therapies (Omar 2019) and male infertility reports no improvement of pregnancy rates following treatment with L‐carnitine or L‐carnitine combined with L‐acetyl‐carnitine, which is in line with our review.

It should be noted that some of these reviews are relatively outdated, given the newly published studies in the past decade.
The above‐mentioned systematic reviews mainly reported on overall pregnancy rates, whereas this updated Cochrane Review reported specifically on clinical pregnancy rates (as confirmed by the identification of a gestational sac on ultrasound) so fewer studies were available for analysis.

A Cochrane Review of antioxidants for female subfertility has been published (Showell 2020) showing that there is limited evidence for a beneficial effect of antioxidants for subfertile women. Furthermore, a recent systematic review and meta‐analysis looking at the effect of micronutrient supplementation, in both male and females, on IVF outcomes showed a positive influence on clinical outcomes in terms of pregnancy rate and/or live birth rate (Kofi Arhin 2017). However, only five RCTs could be included, with significant heterogeneity among the interventions and study designs.

Figure 1

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Figure 2

Methodological risk of bias summary: review authors' judgements about each methodological bias item for each included study.

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Figure 3

Methodological risk of bias graph: review authors' judgements about each methodological bias item presented as percentages across all included studies.

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Figure 4

Forest plot of comparison: 1 Antioxidant(s) versus placebo or no treatment, outcome: 1.1 Live birth; type of antioxidant.

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Figure 5

Funnel plot of comparison: 1 Antioxidant(s) versus placebo or no treatment, outcome: 1.1 Live birth; type of antioxidant.

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Figure 6

Forest plot of comparison: 1 Antioxidant(s) versus placebo or no treatment, outcome: 1.3 Clinical pregnancy; type of antioxidant.

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Figure 7

Funnel plot of comparison: 1 Antioxidant(s) versus placebo or no treatment, outcome: 1.5 Clinical pregnancy; type of antioxidant.

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Figure 8

Forest plot of comparison: 1 Antioxidant(s) versus placebo or no treatment, outcome: 1.5 Adverse events.

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Figure 9

Forest plot of comparison: 1 Antioxidant(s) versus placebo or no treatment, outcome: 1.6 Sperm DNA fragmentation; type of antioxidant.

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Figure 10

Forest plot of comparison: 1 Antioxidant(s) versus placebo or no treatment, outcome: 1.9 Total sperm motility at 3 months or less; type of antioxidant.

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Figure 11

Forest plot of comparison: 1 Antioxidant(s) versus placebo or no treatment, outcome: 1.14 Progressive sperm motility at 3 months or less; type of antioxidant.

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Figure 12

Forest plot of comparison: 1 Antioxidant(s) versus placebo or no treatment, outcome: 1.19 Sperm concentration at 3 months or less; type of antioxidant.

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Analysis 1.1

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 1: Live birth; type of antioxidant

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Analysis 1.2

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 2: Live birth; IVF/ICSI

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Analysis 1.3

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 3: Clinical pregnancy; type of antioxidant

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Analysis 1.4

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 4: Clinical pregnancy; IVF/ICSI

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Analysis 1.5

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 5: Adverse events

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Analysis 1.6

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 6: Sperm DNA fragmentation at 3 months or less; type of antioxidant

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Analysis 1.7

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 7: Sperm DNA fragmentation at 6 months; type of antioxidant

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Sperm DNA fragmentation (data not suitable for meta‐analysis)

Study

Intervention

Control

P‐value

Folic acid

Boonyarangkul 2015

Folic acid

DNA tail length, COMET assay

3 month:

Mean = 4.04 (n = 15)

SE = 0.94

6 month:

Mean = 6.01

SE = 1.49

Placebo

DNA tail length, COMET assay

3 month:

Mean = 10.08 (n = 15)

SE = 3.39

6 month:

Mean = 8.69

SE = 4.28

Not provided

Analysis 1.8

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 8: Sperm DNA fragmentation (data not suitable for meta‐analysis)

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Analysis 1.9

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 9: Total sperm motility at 3 months or less; type of antioxidant

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Study	Intervention	Control	P value
Total sperm motility at 3 months or less (data not suitable for meta analysis)
Vitamin E
Kessopoulou 1995	Vitamin E Median difference = 7 (n = 15) Min/max difference = ‐27 ‐ 34	Placebo Median difference = 7 (n = 15) Min/max difference = ‐33 ‐ 36	Not provided
Combined antioxidants
Galatioto 2008	N‐acetylcysteine (NAC) 600 mg + vitamins‐minerals % of motile sperm (Class A WHO) = 58% (n = 20)	No treatment % of motile sperm (Class A WHO) = 51% (n = 22)	P = 0.847

Analysis 1.10

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 10: Total sperm motility at 3 months or less (data not suitable for meta analysis)

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Analysis 1.11

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 11: Total sperm motility at 6 months; type of antioxidant

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Analysis 1.12

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 12: Total sperm motility at 9 months or more; type of antioxidant

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Analysis 1.13

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 13: Total sperm motility over time

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Analysis 1.14

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 14: Progressive sperm motility at 3 months or less; type of antioxidant

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Analysis 1.15

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 15: Progressive sperm motility at 6 months; type of antioxidant

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Study	Intervention	Control	P value
Progressive sperm motility at 6 months (data not suitable for meta analysis)
Coenzyme Q10
Saeed Alkumait 2020	Coenzyme Q10 200 mg % improvement = 36 (n = 50)	Placebo % improvement = 4 (n = 50)	0.01
Glutathione
Saeed Alkumait 2020	Glutathione 250 mg % improvement = 38 (n = 51)	Placebo % improvement = 4 (n = 50)	0.01

Analysis 1.16

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 16: Progressive sperm motility at 6 months (data not suitable for meta analysis)

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Analysis 1.17

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 17: Progressive sperm motility at 9 months or more; type of antioxidant

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Analysis 1.18

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 18: Progressive sperm motility over time

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Analysis 1.19

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 19: Sperm concentration at 3 months or less; type of antioxidant

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Study	Intervention	Control	P value
Sperm concentration at 3 months or less (data not suitable for meta analysis)
Carnitines
Lenzi 2003	L‐carnitine Mean = 9 (1st phase data) (n = 43) No SD given	Placebo Mean = 5.3 (n = 43) No SD given	P = 0.03
Vitamin E
Kessopoulou 1995	Vitamin E Median difference = ‐15 (n = 15) Min/max difference = ‐58 ‐ 59	Placebo Median difference = 0 (n = 15) Min/max difference = ‐37 ‐ 160	Not provided

Analysis 1.20

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 20: Sperm concentration at 3 months or less (data not suitable for meta analysis)

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Analysis 1.21

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 21: Sperm concentration at 6 months; type of antioxidant

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Study	Intervention	Control	P value
Sperm concentration at 6 months (data not suitable for meta analysis)
Glutathione
Saeed Alkumait 2020	Glutathione 250 mg % improvement = 26 (n = 51)	Placebo % improvement = 2 (n = 50)	0.01
Coenzyme Q10
Saeed Alkumait 2020	Coenzyme Q10 200 mg % improvement = 24 (n = 50)	Placebo % improvement = 2 (n = 50)	0.01

Analysis 1.22

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 22: Sperm concentration at 6 months (data not suitable for meta analysis)

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Analysis 1.23

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 23: Sperm concentration at 9 months or more; type of antioxidant

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Analysis 1.24

Comparison 1: Antioxidant(s) versus placebo or no treatment, Outcome 24: Sperm concentration over time

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Analysis 2.1

Comparison 2: Head‐to‐head antioxidant(s), Outcome 1: Live birth; type of antioxidant

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Analysis 2.2

Comparison 2: Head‐to‐head antioxidant(s), Outcome 2: Clinical pregnancy; type of antioxidant

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Analysis 2.3

Comparison 2: Head‐to‐head antioxidant(s), Outcome 3: Sperm DNA fragmentation; type of antioxidant

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Analysis 2.4

Comparison 2: Head‐to‐head antioxidant(s), Outcome 4: Total sperm motility at 3 months or less; type of antioxidant

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Analysis 2.5

Comparison 2: Head‐to‐head antioxidant(s), Outcome 5: Total sperm motility at 6 months; type of antioxidant

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Analysis 2.6

Comparison 2: Head‐to‐head antioxidant(s), Outcome 6: Total sperm motility at 9 months or more; type of antioxidant

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Analysis 2.7

Comparison 2: Head‐to‐head antioxidant(s), Outcome 7: Progessive sperm motility at 3 months or less; type of antioxidant

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Analysis 2.8

Comparison 2: Head‐to‐head antioxidant(s), Outcome 8: Progressive sperm motility at 6 months; type of antioxidant

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Study	Coenzyme Q10 (n=50	Glutathione (n=51)	P value
Progressive motility at 6 months (data not suitable for meta‐analysis)
Saeed Alkumait 2020	% improvement = 36	% improvement = 38	Not provided

Analysis 2.9

Comparison 2: Head‐to‐head antioxidant(s), Outcome 9: Progressive motility at 6 months (data not suitable for meta‐analysis)

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Analysis 2.10

Comparison 2: Head‐to‐head antioxidant(s), Outcome 10: Progressive sperm motility at 9 months; type of antioxidant

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Analysis 2.11

Comparison 2: Head‐to‐head antioxidant(s), Outcome 11: Sperm concentration at 3 months or less; type of antioxidant

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Analysis 2.12

Comparison 2: Head‐to‐head antioxidant(s), Outcome 12: Sperm concentration at 6 months; type of antioxidant

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Study	Coenzyme Q10 (n=50)	Glutathione (n=51)	P value
Sperm concentration at 6 months (data not suitable for meta‐analysis)
Saeed Alkumait 2020	% improvement = 24	% improvement = 26	Not provided

Analysis 2.13

Comparison 2: Head‐to‐head antioxidant(s), Outcome 13: Sperm concentration at 6 months (data not suitable for meta‐analysis)

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Analysis 2.14

Comparison 2: Head‐to‐head antioxidant(s), Outcome 14: Sperm concentration at 9 months or more; type of antioxidant

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Summary of findings 1. Antioxidants compared to placebo or no treatment for patients with male subfertility

Outcomes	*Anticipated absolute effects^ (95% CI)**		Relative effect (95% CI)	№ of participants (studies)	Certainty of the evidence (GRADE)	Comments
Antioxidants compared to placebo or no treatment for patients with male subfertility
Patient or population: patients with male subfertility Setting: clinic Intervention: antioxidants Comparison: placebo or no treatment
Outcomes	Risk with placebo or no treatment	Risk with antioxidants	Relative effect (95% CI)	№ of participants (studies)	Certainty of the evidence (GRADE)	Comments
Live birth rate per couple randomised	162 per 1000	216 per 1000 (171 to 269)	OR 1.43 (1.07 to 1.91)	1283 (12 RCTs)	⊕⊝⊝⊝ VERY LOW ^{1 2 3}
Clinical pregnancy rate per couple randomised	146 per 1000	245 per 1000 (199 to 297)	OR 1.89 (1.45 to 2.47)	1706 (20 RCTs)	⊕⊕⊝⊝ LOW ^{1 3}
Adverse events ‐ Miscarriage	48 per 1000	68 per 1000 (36 to 125)	OR 1.46 (0.75 to 2.83)	664 (6 RCTs)	⊕⊝⊝⊝ VERY LOW ^{1 3 4}
Adverse events ‐ Gastrointestinal	15 per 1000	39 per 1000 (22 to 71)	OR 2.70 (1.46 to 4.99)	1355 (16 RCTs)	⊕⊕⊝⊝ LOW ^{1 3}
*The risk in the intervention group (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). CI: Confidence interval; OR: Peto Odds ratio;
GRADE Working Group grades of evidence High certainty: We are very confident that the true effect lies close to that of the estimate of the effect. Moderate certainty: We are moderately confident in the effect estimate; the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different. Low certainty: Our confidence in the effect estimate is limited; the true effect may be substantially different from the estimate of the effect. Very low certainty: We have very little confidence in the effect estimate; the true effect is likely to be substantially different from the estimate of effect.
¹ Downgraded one level for serious risk of bias: lack of blinding and incomplete accounting of patients and outcome events ² Downgraded one level for suspected publication bias based on the funnel plot ³ Downgraded one level for serious imprecision: less than 400 events ⁴ Downgraded one level for serious imprecision: crossing the line of no effect

