Yoga para la prevención primaria de las enfermedades cardiovasculares
Resumen
Antecedentes
Un modo de vida sedentario y el estrés son factores de riesgo principales de enfermedades cardiovasculares (EC). Debido a que el yoga incluye ejercicio y se cree que ayuda en la reducción del estrés puede ser una estrategia efectiva en la prevención primaria de las EC.
Objetivos
Determinar el efecto de cualquier tipo de yoga en la prevención primaria de EC.
Métodos de búsqueda
Se realizaron búsquedas en las siguientes bases de datos electrónicas: el Registro Cochrane Central de Ensayos Controlados (Cochrane Central Register of Controlled Trials) (CENTRAL) (2013, número 11) en The Cochrane Library; MEDLINE (Ovid) (1946 hasta noviembre, semana 3, 2013); EMBASE Classic + EMBASE (Ovid) (1947 hasta 2013, semana 48); Web of Science (Thomson Reuters) (1970 hasta el 4 de diciembre de 2013); Database of Abstracts of Reviews of Effects (DARE), Health Technology Assessment Database and Health Economics Evaluations Database (Número 4 de 4, 2013) en The Cochrane Library. También se hicieron búsquedas en varias bases de datos asiáticas y en la Allied and Complementary Medicine Database (AMED) (inicio hasta diciembre 2012). Se realizaron búsquedas en los registros de ensayos y las listas de referencias de revisiones y artículos y se estableció contacto expertos en el campo. No se aplicaron restricciones de idioma.
Criterios de selección
Ensayos controlados aleatorizados con una duración de al menos tres meses que incluyeran adultos sanos o con alto riesgo de EC. Los ensayos examinaron cualquier tipo de yoga y el grupo de comparación fue ninguna intervención o una intervención mínima. Los resultados de interés fueron los eventos clínicos de EC y los principales factores de riesgo de EC. No se incluyeron ensayos con intervenciones multifactoriales del modo de vida o sobre la pérdida de peso.
Obtención y análisis de los datos
Dos autores de la revisión, de forma independiente, seleccionaron los ensayos para su inclusión, extrajeron los datos y evaluaron el riesgo de sesgo.
Resultados principales
Se identificaron 11 ensayos (800 participantes) y dos estudios en curso. El estilo y la duración de la yoga difirieron entre los ensayos. La mitad de los participantes seleccionados en los estudios estuvieron en riesgo alto de EC. La mayoría de los estudios estuvieron en riesgo de sesgo de realización, y muchos de ellos informaron detalles inadecuados para juzgar el riesgo de sesgo de selección.
Ningún estudio informó la mortalidad cardiovascular, la mortalidad por cualquier causa o los eventos no mortales, y la mayoría de los estudios fueron pequeños y a corto plazo. Hubo heterogeneidad apreciable entre los estudios lo cual imposibilitó la combinación de los estudios estadísticamente para la presión arterial sistólica y el colesterol total. Se encontró que el yoga produjo reducciones en la presión arterial diastólica (diferencia de medias [DM] ‐2,90 mmHg, intervalo de confianza [IC] del 95%: ‐4,52 a ‐1,28), que fue estable en el análisis de sensibilidad, los triglicéridos (DM ‐0,27 mmol/l, IC del 95%: ‐0,44 a ‐0,11) y el colesterol de lipoproteínas de alta densidad (HDL) (DM 0,08 mmol/l, IC del 95%: 0,02 a 0,14). Sin embargo, los estudios que contribuyeron con datos fueron pequeños, a corto plazo y en riesgo incierto o alto de sesgo. No hubo evidencia clara de una diferencia entre los grupos para el colesterol de lipoproteínas de baja densidad (LDL) (DM ‐0,09 mmol/l, IC del 95%: ‐0,48 a 0,30), aunque hubo una heterogeneidad estadística moderada. Los eventos adversos, la aparición de diabetes de tipo 2 y los costos no se informaron en ninguno de los estudios incluidos. La calidad de vida se midió en tres ensayos, pero los resultados no fueron concluyentes.
Conclusiones de los autores
La evidencia limitada proviene de estudios pequeños, a corto plazo y de baja calidad. Hay alguna evidencia de que la yoga tiene efectos favorables sobre la presión arterial diastólica, el colesterol HDL y los triglicéridos, y efectos inciertos sobre el colesterol LDL. Estos resultados deben considerarse exploratorios y deben interpretarse con cuidado.
Resumen en términos sencillos
Yoga para prevenir las enfermedades cardiovasculares
Antecedentes
Las enfermedades cardiovasculares (EC) son una carga para la asistencia sanitaria a nivel global. No obstante, se cree que el riesgo de EC puede disminuir al cambiar varios factores de riesgo, como el aumento de la actividad física y el uso de relajación para reducir el estrés, que son componentes del yoga. Esta revisión evaluó la efectividad de cualquier tipo de yoga en adultos sanos y en alto riesgo de EC.
Características de los estudios
En las bases de datos científicas se buscaron los ensayos controlados aleatorizados (ensayos clínicos que asignaron al azar a los pacientes a uno de dos o más tratamientos) que consideraron los efectos del yoga en adultos con alto riesgo de desarrollar EC. No se incluyeron los pacientes que ya habían tenido una EC (p.ej., ataques cardíacos y accidentes cerebrovasculares). La evidencia está actualizada hasta diciembre 2013.
Resultados clave
Se encontraron 11 ensayos (800 participantes) de los cuales ninguno fue lo suficientemente grande ni de duración suficiente para examinar los efectos del yoga sobre la disminución de la muerte o las variables de evaluación no mortales. Hubo variaciones en el estilo y la duración del yoga y el seguimiento de las intervenciones varió de tres a ocho meses. Los resultados mostraron que el yoga tiene efectos favorables sobre la presión arterial diastólica, el colesterol de lipoproteínas de alta densidad (HDL) y los triglicéridos (lípidos en sangre) y que tuvo efectos inciertos sobre el colesterol de lipoproteínas de baja densidad (LDL). Ninguno de los ensayos incluidos informó los eventos adversos, la aparición de diabetes de tipo 2 o los costos. Se necesitan ensayos de alta calidad a más largo plazo para determinar la efectividad del yoga para la prevención de las EC.
Calidad de la evidencia
Estos resultados deben considerarse exploratorios y deben interpretarse con cuidado. Lo anterior se debe a que los estudios incluidos fueron pequeños, de corta duración y en riesgo de sesgo (hubo riesgo de establecer conclusiones equivocadas debido al favoritismo por parte de los participantes o los investigadores).
Authors' conclusions
Background
Description of the condition
The number one cause of death and disability worldwide is cardiovascular disease (CVD) (WHO 2011). Around 29.6% of total global deaths can be attributed to CVD (WHO 2003). Indeed, in the United Kingdom 88,000 deaths per year are attributable to coronary heart disease and 43,000 deaths are caused by stroke (BHF 2012). In the United States, 35% of total deaths in 2010 were accounted for by CVD compared to 45% in Germany, 30% in Costa Rica, 31% in Denmark, 15% in Ethiopia, 48% in Greece, 32% in Japan, 26% in Mexico and 38% in China (WHO 2011a). It is estimated that by 2030 CVDs will account for almost 23.6 million deaths (WHO 2011).
There are many risk factors that contribute to the development of CVD. Many of these are related to lifestyle choices, such as physical inactivity, smoking, alcohol use and an unhealthy diet (WHO 2011). Whilst clinical treatments are effective in preventing and treating CVD, other strategies involving a healthy lifestyle and stress management have also been shown to be beneficial (Frishman 2005). One such strategy that involves both stress management and exercise is yoga.
As yoga has become increasingly popular as a form of exercise it may be an important intervention for the primary and secondary prevention of CVDs, since high levels of physical activity play a major role in preventing obesity and CVDs (Mendelson 2008). Indeed, there are a number of publications that show yoga to be beneficial for hypertension (Murugesan 2000), dyslipidaemia and atherosclerosis (Singh 2004). The systematic review by Innes et al also demonstrated that yoga improved metabolic risk factors such as blood pressure and lipids (Innes 2005). Furthermore, the psychological benefits of yoga have been established (Saper 2004), with yoga being used in stress reduction, a determinant for CVD (WHO 2011).
Description of the intervention
Yoga is defined in the Oxford Handbook of Complementary Medicine as an "ancient Indian practice involving postural exercises, breathing control and meditation" (Ernst 2008). It was founded in India over 5000 years ago and is believed to give the practitioner a union between their mind, body and spirit (Kappmeier 2006). It has its foundations in three basic components: Asana (posture), Pranayama (breathing) and Dhyana (mediation and devotion), all of which are integrated with each other (Riley 2004). There are six main branches of yoga: Bhakti yoga, Hatha yoga, Jnana yoga, Karma yoga, Mantra yoga and Raja yoga (Feuerstein 2011). Jnana yoga is the path of knowledge in which enlightenment is developed through the philosophical investigation of traditional yogic texts and scriptures (Carrico 2012). In Jnana yoga the practitioner contemplates life using rational thought which, it is believed, leads the mind towards higher levels of consciousness (Simpkins 2003). Karma yoga, however, teaches practitioners how to maintain their focus in everyday life (Simpkins 2003), by analysing their actions and thoughts in order to learn about different qualities of thinking and behaving. In doing this, the practitioner will learn how to channel their actions in the correct way and ultimately achieve an enlightened way of living (Simpkins 2003). Mantra yoga influences the human consciousness through sound and vibration (Simpkins 2003). It is thought that after many repetitions and practice the mantra will begin to recite itself within the practitioner's mind, leading them to enlightenment. Bhakti yoga is known as the yoga of devotion (Koay 2009). It involves "cultivating selfless love, pure giving and long devotion to God", which is expressed through chants, prayers and also services to humanity (Koay 2009). In Raja yoga, on the other hand, all three basic yoga components are used so that the practitioner can seek enlightenment through the mind. In this type of yoga enlightenment is sought through using meditation and techniques of attention, concentration and contemplation (Simpkins 2003). However, the most practised style of yoga in the West is Hatha (Hewitt 2001). Hatha yoga seeks enlightenment not through the mind, as in Raja yoga, but through the body. It can be practised in many styles, such as Bikram, Integral, Kundalini, Ashtanga and Iyengar, all of which have unique characteristics (Cook 2014).