Summary of findings 1. Antioxidants compared to placebo or no treatment for patients with male subfertility

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Table 1. Data for undefined or biochemical pregnancy

Undefined or biochemical pregnancy	Antioxidant		Control		Peto OR [CI]
Antioxidant(s) versus placebo or no treatment
Combined antioxidants	Events	Total	Events	Total
	35	234	32	194
Galatioto 2008	1	20	0	22	8.17 [0.16 to 413.39]
Gopinath 2013	13	92	2	46	2.72 [0.88 to 8.46]
Steiner 2020	18	85	26	86	0.62 [0.32 to 1.24]
Stenqvist 2018	3	37	4	40	0.80 [0.17 to 3.74]
Arginine
Pryor 1978	2	35	2	29	0.82 [0.11 to 6.16]
Carnitines	25	154	3	145
Sigman 2006	1	12	1	9	0.74 [0.04 to 13.02]
Peivandi 2010	3	15	0	15	8.57 [0.82 to 89.45]
Lenzi 2003	6	43	0	43	8.37 [1.61 to 43.58]
Lenzi 2004	4	30	0	26	7.20 [0.95 to 54.34]
Cavallini 2004	9	39	1	47	7.50 [2.01 to 27.98]
Coenzyme Q10	6	136	3	136
Safarinejad 2009a	0	106	0	106	Not estimable
Nadjarzadeh 2011	0	23	0	24	Not estimable
Vitamin C + Vitamin E
Rolf 1999	0	15	0	16	Not estimable
Vitamin E
Ener 2016	5	28	5	28	1.00 [0.26 to 3.88]
Head‐to‐head antioxidant(s)	Events	Total	Events	Total
L‐acetyl carnitine + L‐carnitine vs Vitamin E + Vitamin C
Li 2005	10	85	2	53	2.72 [0.81 to 9.14]
L‐carnitine + Vitamin E vs Vitamin E
Wang 2010	21	68	3	67	6.01 [2.49 to 14.47]
Vitamin E + amino acids vs Vitamin E
Zhou 2016	4	70	1	50	2.52 [0.41 to 15.35]

Table 1. Data for undefined or biochemical pregnancy

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Table 2. Outcomes and conclusions from all included studies