The physical and psychological benefits of yoga have been acknowledged (Saper 2004). Evidence suggests that yoga can be applied to manage and reduce the symptoms of a number of conditions. For instance, yoga has been found to relieve perceived pain in those with rheumatoid arthritis (Bosch 2009) and chronic back pain (Groessl 2008), as well as aiding in the management of those with urological disorders (Ripoll 2002). Yoga has also been found to be beneficial for depression, stress and insomnia (Granath 2006; Khalsa 2004; Lavey 2005; Woolery 2004).
How the intervention might work
High levels of exercise have been found to play a major role in preventing obesity, diabetes and CVD (Mendelson 2008; Mittal 2008). Therefore, as some forms of yoga, such as Hatha yoga, are considered a form of exercise it may help in CVD prevention. Evidence suggests that regular exercise can decrease the risk of dying from CVD (USHHS 1996), and it has been found to be beneficial when included in primary and secondary prevention strategies (Lavie 2009). This is because physical activity has favourable effects not only on CVD risk factors but also on CVD symptoms, a person's functional capacity, physiology and quality of life (ACSM 2010; AHA 2011). Furthermore, yoga is a moderately safe, gentle choice for promoting general health and emotional well‐being, especially for those whose lifestyle is sedentary (Lau 2012).
However, the benefits and degree of improvement in physical health from yogic practices will vary according to how the individual practises yoga. For instance, if the individual practises all poses intensely and continuously then cardiorespiratory fitness will improve, whereas if the individual only practises a single pose then improvements may be made in muscular strength, flexibility and posture (Lau 2012). Furthermore, Hagins et al state that when the metabolic cost is averaged out across a whole session of Hatha yoga, it only represents a low level of exercise. This does not meet the recommendations for levels of physical activity developed for improving or maintaining general health and CVD fitness (Hagins 2007). However, studies suggests that regular yogic training leads to significant improvements in cardiovascular endurance and anaerobic power and therefore is useful in improving exercise tolerance (Bera 1993; Madanmohan 2004; Muralidhara 1982). As such, yoga can be used in patients who are sedentary to improve exercise tolerance gradually and, with regular practice, strengthen the cardiovascular system.
As well as being beneficial in terms of exercise, yoga may also help to reduce stress (AHA 2011), a determinant of CVD (WHO 2011). It does this by creating a physiological sequence of events in the body as a result of changes in posture, breathing or both, which reduces the stress response (Woodyard 2011). Proposed physiological mechanisms include reducing the reactivity of the sympathetic nervous system (Innes 2007), counteracting the activity of the aroused autonomic nervous system (ANS), reversing it back to a relaxed state, or both (Levine 2000; Michalsen 2005).
Why it is important to do this review
Few randomised controlled trials have directly investigated the effectiveness of yoga for the primary prevention of CVD, even though yoga has the potential to help in CVD prevention. Furthermore, few systematic reviews examining the effectiveness of yoga are available. Those that do exist mainly focus on secondary prevention (Innes 2005; Lau 2012). Therefore, a comprehensive systematic review is needed, which thoroughly examines the most up to date evidence to determine whether yoga is effective in preventing CVD in those who are healthy or who have CVD risk factors.
Objectives
To determine the effect of any type of yoga on the primary prevention of CVD.
Methods
Criteria for considering studies for this review
Types of studies
Randomised controlled trials.
Types of participants
Healthy adults (18 years old or over) of all ages from the worldwide general population and those at moderate to high risk of CVD (Perk 2012). As the review focused on the primary prevention of CVD we excluded those who had experienced a previous myocardial infarction, stroke, revascularisation procedure (coronary artery bypass grafting (CABG) or percutaneous transluminal coronary angioplasty (PTCA)), and those with angina or angiographically defined coronary heart disease.
Types of interventions
Trials investigating any type of yoga. The definition of yoga was that provided by Ernst 2008, in which yoga is seen as involving three components: postural exercises, breathing control and meditation.
Multifactorial intervention studies (including dietary or other additional interventions) were not included in this review in order to avoid confounding. We also focused on follow‐up periods of three months or more. We considered follow‐up to be the time elapsed since the start of the intervention and therefore we excluded any trials with an intervention period of less than 12 weeks. We also only considered trials where the comparison group was either given no intervention or minimal intervention (for example, leaflets with no person to person intervention or reinforcement).
Types of outcome measures
Primary outcomes
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Cardiovascular mortality
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All‐cause mortality
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Non‐fatal endpoints such as myocardial infarction, CABG, PTCA, angina or angiographically defined coronary heart disease, stroke, carotid endarterectomy or peripheral arterial disease (PAD)
Secondary outcomes
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Changes in blood pressure (systolic and diastolic blood pressure) and blood lipids (total cholesterol, HDL cholesterol, LDL cholesterol, triglycerides)
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Occurrence of type 2 diabetes as a major CVD risk factor
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Quality of life
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Adverse effects (as defined by the authors of the included trials)
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Costs
Search methods for identification of studies
Electronic searches
We searched the following electronic databases:
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Cochrane Central Register of Controlled Trials (CENTRAL) (2013, Issue 11) in The Cochrane Library;
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MEDLINE (Ovid) (1946 to November week 3 2013);
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EMBASE Classic + EMBASE (Ovid) (1947 to 2013 Week 48);
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Web of Science (Thomson Reuters, including the Social Sciences Citation Index (SSCI) and the Conference Proceedings Citation Index ‐ Science (CPCI‐S) (1970 to 4 December 2013);
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Database of Abstracts of Reviews of Effects (DARE), Health Technology Assessment Database and Health Economics Evaluations Database (Issue 4 of 4, 2013) in The Cochrane Library;
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AMED (Inception to 22 October 2012).
We used medical subject headings (MeSH) or equivalent and text word terms. We used the Cochrane sensitivity‐maximising randomised controlled trial (RCT) filter for MEDLINE (Lefebvre 2011), and used adaptations of it for EMBASE and Web of Science.
There were no language restrictions.
We tailored the searches to individual databases (Appendix 1).
Searching other resources
In addition, we checked reference lists of reviews and retrieved articles for additional studies.
We searched the metaRegister of controlled trials (mRCT) (www.controlled‐trials.com/mrct), Clinicaltrials.gov (www.clinicaltrials.gov) and the World Health Organization (WHO) International Clinical Trials Registry Platform (ICTRP) (http://apps.who.int/trialsearch/) for ongoing trials (search date 5 November 2012). We also searched OpenGrey for grey literature (http://www.opengrey.eu/) and the following Asian databases: KoreaMed (www.koreamed.org), DBPIA (www.dbpia.co.kr/), Korean studies information service system (http://kiss.kstudy.com/index.asp), Oasis (http://oasis.kiom.re.kr) and China National Knowledge Infrastructure (CNKI) (www.cnki.net/) from their inception to 21 December 2012.
We performed citation searches on key articles and used Google Scholar to search for further studies. We contacted experts in the field for unpublished and ongoing trials and contacted authors where necessary for any additional information.
Data collection and analysis
Selection of studies
From the searches, two review authors (LH, JH, MD) independently reviewed the title and abstract of each paper. Following the initial screen, we obtained the full text of potentially relevant studies and two review authors (LH, MD) independently selected relevant studies using predetermined inclusion criteria. In all cases we resolved disagreements about study inclusion by consensus and consulted a third review author (KR) if disagreements persisted.
Data extraction and management
Two review authors (LH, MD) independently extracted data using a proforma. We contacted chief investigators to provide additional relevant information if necessary. We extracted details of the study design, participant characteristics, study setting, intervention and outcome data (including details of outcome assessment, adverse effects and methodological quality (randomisation, blinding and attrition)) from each included study. We resolved disagreements about extracted data by consensus with a third review author (KR) being consulted if disagreements persisted.
Assessment of risk of bias in included studies
Two review authors (LH, MD) assessed risk of bias independently by examining the random sequence generation and allocation concealment, description of drop‐outs and withdrawals (including analysis by intention‐to‐treat), blinding (participants, personnel and outcome assessment) and selective outcome reporting in each trial (Higgins 2011).
Measures of treatment effect
We processed data in accordance with the Cochrane Handbook for Systematic Reviews of Interventions (Higgins 2011). For continuous outcomes we compared net changes (i.e. intervention group minus control group differences) and calculated a mean difference (MD) and 95% CI for each study.
Assessment of heterogeneity
For each outcome we conducted tests of heterogeneity (using the Chi2 test of heterogeneity and the I2 statistic). Where there was no heterogeneity we performed a fixed‐effect meta‐analysis. If we detected substantial heterogeneity (I2 greater than 50%), we looked for possible explanations for this (for example, participants and intervention). If the heterogeneity could not be explained, we considered the following options: providing a narrative overview and not aggregating the studies at all or using a random‐effects model with appropriate cautious interpretation.
Subgroup analysis and investigation of heterogeneity
It was our intention to stratify results according to baseline risk, type of yoga, and time and duration of yoga, but there were not sufficient trials for us to perform these analyses.
Sensitivity analysis
We performed sensitivity analysis by excluding studies at high risk of bias (e.g. those with loss to follow‐up of more than 20% without intention‐to‐treat analysis). We intended to examine the effects of publication bias using funnel plots and tests of asymmetry (Egger 1997), but these could not be carried out due to the small number of trials included in the review.
Results
Description of studies
Results of the search
The electronic searches generated 2151 hits (1465 after de‐duplication). Screening of titles and abstracts identified 204 papers to go forward for formal inclusion and exclusion. Of these, 11 randomised controlled trials (16 papers) met the inclusion criteria. We also identified two ongoing trials. The study flow diagram is presented in Figure 1.
Study flow diagram.
Included studies
Details of studies included in the review are provided in the Characteristics of included studies table. Eleven trials (16 papers) with 800 participants randomised met the inclusion criteria. Four of the 11 trials recruited only female participants (Kim 2012; Kim 2013; Lee 2012; Monika 2012), and one trial recruited only male participants (Mahajan 1999). The health status of participants varied between studies; one study recruited females with metabolic syndrome (Kim 2013); one study recruited essential hypertensives (Latha 1991); one study recruited women with menstrual irregularities (Monika 2012); one study recruited healthy premenopausal women (Kim 2012); one study recruited males with hypertension and high cholesterol (Mahajan 1999); one study recruited HIV‐infected participants (Cade 2010); another study recruited obese postmenopausal women (Lee 2012); one study recruited non‐exercisers with a family history of type 2 diabetes and at least one cardio‐metabolic risk factor (Yang 2011), and the remaining three studies recruited healthy participants (394 participants) (Blumenthal 1989; Ray 2001; Wolever 2012).