Study ID	Design, population	Outcomes described in methods section	Outcomes reported on in results	In meta‐analysis Y or N	Results	Conclusions + = positive effect ‐ = negative or no effect
Abbasi 2020	Parallel, placebo Men post‐varicocelectomy N = 60	Sperm parameters, DNA fragmentation	Sperm parameters, DNA fragmentation	Y ‐ sperm parameters Y ‐ DNA fragmentation	ALA improved sperm motility compared to baseline. No significant difference in sperm parameters between ALA and placebo.	‐ ALA does not improve semen quality compared to placebo after varicocelectomy
Akiyama 1999	Cross‐over, head‐to‐head Infertile men, high ROS levels N = 10	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Ethylcystein did not improve sperm density and motility but "sperm function" increased and ROS levels decreased, compared to vitamin E	+ Ethylcysteine shown to be effective for improvement of sperm parameters when compared to vitamin E
Alahmar 2019	Parallel, head‐to‐head Idiopathic OAT N = 65	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	CoQ10 200 and 400 mg improved sperm concentration and motility, greater improvement with 400 mg	+ CoQ10 improves sperm parameters, greater improvement with a 400 mg dose compared to 200 mg
Alahmar 2020	Parallel, head‐to‐head Idiopathic OAT N = 70	Sperm parameters	Sperm parameters	N ‐ number of drop‐outs unclear	CoQ10 and selenium each improved sperm concentration and motility, greater improvement with CoQ10	+ CoQ10 and selenium improve sperm parameters, greater improvement with CoQ10
Amini 2020	Parallel, placebo Infertile men N = 72	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Vitamin D did not improve sperm parameters	‐ Vitamin D does not improve sperm parameters
Ardestani 2019	Parallel, no treatment Men post‐varicocelectomy N = 64	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Co‐administration of folic acid, selenium and vitamin E improved sperm concentration and motility	+ A combination of folic acid + selenium + vitamin E improves sperm parameters after varicocelectomy
Attallah 2013	Parallel, no treatment Idiopathic asthenozospermia, IUI N = 30 Conference abstract	Sperm parameters, chemical and clinical pregnancy	Sperm parameters, chemical and clinical pregnancy	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical	NAC increased sperm concentration and motility. Clinical pregnancy was not significantly different between the groups	+ NAC improves semen quality and improves pregnancy rates prior to IUI, no improvement of pregnancy rate
Azizollahi 2013	Multiple arm trial Men post‐varicocelectomy N = 160	Sperm parameters	Sperm parameters	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical	Mild improvement in sperm parameters with the use of antioxidants zinc, folic acid or both	+ Co‐administration of zinc and folic acid improved sperm parameters and increased varicocelectomy outcomes, only zinc an improvement in pregnancy rate
Bahmyari 2021	Parallel, placebo Idiopathic OAT N = 70	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	No improvement of sperm parameters with the use of selenium, folic acid and vitamin E	‐ Co‐administration of selenium, folic acid and vitamin E were not effective to improve sperm parameters
Balercia 2005	Multiple arm, placebo Infertile men N = 60	Sperm parameters	Sperm parameters, pregnancy rate	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical Y ‐ live birth	Improvement in motility in LAC group.	+ Long‐term carnitine is effective in increasing sperm motility. No evidence of increased live birth or clinical pregnancy.
Balercia 2009	Parallel, placebo Infertile and unexplained N = 60	Sperm parameters	Sperm parameters, pregnancy rate	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical	Co enzyme Q10 increased sperm motility.	+ Q10 is effective in improving sperm kinetic features in asthenospermia. No evidence of increased live birth or clinical pregnancy.
Barekat 2016	Parallel, no treatment Subfertile men with varicocele N = 40	Sperm parameters, DNA fragmentation	Sperm parameters, DNA fragmentation, clinical spontaneous pregnancies	Y ‐ sperm parameters Y ‐ DNA fragmentation Y ‐ pregnancy rate, clinical (SEs converted to SDs)	Sperm parameters significantly improved after surgery compared to before surgery in both the NAC and control groups. NAC might have an additional value by improving sperm motility post‐varicocelectomy	+ The results of this study revealed that NAC improved chromatin integrity and pregnancy rate when administered as adjunct therapy post‐varicocelectomy
Biagiotti 2003	Multiple arm, no treatment Severe idiopathic oligoasthenospermia N = 42 Conference abstract	Sperm parameters	Sperm parameters	N ‐ no data available	A significant improvement in morphology concentration, motility in the carnitine group No side effects	+ Quality of semen is positively associated with fertilisation and implantation rates in assisted reproduction
Blomberg Jensen 2018	Parallel, placebo Infertile men with impaired semen quality N = 307	Sperm parameters, reproductive hormones, live birth rate	Sperm parameters, reproductive hormones, live birth rate	Y ‐ sperm parameters, concentration provided as median + IQR and converted to mean + SD Y ‐ live birth rate	Vitamin D was not associated with changes in semen parameters, although spontaneous pregnancies tended to be higher in couples in which the man was in the treatment group	± Vitamin D did not improve semen quality. The positive impact of vitamin D supplementation on live birth rate and serum inhibin B in oligozoospermic and vitamin D–deficient men may be of clinical importance and warrant verification by others.
Boonyarangkul 2015	Multiple arm, placebo, tamoxifen excluded Men with abnormal semen analysis N = 68	Sperm parameters, DNA damage (Comet assay)	Sperm parameters, DNA tail length	Y ‐ sperm parameters	Folate alone significantly decreased DNA tail length at 3‐months. Sperm motility was significantly increased after 3‐months Folate alone.	+ Our study indicated that folate in combination with Tamoxifen citrate could improve sperm quality including semen parameters and sperm DNA integrity
Busetto 2018	Parallel, placebo Infertile men with OAT, 50% included with varicocele N = 104	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical	Sperm concentration, total sperm count, progressive and total motility were significantly increased in supplemented (Proxeed Plus) patients. Increased pregnancy rate	+ Supplementation with metabolic and antioxidant compounds could be efficacious when included in strategies to improve fertility
Cavallini 2004	Multiple arm, placebo Idiopathic OAT men with varicocele N = 325	Sperm parameters, pregnancy rate, adverse events	Sperm parameters, pregnancy rate, adverse events	Y ‐ sperm parameters (median +IQR converted to mean + SD) N ‐ pregnancy rate, unclear if clinical Table 1 Y ‐ adverse events	Significant increase in sperm parameters for carnitines when compared to placebo. Carnitine groups had a significantly higher pregnancy rate than placebo group	+ The antioxidant plus anti‐inflammatory group was more effective in improving sperm parameters and pregnancy than those of carnitines alone or placebo however carnitines alone were more effective than placebo
Cheng 2018	Multiple arm, head‐to‐head Idiopathic OAT N = 312	Sperm parameters, DNA fragmentation, pregnancy rate	Sperm parameters, DNA fragmentation, pregnancy rate	Y/N ‐ sperm parameters, results not available for all groups and parameters Y ‐ DNA fragmentation Y ‐ pregnancy rate, clinical	Significant improvement of sperm parameters and DNA fragmentation in the L‐carnitine plus CoQ10 group compared to placebo. Combination and L‐carnitine groups had remarkably higher pregnancy rate than placebo group	+ Combination of LC and CoQ10 improve semen parameters and outcome of clinical pregnancy
Conquer 2000	Multiple arm, placebo Asthenozoospermic men N = 28	Sperm parameters	Sperm parameters	Y ‐ sperm parameters (SEs converted to SDs)	DHA showed no effect on sperm motility or concentration	± DHA supplementation increased DHA levels in the sperm but not motility or concentration
Cyrus 2015	Parallel, placebo Infertile men with varicocele N = 115	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Vitamin C was not effective on sperm count but improved sperm motility and morphology significantly	+ Ascorbic acid can play a role as adjuvant treatment after varicocelectomy in infertile men
Dawson 1990	Multiple arm, placebo Men with sperm agglutination N = 30	Sperm parameters	Sperm parameters	Y ‐ sperm parameters (SEs converted to SDs)	The group receiving 1000 mg of AA showed more improvement in parameters than the 200mg group and the placebo	+ Vitamin C can improve sperm parameters, especially dosage of 1000 mg.
Deng 2014	Head‐to‐head Men with idiopathic oligoasthenozoospermia N = 86	Sperm parameters, adverse reactions, pregnancy rate	Sperm parameters, adverse reactions, pregnancy rate	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical	Vitamin D is a safe option for the treatment of idiopathic oligoasthenozoospermia and can effectively improve the semen quality especially the progressive sperm motility	+ Vitamin D can improve forward movement sperm number and percentage, improve the woman's clinical pregnancy rate, and is well tolerated
Dimitriadis 2010	Multiple arm, no treatment, vardenafil/sildenafil arms excluded Men with oligoasthenospermia N = 75	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	An improvement in sperm concentration with carnitine versus no treatment	+ Enhancement of Leydig cell secretory function may increase sperm concentration and motility
Ener 2016	Parallel, no treatment Infertile men with varicocele N = 56	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	Y ‐ sperm parameters N ‐ pregnancy rate, unknown if clinical Table 1	The administration of vitamin E increased all of the parameters; however not statistically significant	‐ Vitamin E supplementation does not improve the sperm parameters after varicocelectomy
Eslamian 2013	Parallel, placebo Asthenoszoospermic men N = 50	Sperm parameters	Sperm parameters, sperm membrane and serum fatty acids	N ‐ sperm parameters, data not usable, no continuous data but categories from 'significantly improvement' to 'worsened'	Sperm parameters improved with DHA + vitamin E supplementation	+ Sperm parameters improve with DHA + vitamin E supplementation
Eslamian 2020	Multiple arm, placebo Asthenozoospermic men N = 180	Sperm parameters	Sperm parameters	N ‐ sperm parameters, only imputed data provided	Significant increase of sperm concentration in the DHA + vitamin E group compared to groups treated with DHA+placebo, vitamin E+placebo and placebo.	+ Combined DHA and vitamin E improve sperm parameters
Exposito 2016	Parallel, placebo Normozoospermic, oligozoospermic and asthenozoospermic men N = 113	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	N ‐ sperm parameters N ‐ pregnancy rate Both not included because data included normospermic men	50% of oligozoospermic men improved sperm concentration and sperm count to normozoospermic levels. This trend was also observed in asthenozoospermic men, but nog significantly	+ Vitamin E treatment by oral administration improves semen parameters
Galatioto 2008	Parallel, no treatment Men with persistent oligospermia after embolisation of varicocele N = 42	Sperm parameters, pregnancy rate, adverse events	Sperm parameters, pregnancy rate, adverse events	N ‐ sperm parameters, only medians given N ‐ pregnancy, unclear if clinical Table 1 N ‐ adverse events	Significant difference in sperm count in combined antioxidant group but not in motility. One pregnancy in the NAC group No significant adverse effects	± NAC does not improve pregnancy rate, no significant adverse events, but do significantly increase sperm count
Gamidov 2017	Multiple arm, no treatment Men with varicocele N = 114	Sperm parameters, DNA fragmentation, adverse events	Sperm parameters, DNA fragmentation, adverse events	Y ‐ sperm parameters (median+IQR converted to mean+SD) Y ‐ DNA fragmentation (median+IQR converted to mean+ SD) Y ‐ adverse events	SpermActine (SA) resulted in a 22.3% decrease in the level of sperm DNA fragmentation at 3 months. SA + vitamin complex resulted in a 27% increase in the sperm concentration at 3 months. There were no side effects of pharmacotherapy.	+ Antioxidant therapy leads to an improvement in the basic sperm parameters (sperm concentration and motility) and a decrease in the level of sperm DNA fragmentation in the short term. There were no side effects
Gamidov 2019	Parallel, placebo Infertile men with high oxidative stress and DNA fragmentation N = 80	Sperm parameters, DNA fragmentation, pregnancy rate, live birth	Sperm parameters, DNA fragmentation, pregnancy rate, live birth, adverse events	Y ‐ sperm parameters Y ‐ DNA fragmentation Y ‐ pregnancy rate, clinical Y ‐ live births Y ‐ adverse events	Spermactin Forte significantly improvement sperm motility and decreased oxidative stress. There were more pregnancies in the intervention group (13 versus 1)	+ The use of the SpermActin Forte antioxidant improves sperm analysis in most patients. SpermActin Forte is an effective and safe method of treating male infertility
Gonzalez‐Ravina 2018	Multiple arm, placebo Infertile men N = 60	Sperm parameters, DNA fragmentation	Sperm parameters, DNA fragmentation	N ‐ sperm parameters, outcomes provided as change + SD Analysis 1.15; Analysis 1.20 N ‐ DNA fragmentation, outcomes provided as change + SD Analysis 1.8	Significant increase of progressive sperm motility in the DHA 1g and 2g groups after 1 month and in the DHA 0.5 group after 3 months. Greater effect in asthenozoospermic men	+ DHA (0.5, 1 and 2g) had beneficial effects on sperm function without producing any adverse effects, obtaining more immediate results with higher doses
Gopinath 2013	Multiple arm, placebo Idiopathic OAT men N = 138	Sperm parameters, pregnancy rate, adverse events	Sperm parameters, pregnancy rate, adverse events	Y ‐ sperm parameters N ‐ pregnancy rate, not clinical Table 1 Y ‐ adverse events	Combined antioxidant significantly improved sperm count and total motility in both treatment arms (1 vs 2 tablets). Mild adverse events were reported, no severe.	+ Exogenous administration of fixed dose combination of antioxidants is safe and effective therapy in improving the male subfertility regarding sperm parameters. Only mild adverse events when using combined antioxidants
Goswami 2015	Multiple arm, placebo Arm treated with diet enriched in antioxidants not used Men with idiopathic infertility and high ROS N = 175 Conference abstract	Sperm parameters, DNA fragmentation	DNA fragmentation	N ‐ sperm parameters, not reported in results N ‐ DNA fragmentation, no results reported besides p‐value	No difference in DNA fragmentation between the study groups	+/‐ No conclusions on antioxidants versus placebo. A diet rich in antioxidants and lifestyle modifications can bring almost the same effect as antioxidant supplements
Greco 2005	Parallel, placebo Infertile males with high DNA fragmentation N = 64	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	No significant difference in concentration or motility however DNA fragmentation was significantly reduced in the vitamin C + E when compared to placebo	+ A short oral treatment of Vitamin C + E can reduce DNA fragmentation
Haghighian 2015	Parallel, placebo Men with idiopathic asthenozoospermia N = 48	Sperm parameters, adverse events	Sperm parameters, adverse events	Y ‐ sperm parameters N ‐ adverse events, reported "none", however not clear which side effects they aimed for	Sperm parameters were significantly higher in ALA group. No side effects due to the oral administration of ALA were observed in any participants.	+ Medical therapy of asthenoteratospermia with ALA supplement could improve quality of semen parameters
Haje 2015	Multiple arm, placebo, tamoxifen arms excluded Infertile men with idiopathic OAT N = 128	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	N ‐ sperm parameters, range of treatment 3 ‐ 6 months and not divided N ‐ pregnancy rate, unclear if pregnancy and no numbers but percentage	L‐carnitine did not improve sperm count or motility. Only tamoxifen or tamoxifen + L‐carnitine improved pregnancy rate, not significantly.	± Administration of tamoxifen or L‐carnitine can improve sperm parameters and ICSI outcomes. Combining those result in maximum therapeutic effect
Huang 2020	Parallel, placebo Oligozoospermic men N = 769	Sperm parameters, evaluation of MTHFR polymorphism, DNA fragmentation, pregnancy rate, live birth	Sperm parameters, evaluation of MTHFR polymorphism, DNA fragmentation, pregnancy rate, live birth	N ‐ sperm parameters N ‐ DNA fragmentation N ‐ pregnancy, clinical N ‐ live births All outcomes reported for MTHFR polymorphism groups only	Folic acid significantly increased sperm parameters, decreased oxidative stress and DNA fragmentation and lead to a higher pregnancy and live birth rate in the MTHFR 677 TT group. Effect not seen in other MTHFR groups.	+ Folic acid has a beneficial effect on oligozoospermia with MTHFR 677 TT genotype in terms of sperm parameters, DNA fragmentation and pregnancy outcomes
Joseph 2020	Parallel, no treatment Infertile men scheduled for ART N = 200	Sperm parameters, pregnancy rate, live birth, adverse events	Sperm parameters, pregnancy rate, live birth, adverse events	Y ‐ sperm parameters (median+IQR converted to mean+SD) Y ‐ pregnancy rate, clinical Y ‐ live births Y ‐ adverse events	No significant difference in clinical pregnancies or live births when combined vitamin C + vitamin E + zinc were compared to no treatment. No improvement of sperm parameters	‐ No difference in clinical pregnancy and live births. No improvement of sperm parameters
Kessopoulou 1995	Cross‐over, placebo Male infertility N = 30	Sperm parameters, adverse events, live birth	Sperm parameters, adverse effects, live birth	N ‐ sperm parameters, only medians given Analysis 1.10; Analysis 1.20 Y ‐ pregnancy rate, clinical Y ‐ live births Y ‐ adverse events	No differences in sperm outcomes were seen between the groups. 1 pregnancy in the vitamin E group and nil in the placebo (first phase data)	+ No difference in semen parameters. There is evidence of increased live birth and clinical pregnancy rate
Kizilay 2019	Parallel, no treatment Varicocele patients with oligozoospermia N = 93	Sperm parameters, clinical pregnancy, adverse events	Sperm parameters, clinical pregnancy, adverse events	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical Y ‐ adverse events	Significant improvement of sperm parameters and higher clinical pregnancy rate in combined antioxidant group compared to no treatment	+ Antioxidant treatment provides an important contribution to varicocelectomy outcomes and improves pregnancy rates
Kopets 2020	Parallel, placebo Idiopathic infertility N = 83	Sperm parameters, clinical pregnancy, adverse events	Sperm parameters, clinical pregnancy, adverse events	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical Y ‐ adverse events	The percentage of normal spermiograms was significantly higher in the combined antioxidant group. Higher spontaneous pregnancy rate in antioxidant group	+ Combined l‐carnitine/l‐acetyl‐carnitine, l‐arginine, glutathione, CoQ10, zinc, folic acid, cyanocobalamin, and selenium improves sperm quality and increases pregnancy rates
Korshunov 2018	Parallel, no treatment Obstructive azoospermia, TESA/ICSI candidates N = 46 Conference abstract	Clinical pregnancy, live births	Clinical pregnancy, live birth, embryo quality, early pregnancy loss	Y ‐ pregnancy rate, clinical Y ‐ live births N ‐ adverse events, miscarriage. No data provided by authors.	Clinical pregnancy and live birth rate were 62,5% vs 59,1% and 54,1% vs 40,9% in the antioxidant and no treatment group, respectively. Higher early pregnancy loss rate in control group	+ Antioxidant therapy may have a positive effect for patients with obstructive azoospermia. It might improve ART outcome and decrease pregnancy loss
Kumalic 2020	Parallel, placebo Infertile men with OAT N = 80	Sperm parameters, DNA fragmentation, adverse events	Sperm parameters, DNA fragmentation, adverse events, after contact with author: clinical pregnancy rate and live births after ICSI	Y ‐ sperm parameters Y ‐ DNA fragmentation Y ‐ adverse events Y ‐ pregnancy rate, clinical Y ‐ live births	No statistical differences in sperm parameters between astaxanthin + vitamin E group and placebo	‐ The oral intake of astaxanthin did not affect any semen parameters in patients with OAT