Five of the trials were conducted in the USA (472 participants) (Blumenthal 1989; Cade 2010; Kim 2012; Wolever 2012; Yang 2011), four trials in India (271 participants) (Latha 1991; Mahajan 1999; Monika 2012; Ray 2001), one trial in South Korea (16 participants) (Lee 2012), and one trial did not provide country information (Kim 2013). The follow‐up periods also varied between studies. Three studies had a follow‐up period of three months (Kim 2013; Wolever 2012; Yang 2011); one study had a follow‐up period of 14 weeks (Mahajan 1999); two studies had a four‐month follow‐up (Blumenthal 1989; Lee 2012); two studies had a follow‐up period of five months (Cade 2010; Ray 2001); two studies had a six‐month follow‐up (Latha 1991; Monika 2012); and the remaining study had a follow‐up period of eight months (Kim 2012).
Duration and type of yoga varied between the 11 included studies. In six studies yoga was practised for one hour either for three days a week (Ray 2001), three days a week on non‐consecutive days for 16 weeks (Lee 2012), two times a week every two to three days for three months (Yang 2011), two to three times a week for 20 weeks (Cade 2010), for 64 sessions on Mondays and Wednesdays for eight months (Kim 2012), or for two or more days a week for four months (Blumenthal 1989). In one study yoga was practised twice weekly for six months (Latha 1991), whilst in another study yoga was practised for 35 to 40 minutes a day for five days a week in the morning for six months (Monika 2012). In another study yoga was practised for 12 weeks (Kim 2013). In the remaining two studies yoga was practised either for 12 hours in 12 weeks (Wolever 2012), or for four days at a residential camp with one hour of practice at home every day (Mahajan 1999). Five studies stated that yoga was taught by a qualified instructor (Kim 2012; Monika 2012; Ray 2001; Wolever 2012; Yang 2011), and four studies stated that participants were encouraged to practise at home (Cade 2010; Mahajan 1999; Wolever 2012; Yang 2011). Two studies used Ashtanga yoga (Cade 2010; Kim 2012), one study used Hatha yoga (Ray 2001), one study practised Vinyasa yoga (Yang 2011), and one study used Viniyoga (Wolever 2012). Five studies did not state which type of yoga they used (Blumenthal 1989; Kim 2013; Latha 1991; Lee 2012; Mahajan 1999). In the remaining study (Monika 2012), the intervention involved yoga Nidra. This differs from the types of yoga performed in the other included studies as the focus was not on yoga as a whole but on yoga meditation whilst in a particular pose. Yoga Nidra was performed whilst in the Shavasana pose and involved an awareness of breath and meditation practices.
One study is awaiting classification because clarification was needed about the control group. We contacted the authors for this information. Details of this study are provided in the Characteristics of studies awaiting classification table. This trial examined 12 weeks of yoga therapy in patients with essential hypertension (Punita 2011). We identified two ongoing trials looking at yoga. Details for these trials are given in the Characteristics of ongoing studies table. The first trial examines the effects of yoga in male and female office and manual workers in Sweden (NCT01305096). The outcomes measured include blood pressure and the anticipated start date was March 2011. The second trial examines the effect of yoga in patients with metabolic syndrome with outcome measures of blood pressure, quality of life, stress and insulin sensitivity. No anticipated end date was provided for this study (Cohen 2008).
Excluded studies
Details and reasons for exclusion of the studies that most closely missed the inclusion criteria are given in the Characteristics of excluded studies table. Reasons for exclusion for the majority of studies included the control group not receiving a minimal intervention or no intervention, no relevant outcomes reported and the studies not being randomised controlled trials. Some studies were also short‐term, with less than three months follow‐up (see Figure 1).
Risk of bias in included studies
Details for each included trial are presented in the 'Risk of bias' tables in the Characteristics of included studies and summaries are presented in Figure 2 and Figure 3.
'Risk of bias' summary: review authors' judgements about each risk of bias item for each included study.
'Risk of bias' graph: review authors' judgements about each risk of bias item presented as percentages across all included studies.
Allocation
The methods of random sequence generation were unclear in nine of the 11 included studies (Blumenthal 1989; Cade 2010; Kim 2012; Kim 2013; Lee 2012; Mahajan 1999; Ray 2001; Wolever 2012; Yang 2011). In the remaining two studies the methods of random sequence generation were stated. In one study, we judged these methods to be at low risk of bias as a statistician not associated with the study generated the randomisation scheme and opaque envelopes were used (Monika 2012). The other study used open allocation and so we judged this to be at high risk of bias (Latha 1991). The methods of allocation concealment were unclear in eight of the 11 included studies (Blumenthal 1989; Cade 2010; Kim 2012; Kim 2013; Lee 2012; Mahajan 1999; Ray 2001; Wolever 2012). In one study, we judged the method of allocation concealment stated to be at low risk of bias (Monika 2012), whilst in the remaining two studies we judged the method of allocation concealment to be at high risk of bias (Latha 1991; Yang 2011).
Blinding
We considered blinding of participants and personnel to be high risk in all 11 of the included studies as it is difficult, if not impossible, to blind participants and personnel to behavioural interventions such as exercise. The blinding of outcome assessment was unclear in 10 of the 11 included studies (Blumenthal 1989; Cade 2010; Kim 2012; Kim 2013; Latha 1991; Lee 2012; Mahajan 1999; Monika 2012; Ray 2001; Wolever 2012). The remaining study stated that outcome assessors were blind and so we judged it to be at low risk of bias (Yang 2011).