Kumamoto 1988	Multiple arm, placebo Men with abnormal sperm count or motility N = 396	Sperm parameters	Sperm parameters	N ‐ sperm parameters, only scales given	No statistical difference in sperm outcomes in vitamin B 12 groups or placebo	‐ No improvement in sperm parameters after use of vitamin B12
Lenzi 2003	Cross‐over, placebo Infertile men with OAT N = 100	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	Y ‐ sperm parameters N ‐ pregnancy rate, no definition of pregnancy given see Table 1	The patient groups showed no differences in sperm outcomes between therapy (carnitine) and placebo groups. Six pregnancies in the carnitine group and nil in the placebo (first phase)	+ The pregnancies obtained during the carnitine therapy period could suggest that carnitines may also lead to improvement in sperm function and fertilisation
Lenzi 2004	Parallel, placebo Infertile men with OAT N = 60	Sperm parameters, pregnancy rate, adverse events	Sperm parameters, pregnancy rate, adverse events	Y ‐ sperm parameters N ‐ pregnancy rate, no definition of pregnancy given Table 1 N ‐ adverse events	Four participants taking carnitine induced a pregnancy in their partner and nil in the placebo	+ No evidence of improved sperm parameters
Li 2005	Head‐to‐head Infertile men with OAT N = 150	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	Y ‐ sperm parameters N ‐ pregnancy rate, no definition given Table 1	L‐carnitine and acetyl carnitine more effective than vitamin E + vitamin C for pregnancy, sperm parameters and no evidence of adverse events	+ L‐carnitine and acetyl carnitine more effective than vitamin E + vitamin C for pregnancy, sperm parameters and no evidence of adverse events
Li 2005a	Head‐to‐head Infertile men with OAT N = 80	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Statistical significance for carnitines over vitamin E + C	+ Improvement of sperm parameters for carnitines compared to vitamin E + C
Lombardo 2002	Cross‐over Infertile men with OAT N = 100 Conference abstract	Sperm parameters	Sperm parameters	N ‐ sperm parameters, no data available	Sperm parameters (concentration, motility) carnitines versus placebo	+ Improvement of sperm parameters
Martinez 2015	Multiple arm, placebo, SG1002 arm excluded Men with idiopathic OAT N = 54	Sperm parameters	Sperm parameters	N ‐ sperm parameters, no SDs given	Resveratrol treatment did not significantly affect any of the parameters.	‐ Resveratrol treatment did not significantly affect any of the parameters. SG1002 may reverse oligoasthenozoospermia. It seems to be more potent antioxidant than resveratrol
Martinez‐Soto 2010	Parallel, placebo Infertile men N = 50 Conference abstract + manuscript from author	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	No differences were found in traditional sperm parameters or lipid composition of the sperm membrane after DHA treatment, only reduction in the percentage of spermatozoa with DNA damage	+ Positive effect only on DNA fragmentation
Mehni 2014	Multiple arm, placebo, pentoxifylline arms excluded Infertile men with OAT N = 235	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	L‐carnitine only improved sperm motility, combined with pentoxifylline it improves all sperm parameters.	+ Positive effect only on sperm motility
Micic 2019	Parallel, placebo Men with OAT N = 175	Sperm parameters, DNA fragmentation	Sperm parameters, DNA fragmentation	Y ‐ sperm parameters Y ‐ DNA fragmentation (median+IQR converted to mean + SD)	Proxeed Plus significantly improved sperm volume, motility and DNA fragmentation compared to baseline.	+ Beneficial effects of carnitine derivatives (Proxeed plus) on progressive motility, vitality and sperm DNA fragmentation
Morgante 2010	Parallel, no treatment Infertile men with idiopathic asthenospermia N = 180	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Significant improvement in sperm motility.	+ Improvement of sexual satisfaction Significant improvement in sperm motility
Nadjarzadeh 2011	Parallel, placebo Men with Idiopathic OAT N = 60	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Non‐significant changes in semen parameters of CoQ10 group.	‐ CoQ10 further evidence suggesting that supplementation is associated with alleviating oxidative stress, although it does not show any significant effects on sperm concentration, motility and morphology
Nouri 2019	Parallel, placebo Men with history of infertility N = 44	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Significant improvement of sperm concentration with lycopene compared to placebo. Increase of total motility in lycopene group compared to baseline.	+ Lycopene improves sperm parameters and oxidative stress biomarkers in infertile men
Nozha 2001	Head‐to‐head Men with OAT N = unclear, 20?	Sperm parameters	Sperm parameters	N ‐ sperm parameters, no data available	Vitamin E + selenium significantly improves sperm motility	+ Vitamin E + selenium associated with improved sperm motility when compared with vitamin B
Omu 1998	Parallel, no treatment Men with asthenozoopermia N = 100	Sperm parameters	Sperm parameters, pregnancy, live birth	N ‐ sperm parameters, only % increase or decrease, not usable Y ‐ pregnancy rate, clinical Y ‐ live birth	Significant improvement in sperm quality by zinc therapy	+ Zinc has a role in improving sperm parameters. Significant increase in pregnancy, not live birth
Omu 2008	Multiple arm, no treatment Men with asthenozoospermia N = 100	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Zinc therapy alone, in combination with vitamin E or with vitamin E+C were associated with comparably improved sperm parameters and less sperm DNA fragmentation	+ Zinc therapy reduces asthenozoospermia
Peivandi 2010	Cross‐over, placebo Infertile men N = 30	Sperm parameters	Sperm parameters, pregnancy rate	Y ‐ sperm parameters N ‐ pregnancy rate, not defined as clinical Table 1	Significant improvements in mean sperm concentration and progressive sperm motility upon two months of L‐carnitine intake but no significant changes were found in sperm volume or morphology.	+ Sperm outcomes and biochemical pregnancies. L‐carnitine intake effectively improved the mean sperm count and progressive sperm motility
Popova 2019	Parallel, no treatment Men planning ART treatment N = 80	Sperm parameters, clinical pregnancy, adverse events	Sperm parameters, clinical pregnancy, adverse events	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical Y ‐ adverse events	No significant change in sperm motility. A pregnancy rate in the combined antioxidants (Androdoz) group was 45% compared to 25% in the control group.	+/‐ Androdoz contributes to an increase in positive outcomes of ART program. "Androdoz improves the main criteria of sperm analysis and functional tests (HBA‐test)". This is based on the improvement of morphology
Pourmand 2014	Parallel, no treatment Men with male factor infertility and varicocele N = 100	Sperm parameters, DNA fragmentation, adverse events	Sperm parameters, DNA fragmentation, adverse events	N ‐ sperm parameters, no SD given N ‐ DNA fragmentation, no SD given Y ‐ adverse events	No statistical difference between the two groups (varicocelectomy with L‐carnitine or with no adjuvant therapy).	‐ Addition of 750 mg of L‐carnitine orally daily to standard inguinal varicocelectomy does not add any extra benefit in terms of improvement in semen analysis parameters or DNA damage
Poveda 2013	Multiple arm, placebo Infertile men N = 60 Conference abstract	Sperm parameters	Sperm parameters	N ‐ sperm parameters, data not available	L‐carnitine significantly improves sperm concentration, Spermotrend and Maca improve sperm motility.	+ Sperm concentration with L‐carnitine and motility with combined antioxidant Spermotrend
Pryor 1978	Cross‐over, placebo Men with severe oligozoospermia N = 64	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	N ‐ sperm parameters, bar graph of % patients showing an increase in motility and density N ‐ pregnancy rate, not clear if clinical. Included in biochemical analysis Table 1	Arginine was no more effective than placebo for sperm parameters and biochemical pregnancy rates	‐ There was no difference in the conception rates of the wives or changes in the quality of the semen during each period of treatment
Raigani 2014	Multiple arm, placebo Men with proven male factor infertility N = 83	Sperm parameters, DNA fragmentation	Sperm parameters, DNA fragmentation	Y ‐ sperm parameters ( median+IQR converted to mean+ SD) Y ‐ DNA fragmentation	Sperm concentration, DNA fragmentation not significantly improved in either group	‐ Zinc sulphate and folic acid supplementation did not ameliorate sperm quality in infertile men with severely compromised sperm parameters, OAT
Rolf 1999	Asthenospermia N = 33	Sperm parameters, pregnancy rates, adverse events	Sperm parameters, pregnancy rate, adverse events	Y ‐ sperm parameters N ‐ pregnancy rate, not stated as clinical pregnancy N ‐ adverse events, not clear which side effects aimed for	No adverse events or pregnancies in either group	‐ Overall no difference vitamin E + C versus placebo
Saeed Alkumait 2020	Multiple arm, placebo Infertile men N = 151	Sperm parameters	Sperm parameters	N ‐ sperm parameters, data provided as percentage improvement, Analysis 1.16; Analysis 1.22	Significantly higher percentage improvement of progressive sperm motility and concentration with glutathione or CoQ10 compared to placebo	+ Both glutathione and CoQ10 are effective treatment options for improving sperm motility, morphology and concentration
Safarinejad 2009	Multiple arm, placebo Men with idiopathic OAT N = 468	Sperm parameters, adverse events	Sperm parameters, adverse events	Y ‐ sperm parameters N ‐ adverse events, not specified which adverse events aimed for	All semen parameters significantly improved with selenium and N‐acetyl‐cysteine treatment. Administering selenium plus N‐acetyl‐cysteine resulted in additive beneficial effects. Zero adverse events	+ Supplemental selenium and N‐acetyl‐cysteine improve semen quality. Zero adverse events
Safarinejad 2009a	Parallel, placebo Men with idiopathic OAT N = 212	Sperm parameters, adverse events	Sperm parameters, adverse events	Y ‐ sperm parameters N ‐ adverse events, not specified which adverse events aimed for	Significant improvement in sperm density and motility after coenzyme Q10 therapy. Zero adverse events	+ Coenzyme Q10 supplementation resulted in a statistically significant improvement in certain sperm parameters. Zero adverse events
Safarinejad 2011b	Parallel, placebo Men with idiopathic OAT N = 238	Sperm parameters, adverse events	Sperm parameters, adverse events	Y ‐ sperm parameters N ‐ adverse events, not clear how many patients had gastrointestinal upsets in total	Significant improvement of sperm concentration and progressive motility after omega‐3 fatty acids therapy. Significantly more adverse events (gastrointestinal and pruritus) in the omega‐3 group	+ These findings suggest a protective effect of omega‐3 fatty acid intake in idiopathic infertile men. More adverse events in omega‐3 group
Safarinejad 2012	Parallel, placebo Infertile men N = 228	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Sperm parameters improved significantly after coenzyme Q10	+ Coenzyme Q10 was significantly effective in men with unexplained oligoasthenoteratozoospermia for improving sperm density, sperm motility and sperm morphology
Schisterman 2020	Parallel, placebo Male partner of couples planning infertility treatment. Data from subfertile men used. N = 2370	Sperm parameters, DNA fragmentation, clinical pregnancy, live births, adverse events	Sperm parameters, DNA fragmentation, clinical pregnancy, live births, adverse events	Y ‐ sperm parameters Y ‐ DNA fragmentation N ‐ pregnancy, clinical; N ‐ live births N ‐ adverse events Data not provided for male factor infertility subgroup	No significant difference in sperm parameters between folic acid + zinc and placebo. No results on clinical outcomes in male factor subgroup	‐ Folic acid and zinc did not significantly improve semen quality. The findings also were similar when restricted to men with known male factor infertility or poor semen quality at baseline
Scott 1998	Multiple arm, placebo Men with subfertility and low sperm motility N = 69	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	Y ‐ sperm parameters N ‐ pregnancy rate, not usable due to pooling of data in the two intervention groups Table 1	Sperm motility increased in both selenium‐treated groups, only significant if both treatment groups were combined. Sperm density unaffected	± Selenium supplementation in subfertile men with low selenium status can improve sperm motility and the chance of successful conception. However, not all patients responded; 56% showed a positive response to treatment
Sharifzadeh 2016	Parallel, placebo Idiopathic subfertile men N = 114	Sperm parameters, adverse events	Sperm parameters, adverse events	Y‐ sperm parameters Y ‐ adverse events	Significant increase in concentration in zinc group	+ Normal sperm percentage and total sperm concentration increased after zinc sulphate treatment
Sigman 2006	Parallel, placebo Infertile men with low sperm motility N = 26	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	Y ‐ sperm parameters N ‐ pregnancy rate, biochemical Table 1	No statistically significant or clinically significant increase in motility or total motile sperm counts between baseline, 12 weeks, or 24 weeks in the carnitine or placebo arms.	‐ Carnitine supplementation demonstrated no clinically or statistically significant effect on sperm motility or total motile sperm counts. No difference in pregnancy rate
Sivkov 2011	Parallel, placebo Men with chronic prostatitis and infertility N = 30	Sperm parameters	Sperm parameters	N ‐ sperm parameters, no SD given Analysis 1.10	One‐month course of therapy produced no side effects, had a positive effect on low fertility of ejaculate.	+ Selenium + zinc improve
Sofikitis 2016	Multiple arm, no treatment, Avanafil excluded Oligoasthenospermic infertile men N = 39 Abstract only	Sperm parameters	Sperm parameters	N ‐ sperm parameters, no data available	No significant difference in L‐carnitine group regarding sperm parameters	‐ No direct conclusion made about L‐carnitine. From result section concluded: no impact on sperm parameters after use of L‐carnitine
Steiner 2020	Parallel, placebo Men with one abnormal semen parameter N = 171	Sperm parameters, DNA fragmentation, clinical pregnancy, live birth	Sperm parameters, DNA fragmentation, clinical pregnancy, live birth	Y ‐ sperm parameters Y ‐ DNA fragmentation (data shared by authors after requested via e‐mail) Y ‐ pregnancy, clinical Y ‐ live birth	No difference in sperm motility, DNA fragmentation, pregnancy rate and live birth rate between combined antioxidants and placebo	‐ No improvement in semen parameters in infertile males. This study suggests that combination antioxidants does not improve pregnancy or live birth rates
Stenqvist 2018	Parallel, placebo Infertile men with DNA fragmentation ≥ 25% N = 79	Sperm parameters, DNA fragmentation, pregnancy rate, adverse events	Sperm parameters, DNA fragmentation, pregnancy rate, adverse events	Y ‐ sperm parameters Y ‐ DNA fragmentation N ‐ pregnancy rate, biochemical Table 1 Y ‐ adverse events	No statistically significant difference between the antioxidant and placebo group was seen for semen parameters including DNA fragmentation	‐ Six months treatment with combined antioxidants had no effect on sperm parameters including DNA fragmentation
Suleiman 1996	Parallel, placebo Asthenospermic men N = 110	Sperm parameters	Sperm parameters, pregnancy rate, live birth, miscarriage	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical Y ‐ live birth Y ‐ adverse events: miscarriage	Vitamin E significantly decreased the MDA concentration in spermatozoa and improved sperm motility. Significant increase pregnancy/live birth rate	+ Vitamin E increases sperm motility, pregnancy rate and live birth rate compared to placebo
Sun 2018	Parallel, head‐to‐head Infertile men with low acrosin activity N = 232	Sperm parameters	Sperm parameters	Y ‐ sperm parameters	Significant increase of progressive sperm otility in men treated with L‐carnitine compared to vitamin E	+ L‐carnitine can effectively elevate sperm acrosin activity in male infertility patients, particularly in those with asthenozoospermia
Tremellen 2007	Parallel, placebo Male factor infertility N = 60	Pregnancy rate, adverse events	Pregnancy rate, adverse events, live birth provided by author	Y ‐ pregnancy rate, clinical Y ‐ live birth Y ‐ adverse events	Antioxidant group recorded a statistically significant improvement in viable pregnancy rate. Side‐effects on the Menevit antioxidant were rare (8%) and mild in nature.	+ Menevit antioxidant appears to be a useful ancillary treatment that significantly improves pregnancy rates in couples undergoing IVF‐ICSI treatment. Side‐effects on the Menevit antioxidant were rare (8%) and mild in nature.
Tsounapi 2018	Multiple arm, head‐to‐head Profertil + avanafil and avafanil only groups not used Idiopathic OAT N = 217	Sperm parameters, DNA fragmentation, pregnancy rate	Sperm parameters, DNA fragmentation, pregnancy rate	N ‐ sperm parameters N ‐ DNA fragmentation Not reported in how many patients sperm outcomes were assessed Y ‐ pregnancy rate, clinical	Significantly higher total and progressive sperm motility in Profertil group compared to L‐carnitine and no treatment. No difference in pregnancy rate	+ Profertil or Profertil combined with avanafil or or avanafil alone improves sperm membrane permeability with an improvement in sperm motility
Vinogradov 2019	Parallel, placebo Infertile men with at least one abnormal sperm parameter N = 109	Sperm parameters, DNA fragmentation	Sperm parameters, DNA fragmentation	N ‐ sperm parameters N ‐ DNA fragmenation Only results after cryotolerance test provided	No statistical differences between results after Brudy plus (combined antioxidant) and placebo	+/‐ No conclusions on outcomes of interest. Brudy Plus increases cryotolerance, promotes the normal formation of the genetic material and reduces the frequency of ultrastructural sperm disorders.
Wang 2010	Head‐to‐head Infertile men with asthenozoospermia N = 135	Sperm parameters, pregnancy rate, adverse events	Sperm parameters, pregnancy rate, adverse events	Y ‐ sperm parameters N ‐ pregnancy rate, not clear if clinical Table 1 N ‐ adverse events, zero found, however not clear which they aimed for	Significant increase in L‐carnitine + vitamin E group for sperm motility, no difference for sperm density and morphology. Pregnancy rate significantly higher in L‐carnitine + vitamin E group	+ L‐carnitine (+vitamin E) significantly improves sperm motility and pregnancy rate
Wong 2002	Multiple arm, placebo Fertile and subfertile men N = 103	Sperm parameters	Sperm parameters	Y ‐ sperm parameters (median+IQR converted to mean+ SD)	Subfertile men demonstrated a significant 74% increase in total normal sperm count and a minor increase of 4% abnormal spermatozoa	+ Total normal sperm count increases after combined zinc sulphate and folic acid treatment in both subfertile and fertile men
Zalata 1998	Head‐to‐head, pilot Men attending andrology clinic N = 22 Conference abstract	Sperm parameters	Sperm parameters	N ‐ sperm parameters, only before and after median data given	No significant difference in sperm parameters after treatment (acetyl‐cysteine or DHA). DNA damage measured by 8‐OHdG (fmol/ug) was significantly decreased after supplementation	‐ No improvement of sperm parameters
Zavaczki 2003	Parallel, placebo Men with idiopathic infertility N = 20	Sperm parameters, clinical pregnancy, adverse events	Sperm parameters, clinical pregnancy, adverse events	Y ‐ sperm parameters Y ‐ pregnancy rate, clinical Y ‐ adverse events	No significant changes in sperm characteristics were detected	‐ Magnesium neither leads to a significant improvement of sperm variables nor does it increase the pregnancy rates
Zhou 2016	Parallel, head‐to‐head Idiopathic asthenozoospermia N = 120	Sperm parameters, pregnancy rate	Sperm parameters, pregnancy rate	Y ‐ sperm parameters N ‐ pregnancy rate, definition unclear Table 1 Y ‐ adverse events	Significant increase of total and progressive sperm motility in vitamin E and vitamin E + compound amino acids group. Greater increase in compound amino acids group. 5.7% pregnancy in combined group, 2% in vitamin E group. No adverse events	+ Compound amino acid combined with vitamin E can safely and effectively improve sperm motility in idiopathic asthenospermia patients.
DHA: docosahexaenoic acid; IUI: intrauterine insemination; NAC: N‐acetylcysteine; OAT:oligoasthenoteratozoospermia; ROS: reactive oxygen species