Incomplete outcome data
In eight of the 11 included studies we judged the reporting of incomplete outcome data to be unclear because there was insufficient information to judge (Cade 2010; Latha 1991; Lee 2012; Mahajan 1999; Yang 2011), no information was provided (Kim 2013), or they did not report the reasons for losses to follow‐up (Monika 2012; Ray 2001). Two of the 11 included studies reported losses to follow‐up and had a similar number of losses between the intervention and control arms (Blumenthal 1989; Wolever 2012). In the remaining study, we judged the reporting of incomplete outcome data to be high risk of bias as there were large numbers of losses to follow‐up with a high number of losses reported in the intervention arm (Kim 2012).
Selective reporting
For five of the 11 included trials the risk of bias of selective reporting was unclear as there was insufficient information available for a judgement to be made (Cade 2010; Kim 2013; Lee 2012; Mahajan 1999; Wolever 2012). We judged five of the included studies to be at low risk of bias as these studies clearly stated the primary and secondary outcomes and reported their results (Blumenthal 1989; Kim 2012; Monika 2012; Ray 2001; Yang 2011). We judged one study to be at high risk of bias as no blood pressure measurements were provided for the control group (Latha 1991).
Other potential sources of bias
For all included studies there was insufficient information to judge the risk of bias from other potential sources.
Effects of interventions
Cardiovascular and all‐cause mortality
None of the included studies provided cardiovascular or all‐cause mortality data.
Non‐fatal Endpoints
None of the included studies provided clinical event data.
Cardiovascular risk factors
Blood pressure
Nine of the 11 included studies measured blood pressure (Cade 2010; Kim 2012; Kim 2013; Latha 1991; Lee 2012; Monika 2012; Ray 2001; Wolever 2012; Yang 2011). Useable data for meta‐analysis were not available for one study (Latha 1991), as no data for the control group were provided, only changes since baseline for the yoga group. In this study, systolic blood pressure was significantly reduced by yoga (mean difference (MD) ‐5.71 mmHg, P < 0.01) and diastolic blood pressure was also reduced but this did not reach statistical significance (mean difference ‐3.71 mmHg, P reported to be non‐significant). The other eight studies could not be combined as there was substantial heterogeneity between trials (I2 = 81%) (Analysis 1.1) (485 participants). Two studies showed a significant reduction in systolic blood pressure with the intervention (Lee 2012; Ray Males 2001), and six studies showed yoga to reduce systolic blood pressure but this did not reach statistical significance (Cade 2010; Kim 2013; Monika 2012; Ray Females 2001; Wolever 2012; Yang 2011). The remaining study showed a significant increase in systolic blood pressure (MD 2.20, 95% confidence interval (CI) 0.16 to 4.24) (Kim 2012).
For diastolic blood pressure (444 participants), we found moderate heterogeneity between studies (I2 = 58%) so we performed a random‐effects meta‐analysis. From the pooled analysis, yoga was found to significantly lower diastolic blood pressure (MD ‐2.90, 95% CI ‐4.52 to ‐1.28) (Analysis 1.2). Results were similar for the fixed‐effect model but we reported the random‐effects results as the effect estimate is more conservative with wider confidence intervals. Sensitivity analysis, removing studies with high risk of bias based on incomplete outcome data (those with loss to follow‐up of more than 20% without intention‐to‐treat analysis), retained seven studies and statistical significance (MD ‐3.38 mmHg, 95% CI ‐4.88 to ‐1.88).
Lipid levels
Five trials measured total cholesterol (207 participants) (Blumenthal 1989; Cade 2010; Lee 2012; Mahajan 1999; Yang 2011). Data for these trials could not be pooled as there was significant heterogeneity (I2 = 71%) (Analysis 1.3). Two studies showed a significant reduction in total cholesterol with yoga (Lee 2012; Mahajan 1999). In the three remaining studies (Blumenthal 1989; Cade 2010; Yang 2011), there was some variation in the direction of effect, but individually each found no clear evidence of an effect of yoga on total cholesterol (MD 0.33 mmol/l, 95% CI ‐0.13 to 0.79 (Blumenthal 1989), MD ‐0.16 mmol/l, 95% CI ‐0.68 to 0.36 (Cade 2010), MD ‐0.11 mmol/l, 95% CI ‐0.93 to 0.71 (Yang 2011)). Five trials also measured low‐density lipoprotein (LDL) cholesterol (Blumenthal 1989; Cade 2010; Lee 2012; Mahajan 1999; Yang 2011). We found moderate heterogeneity between studies (I2 = 62%) so we performed a random‐effects meta‐analysis. From the pooled analysis, yoga was found to have no effect on LDL cholesterol (MD ‐0.09 mmol/l, 95% CI ‐0.48 to 0.30) (Analysis 1.4). Results were similar for the fixed‐effect model but, as with diastolic blood pressure, we reported the random‐effects results as the effect estimate is more conservative with wider confidence intervals.