Table 2. Outcomes and conclusions from all included studies

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Comparison 1. Antioxidant(s) versus placebo or no treatment

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
1.1 Live birth; type of antioxidant Show forest plot	12	1283	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.43 [1.07, 1.91]

1.1.1 Astaxanthin + Vitamin E	1	36	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.63 [0.34, 7.69]
1.1.2 Carnitines	1	60	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.00 [0.24, 4.25]
1.1.3 Coenzyme Q10	1	60	Peto Odds Ratio (Peto, Fixed, 95% CI)	2.16 [0.53, 8.82]
1.1.4 Vitamin D + Calcium	1	330	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.03 [0.59, 1.80]
1.1.5 Vitamin E	2	140	Peto Odds Ratio (Peto, Fixed, 95% CI)	8.51 [2.36, 30.70]
1.1.6 Zinc	1	100	Peto Odds Ratio (Peto, Fixed, 95% CI)	3.74 [1.02, 13.74]
1.1.7 Combined antioxidants	5	557	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.28 [0.86, 1.91]
1.2 Live birth; IVF/ICSI Show forest plot	5	372	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.63 [1.01, 2.61]

1.3 Clinical pregnancy; type of antioxidant Show forest plot	20	1706	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.89 [1.45, 2.47]

1.3.1 Astaxanthin + Vitamin E	1	36	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.32 [0.35, 4.96]
1.3.2 Carnitines	2	125	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.17 [0.30, 4.59]
1.3.3 Coenzyme Q10	1	60	Peto Odds Ratio (Peto, Fixed, 95% CI)	2.16 [0.53, 8.82]
1.3.4 Folic acid	1	53	Peto Odds Ratio (Peto, Fixed, 95% CI)	Not estimable
1.3.5 Magnesium	1	26	Peto Odds Ratio (Peto, Fixed, 95% CI)	8.73 [0.17, 445.08]
1.3.6 N‐acetylcysteine (NAC)	2	100	Peto Odds Ratio (Peto, Fixed, 95% CI)	2.00 [0.71, 5.63]
1.3.7 Vitamin E	2	117	Peto Odds Ratio (Peto, Fixed, 95% CI)	6.71 [1.98, 22.69]
1.3.8 Zinc	2	153	Peto Odds Ratio (Peto, Fixed, 95% CI)	4.43 [1.39, 14.14]
1.3.9 Zinc + Folic acid	1	53	Peto Odds Ratio (Peto, Fixed, 95% CI)	3.86 [0.15, 99.84]
1.3.10 Combined antioxidants	10	983	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.67 [1.22, 2.28]
1.4 Clinical pregnancy; IVF/ICSI Show forest plot	6	452	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.73 [1.15, 2.61]

1.5 Adverse events Show forest plot	21		Peto Odds Ratio (Peto, Fixed, 95% CI)	Subtotals only

1.5.1 Miscarriage	6	664	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.46 [0.75, 2.83]
1.5.2 Ectopic pregnancy	2	260	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.59 [0.16, 16.01]
1.5.3 Stillbirth	1	200	Peto Odds Ratio (Peto, Fixed, 95% CI)	0.14 [0.00, 6.82]
1.5.4 Gastrointestinal	16	1355	Peto Odds Ratio (Peto, Fixed, 95% CI)	2.70 [1.46, 4.99]
1.5.5 Euphoria	1	86	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.21 [0.16, 9.01]
1.5.6 Headache	1	171	Peto Odds Ratio (Peto, Fixed, 95% CI)	2.32 [0.95, 5.67]
1.5.7 Upper respiratory infection	1	171	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.01 [0.25, 4.17]
1.5.8 Nasofaryngitis	1	171	Peto Odds Ratio (Peto, Fixed, 95% CI)	0.57 [0.17, 1.92]
1.6 Sperm DNA fragmentation at 3 months or less; type of antioxidant Show forest plot	12		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

1.6.1 Astaxanthin + Vitamin E	1	72	Mean Difference (IV, Fixed, 95% CI)	1.40 [‐6.64, 9.44]
1.6.2 Folic acid	1	38	Mean Difference (IV, Fixed, 95% CI)	‐5.80 [‐13.40, 1.80]
1.6.3 Folic acid + Zinc	1	39	Mean Difference (IV, Fixed, 95% CI)	‐1.20 [‐9.36, 6.96]
1.6.4 N‐acetylcysteine (NAC)	1	35	Mean Difference (IV, Fixed, 95% CI)	3.90 [‐0.42, 8.22]
1.6.5 PUFAs	3	137	Mean Difference (IV, Fixed, 95% CI)	‐1.16 [‐4.00, 1.68]
1.6.6 Vitamin C + Vitamin E	1	64	Mean Difference (IV, Fixed, 95% CI)	‐13.80 [‐17.50, ‐10.10]
1.6.7 Zinc	1	42	Mean Difference (IV, Fixed, 95% CI)	1.30 [‐8.62, 11.22]
1.6.8 Combined antioxidants	5	569	Mean Difference (IV, Fixed, 95% CI)	‐0.52 [‐2.00, 0.96]
1.7 Sperm DNA fragmentation at 6 months; type of antioxidant Show forest plot	4		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

1.7.1 Combined antioxidants	3	320	Mean Difference (IV, Fixed, 95% CI)	‐4.57 [‐6.49, ‐2.66]
1.7.2 Zinc + Folic acid	1	853	Mean Difference (IV, Fixed, 95% CI)	3.00 [0.02, 5.98]
1.8 Sperm DNA fragmentation (data not suitable for meta‐analysis) Show forest plot	1		Other data	No numeric data

1.8.1 Folic acid	1		Other data	No numeric data
1.9 Total sperm motility at 3 months or less; type of antioxidant Show forest plot	25		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