Five of the 11 included studies measured high‐density lipoprotein (HDL) cholesterol (Blumenthal 1989; Cade 2010; Lee 2012; Mahajan 1999; Yang 2011). From the pooled analysis (207 participants) we found that HDL cholesterol significantly increased with yoga (MD 0.08 mmol/l, 95% CI 0.02 to 0.14) (Analysis 1.5). All five trials also measured triglyceride levels and we pooled data for these five (207 participants). Overall, yoga was found to lower triglyceride levels significantly (mean difference ‐0.27 mmol/l, 95% CI ‐0.44 to ‐0.11) (Analysis 1.6).
Occurrence of type 2 diabetes
None of the 11 included studies provided data on the occurrence of type 2 diabetes.
Quality of life
Quality of life was measured in three of the included studies (Blumenthal 1989; Cade 2010; Ray 2001). In one study (Ray 2001), subjective well‐being at five months was better for women participants in the yoga group (mean 100.0, standard error of the mean (SEM) 11.44) than in the control group (mean 99.04, SEM 9.65), whilst for men subjective well‐being was better in the control group (mean 101.75, SEM 12.55) than in the yoga group (mean 98.4, SEM 12.72). In another study (Cade 2010), absolute Medical Outcome Study ‐ SF36 scores at 20 weeks were the same between the standard care group and the yoga group (73 ± 25 versus 75 ± 24). In the remaining study (Blumenthal 1989), quality of life was measured by a perceived change questionnaire that measured perceived change in mood, personality, physical and social functioning. In this study, participants in the yoga group saw themselves as improving on a number of psychological and behavioural dimensions compared to participants in the wait‐list control group after four months.
Adverse effects
None of the 11 included studies provided information about adverse events.
Costs
None of the 11 included studies provided data on costs.
Discussion
Summary of main results
We identified 11 trials that randomised 800 participants in studies with a duration of three months or more from the 1465 papers screened; 10 of them provided useable data for meta‐analysis. For the remaining study no control group data were provided and its report gave no contact details to elicit further information. We identified two ongoing trials.
None of the trials measured clinical events as they were relatively short‐term and conducted in mainly healthy participants. Small beneficial effects were seen in the five trials that measured triglycerides, the five trials that measured diastolic blood pressure and the five trials that measured high‐density lipoprotein (HDL) cholesterol. For low‐density lipoprotein (LDL) cholesterol, yoga had uncertain effects. For systolic blood pressure and total cholesterol, heterogeneity between trials precluded meta‐analyses. Three trials looked at the effects of yoga on quality of life but the results were inconclusive. No studies reported adverse events and the occurrence of type 2 diabetes. Costs were also not examined in any of the included studies.
The gaps in the published literature are highlighted by the results of this review. There is a lack of reliable, strong evidence on the effects of yoga on clinical events, blood pressure and lipid levels and for the primary prevention of cardiovascular disease (CVD). As such, more trials are needed in order to clarify the potential benefits of yoga for CVD risk factors. Furthermore, the trials included in this review were quite small, at risk of bias and short‐term. Longer, larger, rigorously conducted trials should be carried out to verify or refute the potential benefits of yoga for the primary prevention of CVD.
Overall completeness and applicability of evidence
The review included adults participants who were at varying levels of CVD risk and included both men and women. Most of the trials were conducted in developed countries. None of the 11 included studies examined our primary outcomes as participants were predominantly healthy and trials were small and relatively short‐term in follow‐up. We were also not able to examine the effects of baseline CVD risk, type of yoga or intervention intensity due to the limited number of included studies.
The effectiveness of yoga could not be rigorously assessed since only three trials (174 participants) assessed cardiovascular risk factors at six or more months. The remaining eight studies were shorter‐term so it is unclear if any effects of yoga could be sustained. Furthermore, the 11 included trials were limited in the information that they provided. The majority of the included trials did not state what type of yoga was being assessed or which yoga components were included, and had few details about intervention frequency, duration or physiological intensity. Consequently, we are unable to say which types of yoga are effective in preventing CVD and if the frequency, duration and physiological intensity of yoga impact on its effectiveness.
The 11 included studies varied in the health status of participants. Around half of participants (399 randomised) were classified as at high risk of CVD and were recruited in seven trials (Cade 2010; Kim 2013: Latha 1991; Lee 2012; Mahajan 1999; Monika 2012; Yang 2011), whilst the remaining four trials recruited healthy participants. Due to this variability the applicability of this review's findings to different populations is uncertain. There was also considerable variability in the yoga interventions in the included studies. Studies differed in the intensity of the yoga intervention, with practice ranging from two times a week to five times a week, for 35 minutes to an hour for 12 to 32 weeks. Yoga instructors were also used in some of the included trials and some encouraged practice at home.The type of yoga taught also differed between studies with two studies using Ashtanga yoga (Cade 2010; Kim 2012), one study using Hatha yoga (Ray 2001), one study using yoga Nidra (Monika 2012), one study using Vinyasa yoga (Yang 2011), and one study using Viniyoga (Wolever 2012). The remaining studies did not state which type of yoga they used (Blumenthal 1989; Kim 2013; Latha 1991; Lee 2012; Mahajan 1999). Due to the variability in yoga interventions and the lack of reported detail, we were unable to assess the effectiveness of each type of yoga or to assess intervention intensity. Therefore we cannot say which type of yoga is best at reducing CVD risk and by how much, or for how long yoga should be practised to be able to see reductions in CVD risk factors.
The two ongoing trials will add to the evidence but more longer‐term trials are needed.