1.9.1 Astaxanthin + Vitamin E	1	72	Mean Difference (IV, Fixed, 95% CI)	‐5.20 [‐11.56, 1.16]
1.9.2 Carnitines	5	244	Mean Difference (IV, Fixed, 95% CI)	31.28 [31.19, 31.37]
1.9.3 Carotenoids	1	36	Mean Difference (IV, Fixed, 95% CI)	3.50 [‐6.95, 13.95]
1.9.4 Coenzyme Q10	1	47	Mean Difference (IV, Fixed, 95% CI)	3.61 [‐6.13, 13.35]
1.9.5 Folic acid	2	89	Mean Difference (IV, Fixed, 95% CI)	4.56 [‐5.63, 14.74]
1.9.6 Magnesium	1	20	Mean Difference (IV, Fixed, 95% CI)	14.50 [‐6.01, 35.01]
1.9.7 N‐acetylcysteine (NAC)	1	35	Mean Difference (IV, Fixed, 95% CI)	14.60 [0.32, 28.88]
1.9.8 PUFAs	3	105	Mean Difference (IV, Fixed, 95% CI)	‐2.40 [‐9.89, 5.09]
1.9.9 Selenium	1	34	Mean Difference (IV, Fixed, 95% CI)	14.90 [1.14, 28.66]
1.9.10 Vitamin C + Vitamin E	1	64	Mean Difference (IV, Fixed, 95% CI)	2.90 [‐7.76, 13.56]
1.9.11 Vitamin E	1	45	Mean Difference (IV, Fixed, 95% CI)	18.90 [4.90, 32.90]
1.9.12 Zinc	3	118	Mean Difference (IV, Fixed, 95% CI)	12.85 [5.40, 20.29]
1.9.13 Zinc + Folic acid	2	93	Mean Difference (IV, Fixed, 95% CI)	5.26 [‐3.64, 14.16]
1.9.14 Zinc + Vitamin E	1	20	Mean Difference (IV, Fixed, 95% CI)	26.00 [12.85, 39.15]
1.9.15 Zinc + Vitamin E + Vitamin C	1	22	Mean Difference (IV, Fixed, 95% CI)	26.00 [12.62, 39.38]
1.9.16 Combined antioxidants	7	684	Mean Difference (IV, Fixed, 95% CI)	12.71 [11.33, 14.08]
1.10 Total sperm motility at 3 months or less (data not suitable for meta analysis) Show forest plot	2		Other data	No numeric data

1.10.1 Vitamin E	1		Other data	No numeric data
1.10.2 Combined antioxidants	1		Other data	No numeric data
1.11 Total sperm motility at 6 months; type of antioxidant Show forest plot	17		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

1.11.1 Carnitines	3	136	Mean Difference (IV, Fixed, 95% CI)	10.09 [5.99, 14.19]
1.11.2 Coenzyme Q10	3	479	Mean Difference (IV, Fixed, 95% CI)	7.28 [6.85, 7.72]
1.11.3 Folic acid	2	98	Mean Difference (IV, Fixed, 95% CI)	0.16 [‐6.96, 7.29]
1.11.4 N‐acetylcysteine (NAC)	1	211	Mean Difference (IV, Fixed, 95% CI)	1.90 [1.20, 2.60]
1.11.5 Selenium	1	211	Mean Difference (IV, Fixed, 95% CI)	3.20 [2.50, 3.90]
1.11.6 Selenium + N‐acetylcysteine (NAC)	1	210	Mean Difference (IV, Fixed, 95% CI)	6.30 [5.60, 7.00]
1.11.7 Vitamin D + Calcium	1	260	Mean Difference (IV, Fixed, 95% CI)	‐4.00 [‐9.57, 1.57]
1.11.8 Vitamin E	2	132	Mean Difference (IV, Fixed, 95% CI)	11.60 [6.18, 17.02]
1.11.9 Zinc	2	105	Mean Difference (IV, Fixed, 95% CI)	0.00 [‐6.95, 6.95]
1.11.10 Zinc + Folic acid	3	956	Mean Difference (IV, Fixed, 95% CI)	0.24 [‐2.54, 3.02]
1.11.11 Combined antioxidants	4	394	Mean Difference (IV, Fixed, 95% CI)	6.76 [4.77, 8.75]
1.12 Total sperm motility at 9 months or more; type of antioxidant Show forest plot	5		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

1.12.1 Carnitines	1	59	Mean Difference (IV, Fixed, 95% CI)	8.54 [3.01, 14.07]
1.12.2 Coenzyme Q10	3	479	Mean Difference (IV, Fixed, 95% CI)	3.33 [2.91, 3.76]
1.12.3 Vitamin E	1	45	Mean Difference (IV, Fixed, 95% CI)	2.20 [‐8.48, 12.88]
1.13 Total sperm motility over time Show forest plot	36		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

1.13.1 Total sperm motility at 3 months or less	25	1638	Mean Difference (IV, Fixed, 95% CI)	31.17 [31.07, 31.26]
1.13.2 Total sperm motility at 6 months	17	2880	Mean Difference (IV, Fixed, 95% CI)	5.77 [5.45, 6.10]
1.13.3 Total sperm motility at 9 months or more	5	583	Mean Difference (IV, Fixed, 95% CI)	3.36 [2.94, 3.78]
1.14 Progressive sperm motility at 3 months or less; type of antioxidant Show forest plot	28		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

1.14.1 Astaxanthin + Vitamin E	1	72	Mean Difference (IV, Fixed, 95% CI)	‐5.10 [‐11.46, 1.26]
1.14.2 Carnitines	4	285	Mean Difference (IV, Fixed, 95% CI)	20.92 [20.52, 21.32]
1.14.3 Carotenoids	1	36	Mean Difference (IV, Fixed, 95% CI)	‐0.20 [‐7.27, 6.87]
1.14.4 Coenzyme Q10	1	47	Mean Difference (IV, Fixed, 95% CI)	4.60 [‐3.54, 12.74]
1.14.5 Folic acid	2	81	Mean Difference (IV, Fixed, 95% CI)	5.08 [‐4.00, 14.16]
1.14.6 N‐acetylcysteine (NAC)	1	60	Mean Difference (IV, Fixed, 95% CI)	3.80 [‐1.03, 8.63]
1.14.7 PUFAs	4	181	Mean Difference (IV, Fixed, 95% CI)	1.53 [0.32, 2.74]
1.14.8 Vitamin C	2	145	Mean Difference (IV, Fixed, 95% CI)	10.95 [4.10, 17.80]
1.14.9 Vitamin C + Vitamin E	1	31	Mean Difference (IV, Fixed, 95% CI)	0.20 [‐9.77, 10.17]
1.14.10 Vitamin D	1	62	Mean Difference (IV, Fixed, 95% CI)	‐0.84 [‐7.65, 5.97]
1.14.11 Zinc	2	157	Mean Difference (IV, Fixed, 95% CI)	1.14 [‐3.37, 5.64]
1.14.12 Zinc + Folic acid	1	54	Mean Difference (IV, Fixed, 95% CI)	3.80 [‐13.66, 21.26]
1.14.13 Combined antioxidants	9	993	Mean Difference (IV, Fixed, 95% CI)	11.16 [9.91, 12.41]
1.15 Progressive sperm motility at 6 months; type of antioxidant Show forest plot	12		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

1.15.1 Carnitines	2	145	Mean Difference (IV, Fixed, 95% CI)	11.66 [8.68, 14.64]
1.15.2 Coenzyme Q10	1	60	Mean Difference (IV, Fixed, 95% CI)	5.00 [2.13, 7.87]
1.15.3 Folic acid	2	81	Mean Difference (IV, Fixed, 95% CI)	‐1.77 [‐10.21, 6.67]
1.15.4 PUFAs	1	227	Mean Difference (IV, Fixed, 95% CI)	8.80 [8.11, 9.49]
1.15.5 Vitamin D + Calcium	1	260	Mean Difference (IV, Fixed, 95% CI)	‐4.00 [‐9.59, 1.59]
1.15.6 Zinc	1	57	Mean Difference (IV, Fixed, 95% CI)	2.00 [‐13.56, 17.56]
1.15.7 Zinc + Folic acid	1	54	Mean Difference (IV, Fixed, 95% CI)	2.70 [‐14.58, 19.98]
1.15.8 Combined antioxidants	5	470	Mean Difference (IV, Fixed, 95% CI)	4.01 [2.05, 5.96]
1.16 Progressive sperm motility at 6 months (data not suitable for meta analysis) Show forest plot	1		Other data	No numeric data

1.16.1 Coenzyme Q10	1		Other data	No numeric data
1.16.2 Glutathione	1		Other data	No numeric data
1.17 Progressive sperm motility at 9 months or more; type of antioxidant Show forest plot	2		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

1.17.1 Carnitines	1	59	Mean Difference (IV, Fixed, 95% CI)	7.77 [2.68, 12.87]
1.17.2 Coenzyme Q10	1	60	Mean Difference (IV, Fixed, 95% CI)	‐0.90 [‐2.68, 0.88]
1.18 Progressive sperm motility over time Show forest plot	32		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

1.18.1 Progressive sperm motility at 3 months or less	27	2054	Mean Difference (IV, Fixed, 95% CI)	17.98 [17.62, 18.34]
1.18.2 Progressive sperm motility at 6 months	12	1304	Mean Difference (IV, Fixed, 95% CI)	8.05 [7.43, 8.66]
1.18.3 Progressive sperm motility at 9 months or more	2	119	Mean Difference (IV, Fixed, 95% CI)	0.04 [‐1.64, 1.72]
1.19 Sperm concentration at 3 months or less; type of antioxidant Show forest plot	36		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

1.19.1 Astaxathin + Vitamin E	1	72	Mean Difference (IV, Fixed, 95% CI)	‐1.00 [‐6.79, 4.79]
1.19.2 Carnitines	5	333	Mean Difference (IV, Fixed, 95% CI)	8.71 [8.09, 9.34]
1.19.3 Carotenoids	1	36	Mean Difference (IV, Fixed, 95% CI)	6.30 [0.62, 11.98]
1.19.4 Coenzyme Q10	1	47	Mean Difference (IV, Fixed, 95% CI)	‐0.10 [‐12.37, 12.17]
1.19.5 Folic acid	3	119	Mean Difference (IV, Fixed, 95% CI)	3.72 [‐4.01, 11.44]
1.19.6 Magnesium	1	20	Mean Difference (IV, Fixed, 95% CI)	5.20 [‐2.61, 13.01]
1.19.7 N‐acetylcysteine (NAC)	2	95	Mean Difference (IV, Fixed, 95% CI)	4.59 [‐0.27, 9.46]
1.19.8 PUFAs	5	209	Mean Difference (IV, Fixed, 95% CI)	3.42 [1.69, 5.15]
1.19.9 Selenium	1	34	Mean Difference (IV, Fixed, 95% CI)	21.20 [‐4.90, 47.30]
1.19.10 Vitamin C	1	115	Mean Difference (IV, Fixed, 95% CI)	9.70 [0.09, 19.31]
1.19.11 Vitamin C + Vitamin E	2	95	Mean Difference (IV, Fixed, 95% CI)	1.31 [‐6.58, 9.20]
1.19.12 Vitamin D	1	62	Mean Difference (IV, Fixed, 95% CI)	‐2.12 [‐8.85, 4.61]
1.19.13 Vitamin E	1	45	Mean Difference (IV, Fixed, 95% CI)	18.90 [3.92, 33.88]
1.19.14 Zinc	3	199	Mean Difference (IV, Fixed, 95% CI)	6.74 [2.81, 10.68]
1.19.15 Zinc + Folic acid	2	93	Mean Difference (IV, Fixed, 95% CI)	0.48 [‐6.79, 7.75]
1.19.16 Combined antioxidants	11	1165	Mean Difference (IV, Fixed, 95% CI)	0.53 [‐0.33, 1.40]
1.20 Sperm concentration at 3 months or less (data not suitable for meta analysis) Show forest plot	2		Other data	No numeric data

1.20.1 Carnitines	1		Other data	No numeric data
1.20.2 Vitamin E	1		Other data	No numeric data
1.21 Sperm concentration at 6 months; type of antioxidant Show forest plot	20		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