Quality of the evidence
Overall, the trials included in this review were at some risk of bias meaning that the results should be treated with caution. In nine of the 11 included trials the methods of random sequence generation were not stated or unclear, whilst in eight trials the details of allocation concealment were not stated. All 11 trials did not state if participants and personnel were blinded and in 10 trials it was unclear whether outcome assessors were blinded. However, it is difficult, if not impossible, to blind participants and personnel to behavioural lifestyle interventions such as yoga. Risk of bias related to incomplete outcome data was unclear in eight trials, low in two trials but high in one trial. We judged bias due to selective outcome reporting to be high in one study, low in five studies and unclear in five studies. For all studies there was insufficient information to judge the risk of other biases.
Small study bias is also a risk in this review since most of the included trials were very small. Furthermore, we were unable to examine the effects of publication bias in funnel plots due to the limited number of trials included in the review. However, small trials are often conducted with less methodological rigour, more likely to be carried out in selected populations and have been shown to report larger beneficial effects than larger trials (Nüesch 2010; Sterne 2000; Sterne 2001). Therefore, the results of this review need to be interpreted with this in mind.
There was a high degree of heterogeneity between trials, from different sources (participants, type of yoga, duration of intervention and follow‐up, outcome data), which precluded statistical pooling for some outcomes. Therefore, our findings should be interpreted with the inconsistency between studies in mind.
Potential biases in the review process
We conducted a comprehensive search across major databases for interventions involving yoga. We also screened systematic review reference lists and contacted authors when necessary. However, from corresponding with authors we did not receive further unreported information or data from three trials, which limited our analysis. Two review authors carried out all screening, inclusion and exclusion and data abstraction independently. Two review authors also conducted data entry and analysis.
Our decision to restrict this review to interventions only investigating yoga avoided the potential confounding effects of other behavioural interventions on our outcomes, for instance interventions that focused on weight loss. However, this limited the number of trials eligible for inclusion. Furthermore, the small number of trials on which this review is based, limitations in reporting methodological rigour, an unclear risk of bias in most trials and sparse or no data for primary and secondary outcomes means that the findings of this review, to date, are limited.
Agreements and disagreements with other studies or reviews
To our knowledge, no other systematic review including only randomised controlled trials has been carried out solely to examine the effects of yoga in adults for the primary prevention of CVD. Other systematic reviews have looked at yoga for secondary prevention of coronary heart disease in adults (Innes 2005; Lau 2012), or have looked at yoga in relation to type 2 diabetes, cancer or physical functioning and health‐related quality of life in older adults (Aljasir 2010; Harder 2012; Patel 2012). One review did not find any randomised controlled trials that examined the effectiveness of yoga for patients with coronary heart disease that were eligible for inclusion (Lau 2012). Another review found some evidence to suggest that yoga reduced insulin resistance syndrome‐related risk factors for CVD and may help in CVD management (Innes 2005). However, this review could not draw any firm conclusions due to the methodological and other limitations of the included studies.
In the current review, trials were also small and heterogeneous. We found substantial heterogeneity between trials for lipid level measurements and for systolic blood pressure. There were differences between the included studies in the characteristics of participants, sample size, intensity of yoga, type of yoga and the duration of follow‐up.
Study flow diagram.
'Risk of bias' summary: review authors' judgements about each risk of bias item for each included study.
'Risk of bias' graph: review authors' judgements about each risk of bias item presented as percentages across all included studies.
Comparison 1 Yoga versus no intervention or minimal control, Outcome 1 Systolic blood pressure, change from baseline (mmHg).
Comparison 1 Yoga versus no intervention or minimal control, Outcome 2 Diastolic blood pressure, change from baseline (mmHg).
Comparison 1 Yoga versus no intervention or minimal control, Outcome 3 Total cholesterol, change from baseline (mmol/l).
Comparison 1 Yoga versus no intervention or minimal control, Outcome 4 LDL cholesterol, change from baseline (mmol/l).
Comparison 1 Yoga versus no intervention or minimal control, Outcome 5 HDL cholesterol, change from baseline (mmol/l).
Comparison 1 Yoga versus no intervention or minimal control, Outcome 6 Triglycerides, change from baseline (mmol/l).
| Outcome or subgroup title | No. of studies | No. of participants | Statistical method | Effect size |
| 1 Systolic blood pressure, change from baseline (mmHg) Show forest plot | 9 | Mean Difference (IV, Fixed, 95% CI) | Totals not selected | |
| 2 Diastolic blood pressure, change from baseline (mmHg) Show forest plot | 8 | 444 | Mean Difference (IV, Random, 95% CI) | ‐2.90 [‐4.52, ‐1.28] |
| 3 Total cholesterol, change from baseline (mmol/l) Show forest plot | 5 | Mean Difference (IV, Fixed, 95% CI) | Totals not selected | |
| 4 LDL cholesterol, change from baseline (mmol/l) Show forest plot | 5 | 207 | Mean Difference (IV, Random, 95% CI) | ‐0.09 [‐0.48, 0.30] |
| 5 HDL cholesterol, change from baseline (mmol/l) Show forest plot | 5 | 207 | Mean Difference (IV, Fixed, 95% CI) | 0.08 [0.02, 0.14] |
| 6 Triglycerides, change from baseline (mmol/l) Show forest plot | 5 | 207 | Mean Difference (IV, Fixed, 95% CI) | ‐0.27 [‐0.44, ‐0.11] |