1.21.1 Carnitines	3	201	Mean Difference (IV, Fixed, 95% CI)	7.42 [4.97, 9.87]
1.21.2 Coenzyme Q10	3	479	Mean Difference (IV, Fixed, 95% CI)	8.80 [7.95, 9.64]
1.21.3 Folic acid	3	128	Mean Difference (IV, Fixed, 95% CI)	17.39 [11.09, 23.69]
1.21.4 N‐acetylcysteine (NAC)	1	211	Mean Difference (IV, Fixed, 95% CI)	3.30 [1.80, 4.80]
1.21.5 PUFAs	1	227	Mean Difference (IV, Fixed, 95% CI)	12.50 [11.39, 13.61]
1.21.6 Selenium	1	211	Mean Difference (IV, Fixed, 95% CI)	4.10 [2.45, 5.75]
1.21.7 Selenium + N‐acetylcysteine (NAC)	1	210	Mean Difference (IV, Fixed, 95% CI)	8.60 [6.89, 10.31]
1.21.8 Vitamin D + Calcium	1	269	Mean Difference (IV, Fixed, 95% CI)	‐2.50 [‐8.18, 3.18]
1.21.9 Vitamin E	1	45	Mean Difference (IV, Fixed, 95% CI)	5.90 [‐10.83, 22.63]
1.21.10 Zinc	2	105	Mean Difference (IV, Fixed, 95% CI)	5.51 [‐4.00, 15.01]
1.21.11 Zinc + Folic acid	3	956	Mean Difference (IV, Fixed, 95% CI)	1.44 [‐6.70, 9.58]
1.21.12 Combined antioxidants	6	534	Mean Difference (IV, Fixed, 95% CI)	3.16 [2.28, 4.05]
1.22 Sperm concentration at 6 months (data not suitable for meta analysis) Show forest plot	1		Other data	No numeric data

1.22.1 Glutathione	1		Other data	No numeric data
1.22.2 Coenzyme Q10	1		Other data	No numeric data
1.23 Sperm concentration at 9 months or more; type of antioxidant Show forest plot	5		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

1.23.1 Carnitines	1	59	Mean Difference (IV, Fixed, 95% CI)	4.17 [‐1.71, 10.06]
1.23.2 Coenzyme Q10	3	479	Mean Difference (IV, Fixed, 95% CI)	3.93 [3.19, 4.67]
1.23.3 Vitamin E	1	45	Mean Difference (IV, Fixed, 95% CI)	11.40 [‐2.56, 25.36]
1.24 Sperm concentration over time Show forest plot	46		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

1.24.1 Sperm concentration at 3 months or less	35	2535	Mean Difference (IV, Fixed, 95% CI)	5.49 [5.02, 5.96]
1.24.2 Sperm concentration 6 months	19	2995	Mean Difference (IV, Fixed, 95% CI)	7.21 [6.73, 7.70]
1.24.3 Sperm concentration at 9 months or more	5	583	Mean Difference (IV, Fixed, 95% CI)	3.95 [3.22, 4.69]

Comparison 1. Antioxidant(s) versus placebo or no treatment

Ir a la tabla de la revisión

Comparison 2. Head‐to‐head antioxidant(s)

Outcome or subgroup title	No. of studies	No. of participants	Statistical method	Effect size
2.1 Live birth; type of antioxidant Show forest plot	1		Peto Odds Ratio (Peto, Fixed, 95% CI)	Subtotals only

2.1.1 L‐carnitine vs L‐acetyl carnitine	1	30	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.00 [0.13, 7.92]
2.1.2 L‐carnitine vs L‐carnitine + L‐acetyl carnitine	1	30	Peto Odds Ratio (Peto, Fixed, 95% CI)	0.34 [0.06, 1.79]
2.1.3 L‐acetyl carnitine vs L‐carnitine + L‐acetyl carnitine	1	30	Peto Odds Ratio (Peto, Fixed, 95% CI)	0.34 [0.06, 1.79]
2.2 Clinical pregnancy; type of antioxidant Show forest plot	4		Peto Odds Ratio (Peto, Fixed, 95% CI)	Subtotals only

2.2.1 L‐carnitine vs L‐acetyl carnitine	1	30	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.00 [0.13, 7.92]
2.2.2 L‐carnitine vs L‐carnitine + L‐acetyl carnitine	1	30	Peto Odds Ratio (Peto, Fixed, 95% CI)	0.34 [0.06, 1.79]
2.2.3 L‐acetyl carnitine vs L‐carnitine + L‐acetyl carnitine	1	30	Peto Odds Ratio (Peto, Fixed, 95% CI)	0.34 [0.06, 1.79]
2.2.4 L‐carnitine vs Coenzyme Q10	1	156	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.48 [0.54, 4.05]
2.2.5 L‐carnitine vs L‐carnitine + Coenzyme Q10	1	156	Peto Odds Ratio (Peto, Fixed, 95% CI)	0.62 [0.27, 1.46]
2.2.6 Coenzyme Q10 vs L‐carnitine + Coenzyme Q10	1	156	Peto Odds Ratio (Peto, Fixed, 95% CI)	0.43 [0.18, 1.06]
2.2.7 Vitamin D + Calcium vs Vitamin E + Vitamin C	1	86	Peto Odds Ratio (Peto, Fixed, 95% CI)	5.13 [1.21, 21.79]
2.2.8 Combined antioxidants vs L‐carnitine	1	89	Peto Odds Ratio (Peto, Fixed, 95% CI)	1.93 [0.20, 19.08]
2.3 Sperm DNA fragmentation; type of antioxidant Show forest plot	1		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

2.3.1 L‐carnitine vs Coenzyme Q10	1	125	Mean Difference (IV, Fixed, 95% CI)	‐0.80 [‐2.22, 0.62]
2.3.2 L‐carnitine vs L‐carnitine + Coenzyme Q10	1	125	Mean Difference (IV, Fixed, 95% CI)	0.40 [‐1.14, 1.94]
2.3.3 Coenzyme Q10 vs L‐carnitine + Coenzyme Q10	1	126	Mean Difference (IV, Fixed, 95% CI)	1.20 [‐0.25, 2.65]
2.3.4 L‐carnitine vs Vitamin B1	1	136	Mean Difference (IV, Fixed, 95% CI)	‐1.50 [‐3.22, 0.22]
2.3.5 Coenzyme Q10 vs Vitamin B1	1	137	Mean Difference (IV, Fixed, 95% CI)	‐0.70 [‐2.34, 0.94]
2.3.6 Vitamin B1 vs L‐carnitine + Coenzyme Q10	1	137	Mean Difference (IV, Fixed, 95% CI)	1.90 [0.16, 3.64]
2.4 Total sperm motility at 3 months or less; type of antioxidant Show forest plot	12		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

2.4.1 Coenzyme Q10 200 mg vs Coenzyme Q10 400 mg	1	65	Mean Difference (IV, Fixed, 95% CI)	‐4.86 [‐10.60, 0.88]
2.4.2 Docosahexaenoic acid (DHA) 400 mg vs Docosahexaenoic acid 800 mg	1	19	Mean Difference (IV, Fixed, 95% CI)	7.40 [‐11.35, 26.15]
2.4.3 DHA vs DHA + Vitamin E	1	90	Mean Difference (IV, Fixed, 95% CI)	‐3.77 [‐5.42, ‐2.12]
2.4.4 DHA versus Vitamin E	1	90	Mean Difference (IV, Fixed, 95% CI)	‐1.60 [‐3.30, 0.10]
2.4.5 DHA + Vitamin E vs Vitamin E	1	90	Mean Difference (IV, Fixed, 95% CI)	2.17 [0.54, 3.80]
2.4.6 Ethylcysteine vs Vitamin E	1	10	Mean Difference (IV, Fixed, 95% CI)	‐1.90 [‐41.97, 38.17]
2.4.7 L‐acetyl carnitine + L‐carnitine vs Vitamin E + Vitamin C	1	138	Mean Difference (IV, Fixed, 95% CI)	23.10 [20.14, 26.06]
2.4.8 L‐carnitine vs L‐acetyl carnitine	1	30	Mean Difference (IV, Fixed, 95% CI)	3.40 [‐3.73, 10.53]
2.4.9 L‐carnitine vs L‐carnitine + L‐acetyl carnitine	1	30	Mean Difference (IV, Fixed, 95% CI)	4.80 [‐1.76, 11.36]
2.4.10 L‐acetyl carnitine vs L‐carnitine + L‐acetyl carnitine	1	30	Mean Difference (IV, Fixed, 95% CI)	1.40 [‐6.42, 9.22]
2.4.11 Selenium vs combined antioxidants	1	46	Mean Difference (IV, Fixed, 95% CI)	3.20 [‐10.13, 16.53]
2.4.12 Vitamin C 200mg vs Vitamin C 1000mg	1	20	Mean Difference (IV, Fixed, 95% CI)	‐43.00 [‐67.10, ‐18.90]
2.4.13 Vitamin E + 'Compound amino acids' vs Vitamin E	1	120	Mean Difference (IV, Fixed, 95% CI)	11.90 [8.71, 15.09]
2.4.14 Zinc vs Folic acid	2	124	Mean Difference (IV, Fixed, 95% CI)	‐3.01 [‐11.38, 5.35]
2.4.15 Zinc vs Zinc + Folic acid	2	125	Mean Difference (IV, Fixed, 95% CI)	‐2.91 [‐10.92, 5.10]
2.4.16 Zinc + Folic acid vs Folic acid	2	121	Mean Difference (IV, Fixed, 95% CI)	0.24 [‐6.17, 6.66]
2.4.17 Zinc vs Zinc + Vitamin E	1	18	Mean Difference (IV, Fixed, 95% CI)	‐1.00 [‐15.00, 13.00]
2.4.18 Zinc vs Zinc + Vitamin E + Vitamin C	1	12	Mean Difference (IV, Fixed, 95% CI)	‐1.00 [‐19.66, 17.66]
2.4.19 Zinc + Vitamin E vs Zinc + Vitamin E + Vitamin C	1	18	Mean Difference (IV, Fixed, 95% CI)	0.00 [‐18.97, 18.97]
2.5 Total sperm motility at 6 months; type of antioxidant Show forest plot	4		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

2.5.1 L‐carnitine vs L‐acetyl carnitine	1	30	Mean Difference (IV, Fixed, 95% CI)	4.10 [‐2.70, 10.90]
2.5.2 L‐carnitine vs L‐carnitine + L‐acetyl carnitine	1	30	Mean Difference (IV, Fixed, 95% CI)	3.40 [‐2.87, 9.67]
2.5.3 L‐acetyl carnitine vs L‐carnitine + L‐acetyl carnitine	1	30	Mean Difference (IV, Fixed, 95% CI)	‐0.70 [‐7.73, 6.33]
2.5.4 N‐acetylcysteine (NAC) vs Selenium + N‐acetylcysteine (NAC)	1	234	Mean Difference (IV, Fixed, 95% CI)	‐4.40 [‐5.14, ‐3.66]
2.5.5 Selenium vs N‐acetylcysteine (NAC)	1	234	Mean Difference (IV, Fixed, 95% CI)	1.30 [0.56, 2.04]
2.5.6 Selenium vs Selenium + N‐acetylcysteine (NAC)	1	232	Mean Difference (IV, Fixed, 95% CI)	‐3.10 [‐3.85, ‐2.35]
2.5.7 Zinc vs Folic acid	2	125	Mean Difference (IV, Fixed, 95% CI)	‐1.03 [‐5.18, 3.13]
2.5.8 Zinc vs Zinc + Folic acid	2	127	Mean Difference (IV, Fixed, 95% CI)	‐1.69 [‐6.95, 3.58]
2.5.9 Zinc + Folic acid vs Folic acid	2	126	Mean Difference (IV, Fixed, 95% CI)	1.03 [‐4.23, 6.29]
2.6 Total sperm motility at 9 months or more; type of antioxidant Show forest plot	1		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

2.6.1 L‐carnitine vs L‐acetyl carnitine	1	30	Mean Difference (IV, Fixed, 95% CI)	3.70 [‐1.69, 9.09]
2.6.2 L‐carnitine vs L‐carnitine + L‐acetyl carnitine	1	30	Mean Difference (IV, Fixed, 95% CI)	5.30 [‐0.73, 11.33]
2.6.3 L‐acetyl carnitine vs L‐carnitine + L‐acetyl carnitine	1	30	Mean Difference (IV, Fixed, 95% CI)	1.60 [‐3.29, 6.49]
2.7 Progessive sperm motility at 3 months or less; type of antioxidant Show forest plot	10		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

2.7.1 Coenzyme Q10 200 mg vs Coenzyme Q10 400 mg	1	65	Mean Difference (IV, Fixed, 95% CI)	‐3.52 [‐9.71, 2.67]
2.7.2 Docosahexaenoic acid (DHA) vs DHA + Vitamin E	1	90	Mean Difference (IV, Fixed, 95% CI)	‐2.22 [‐3.50, ‐0.94]
2.7.3 DHA vs Vitamin E	1	90	Mean Difference (IV, Fixed, 95% CI)	‐0.39 [‐1.67, 0.89]
2.7.4 DHA + Vitamin E vs Vitamin E	1	90	Mean Difference (IV, Fixed, 95% CI)	1.83 [0.68, 2.98]
2.7.5 L‐carnitine vs L‐acetyl carnitine	1	30	Mean Difference (IV, Fixed, 95% CI)	4.00 [‐1.88, 9.88]
2.7.6 L‐carnitine vs L‐carnitine + L‐acetyl carnitine	1	29	Mean Difference (IV, Fixed, 95% CI)	5.00 [‐0.68, 10.68]
2.7.7 L‐acetyl carnitine vs L‐carnitine + L‐acetyl carnitine	1	29	Mean Difference (IV, Fixed, 95% CI)	1.00 [‐5.41, 7.41]
2.7.8 L‐carnitine vs Vitamin B1	1	136	Mean Difference (IV, Fixed, 95% CI)	1.70 [‐1.54, 4.94]
2.7.9 L‐carnitine vs Coenzyme Q10	1	125	Mean Difference (IV, Fixed, 95% CI)	1.30 [‐1.70, 4.30]
2.7.10 L‐carnitine vs L‐carnitine + Coenzyme Q10	1	125	Mean Difference (IV, Fixed, 95% CI)	‐8.20 [‐12.31, ‐4.09]
2.7.11 Coenzyme Q10 vs L‐carnitine + Coenzyme Q10	1	126	Mean Difference (IV, Fixed, 95% CI)	‐9.50 [‐13.54, ‐5.46]
2.7.12 Coenzyme Q10 vs Vitamin B1	1	137	Mean Difference (IV, Fixed, 95% CI)	0.40 [‐2.75, 3.55]
2.7.13 Vitamin B1 vs L‐carnitine + Coenzyme Q10	1	137	Mean Difference (IV, Fixed, 95% CI)	‐9.90 [‐14.12, ‐5.68]
2.7.14 L‐acetyl carnitine + L‐carnitine vs Vitamin E + Vitamin C	1	138	Mean Difference (IV, Fixed, 95% CI)	13.30 [11.21, 15.39]
2.7.15 L‐carnitine vs Vitamin E + Vitamin C	1	63	Mean Difference (IV, Fixed, 95% CI)	30.50 [27.70, 33.30]
2.7.16 L‐carnitine vs Vitamin E	1	212	Mean Difference (IV, Fixed, 95% CI)	1.90 [1.31, 2.49]
2.7.17 L‐carnitine + Vitamin E vs Vitamin E	1	113	Mean Difference (IV, Fixed, 95% CI)	14.10 [10.11, 18.09]
2.7.18 Vitamin D + Calcium vs Vitamin E + Vitamin C	1	86	Mean Difference (IV, Fixed, 95% CI)	6.90 [5.38, 8.42]
2.7.19 Vitamin E + 'Compound amino acids' vs Vitamin E	1	120	Mean Difference (IV, Fixed, 95% CI)	6.10 [3.87, 8.33]
2.8 Progressive sperm motility at 6 months; type of antioxidant Show forest plot	1		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

2.8.1 L‐carnitine vs L‐acetyl carnitine	1	30	Mean Difference (IV, Fixed, 95% CI)	6.30 [0.42, 12.18]
2.8.2 L‐carnitine vs L‐carnitine + L‐acetyl carnitine	1	29	Mean Difference (IV, Fixed, 95% CI)	5.70 [0.10, 11.30]
2.8.3 L‐acetyl carnitine vs L‐carnitine + L‐acetyl carnitine	1	29	Mean Difference (IV, Fixed, 95% CI)	‐0.60 [‐6.93, 5.73]
2.9 Progressive motility at 6 months (data not suitable for meta‐analysis) Show forest plot	1		Other data	No numeric data

2.10 Progressive sperm motility at 9 months; type of antioxidant Show forest plot	1		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

2.10.1 L‐carnitine vs L‐acetyl carnitine	1	30	Mean Difference (IV, Fixed, 95% CI)	3.80 [‐1.50, 9.10]
2.10.2 L‐carnitine vs L‐carnitine + L‐acetyl carnitine	1	29	Mean Difference (IV, Fixed, 95% CI)	5.50 [‐0.11, 11.11]
2.10.3 L‐acetyl carnitine vs L‐carnitine + L‐acetyl carnitine	1	29	Mean Difference (IV, Fixed, 95% CI)	1.70 [‐4.17, 7.57]
2.11 Sperm concentration at 3 months or less; type of antioxidant Show forest plot	11		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

2.11.1 Coenzyme Q10 200 mg vs Coenzyme Q10 400 mg	1	65	Mean Difference (IV, Fixed, 95% CI)	0.20 [‐3.26, 3.66]
2.11.2 Docosahexaenoic acid (DHA) 400 mg vs Docosahexaenoic acid (DHA) 800 mg	1	19	Mean Difference (IV, Fixed, 95% CI)	‐6.80 [‐41.87, 28.27]
2.11.3 DHA vs DHA + Vitamin E	1	90	Mean Difference (IV, Fixed, 95% CI)	‐1.45 [‐2.47, ‐0.43]
2.11.4 DHA vs Vitamin E	1	90	Mean Difference (IV, Fixed, 95% CI)	‐0.24 [‐1.26, 0.78]
2.11.5 DHA + Vitamin E vs Vitamin E	1	90	Mean Difference (IV, Fixed, 95% CI)	1.21 [0.28, 2.14]
2.11.6 Ethylcysteine vs Vitamin E	1	10	Mean Difference (IV, Fixed, 95% CI)	2.20 [‐16.65, 21.05]
2.11.7 L‐carnitine vs L‐acetyl carnitine	1	30	Mean Difference (IV, Fixed, 95% CI)	1.70 [‐10.97, 14.37]
2.11.8 L‐carnitine vs L‐carnitine + L‐acetyl carnitine	1	30	Mean Difference (IV, Fixed, 95% CI)	4.10 [‐9.17, 17.37]
2.11.9 L‐acetyl carnitine vs L‐carnitine + L‐acetyl carnitine	1	30	Mean Difference (IV, Fixed, 95% CI)	2.40 [‐11.14, 15.94]
2.11.10 L‐carnitine vs Vitamin E + Vitamin C	1	63	Mean Difference (IV, Fixed, 95% CI)	15.50 [12.49, 18.51]
2.11.11 L‐carnitine vs Vitamin E	1	212	Mean Difference (IV, Fixed, 95% CI)	0.70 [‐0.34, 1.74]
2.11.12 L‐carnitine + Vitamin E vs Vitamin E	1	113	Mean Difference (IV, Fixed, 95% CI)	1.90 [‐10.52, 14.32]
2.11.13 Selenium vs Combined antioxidants	1	46	Mean Difference (IV, Fixed, 95% CI)	14.70 [‐6.51, 35.91]
2.11.14 Zinc vs Folic acid	2	124	Mean Difference (IV, Fixed, 95% CI)	‐1.30 [‐8.65, 6.06]
2.11.15 Zinc vs Zinc + Folic acid	2	125	Mean Difference (IV, Fixed, 95% CI)	2.93 [‐3.67, 9.54]
2.11.16 Zinc + Folic acid vs Folic acid	2	121	Mean Difference (IV, Fixed, 95% CI)	‐4.11 [‐9.79, 1.57]
2.12 Sperm concentration at 6 months; type of antioxidant Show forest plot	4		Mean Difference (IV, Fixed, 95% CI)	Subtotals only

2.12.1 L‐carnitine vs L‐acetyl carnitine	1	30	Mean Difference (IV, Fixed, 95% CI)	5.90 [‐8.92, 20.72]
2.12.2 L‐carnitine vs L‐carnitine + L‐acetyl carnitine	1	30	Mean Difference (IV, Fixed, 95% CI)	8.10 [‐5.54, 21.74]
2.12.3 L‐acetyl carnitine vs L‐carnitine + L‐acetyl carnitine	1	30	Mean Difference (IV, Fixed, 95% CI)	2.20 [‐10.89, 15.29]
2.12.4 N‐acetylcysteine (NAC) vs Selenium + N‐acetylcysteine (NAC)	1	234	Mean Difference (IV, Fixed, 95% CI)	‐5.30 [‐6.86, ‐3.74]
2.12.5 Selenium vs N‐acetylcysteine (NAC)	1	234	Mean Difference (IV, Fixed, 95% CI)	0.80 [‐0.71, 2.31]
2.12.6 Selenium vs Selenium + N‐acetylcysteine (NAC)	1	232	Mean Difference (IV, Fixed, 95% CI)	‐4.50 [‐6.20, ‐2.80]
2.12.7 Zinc vs Folic acid	2	125	Mean Difference (IV, Fixed, 95% CI)	‐10.10 [‐19.12, ‐1.08]
2.12.8 Zinc vs Zinc + Folic acid	2	127	Mean Difference (IV, Fixed, 95% CI)	‐13.58 [‐25.99, ‐1.17]
2.12.9 Zinc + Folic acid vs Folic acid	2	126	Mean Difference (IV, Fixed, 95% CI)	1.78 [‐9.93, 13.49]
2.13 Sperm concentration at 6 months (data not suitable for meta‐analysis) Show forest plot	1		Other data	No numeric data

2.14 Sperm concentration at 9 months or more; type of antioxidant Show forest plot	1		Mean Difference (IV, Random, 95% CI)	Subtotals only

2.14.1 L‐carnitine vs L‐acetyl carnitine	1	30	Mean Difference (IV, Random, 95% CI)	8.20 [‐0.07, 16.47]
2.14.2 L‐carnitine vs L‐carnitine + L‐acetyl carnitine	1	30	Mean Difference (IV, Random, 95% CI)	6.10 [‐3.74, 15.94]
2.14.3 L‐acetyl carnitine vs L‐carnitine + L‐acetyl carnitine	1	30	Mean Difference (IV, Random, 95% CI)	‐2.10 [‐10.24, 6.04]

Comparison 2. Head‐to‐head antioxidant(s)

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Abstract

Background

Objectives

Search methods

Selection criteria

Data collection and analysis

Main results

Authors' conclusions

PICO

PICO

Population

Intervention

Comparison

Outcome

Plain language summary

Antioxidants for male subfertility

Resumen visual

Authors' conclusions

Implications for practice

Implications for research

Summary of findings

Background

Description of the condition

Description of the intervention

How the intervention might work

Why it is important to do this review

Objectives

Methods

Criteria for considering studies for this review

Types of studies

Inclusion criteria

Exclusion criteria

Types of participants

Inclusion criteria

Exclusion criteria

Types of interventions

Types of outcome measures

Primary outcomes

Secondary outcomes

Search methods for identification of studies

Electronic searches

Searching other resources

Data collection and analysis

Selection of studies

Data extraction and management

Assessment of risk of bias in included studies

Measures of treatment effect

Unit of analysis issues

Dealing with missing data

Assessment of heterogeneity

Assessment of reporting biases

Data synthesis

Subgroup analysis and investigation of heterogeneity

Sensitivity analysis

Summary of findings and assessment of the certainty of the evidence

Results

Description of studies

Results of the search

2011 version of review

2014 update

2018 update

2021 update

Included studies

Study design and setting

Participants

Interventions

Outcomes

Excluded studies

Ongoing studies

Awaiting classification

Risk of bias in included studies

Allocation

Sequence generation

Allocation concealment

Blinding

Performance bias

Detection bias

Incomplete outcome data