The Efficacy of Body-Oriented Yoga in Mental Disorders
A Systematic Review and Meta-Analysis
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Background: The efficacy of body-oriented yoga in the treatment of mental disorders has been investigated in numerous studies. This article is a systematic review and meta-analysis of the relevant publications.
Methods: All studies in which the efficacy of hatha-yoga, i.e., body-oriented yoga with asanas and pranayama, was studied in adult patients suffering from a mental disorder (as diagnosed by ICD or DSM criteria) were included in the analysis. The primary endpoint was disorder-specific symptom severity. The publications were identified by a systematic search in the PubMed, Web of Science, PsycINFO and ProQuest databases, supplemented by a search with the Google Scholar search engine and a manual search in the reference lists of meta-analyses and primary studies, as well as in specialized journals.
Results: 25 studies with a total of 1339 patients were included in the analysis. A large and significant effect of yoga was seen with respect to the primary endpoint (symptom severity) (Hedges’ g = 0.91; 95% confidence interval [0.55; 1.28]; number needed to treat [NNT]: 2.03), with substantial heterogeneity (I2 = 69.8%) compared to untreated control groups. Small but significant effects of yoga were also seen in comparison with attention control (g = 0.39; [0.04; 0.73]; NNT: 4.55) and physical exercise (g = 0.30; [0.01; 0.59]; NNT: 5.75); no difference in efficacy was found between yoga and standard psychotherapy (g = 0.08; [−0.24; 0,40]; NNT: 21.89). In view of the relatively high risk of bias, these findings should be interpreted with caution.
Conclusion: Body-oriented yoga with asanas and pranayama as central components is a promising complementary treatment for mental disorders and should be investigated in further high-quality studies.
Some 2.6 million people in Germany practice yoga. The main reasons for this are improved physical and mental wellbeing (e1). The different types or forms of yoga have in common the fundamental principle of asanas (body posture) and pranayama (breathing techniques); dhyana (meditation) and relaxation are also often components of yoga (e2).
The form of yoga that is practiced most commonly outside India is hatha yoga, a body-oriented variant that includes mostly asanas, pranayama, and dhyana. The different body postures, controlled breathing/breathing exercises, and meditation aim to harmonize a person’s mental and physical state (e3). The first objective is to relax the body before controlling the five senses and reduce mental activity (e4). Consequently, yoga ranges among the “mind-body interventions,” which are based on the idea that physiological states affect emotions, thoughts, and attitudes (e5).
Yoga has been described as a promising interventional approach in treating mental disorders (1–3). One of its advantages lies in the fact that it is an easily accessible therapeutic approach that has become widely accepted (e1). Furthermore, numerous additional benefits are being discussed for yoga in the treatment of mental disorders. In addition to the relatively low costs associated with yoga as a group activity, yoga is associated with hardly any risks nor side effects compared with pharmacological treatment (4). Yoga exercises can be easily integrated into everyday life (5) and therefore possibly contribute to preventing relapses (6).
The effects of yoga on the endocrine system, the nervous system, and physical health are well documented. Lower cortisol concentrations and raised concentrations of serotonin and melatonin after regular yoga have been shown empirically, as has a reduction in pro-inflammatory cytokines (7). Increased satisfaction, self-confidence, and improved self-control after yoga are associated with a lower degree of perceived stress and a higher degree of wellbeing (7).
Some of the fundamental ideas and elements underlying yoga are now also applied in the concept of mind-body medicine in treating physical illness (e6) or in the context of psychotherapy. Approaches such as mindfulness-based stress reduction (MBSR) or acceptance and commitment therapy (ACT) have been developed accordingly (e7).
Several systematic reviews and meta-analyses analyzed the efficacy of yoga in treating mental disorders (1, 8–12), the results are summarized in eTable 1. The meta-analyses reported promising findings and mostly concluded that yoga is effective in treating different mental disorders in regard to diverse disorder-specific outcome measures. Only a meta-analysis of data from schizophrenia patients (10) found no significant effects on symptom-related endpoints, only on quality of life.
In evaluating the available evidence from meta-analyses it needs to be borne in mind that differential effects of yoga have thus far barely been investigated in the treatment of mental disorders. Meta-analyses across mental disorders have not yielded any data; in the disorder-specific analyses, the small number of included studies makes subgroup analyses impossible.
The present article aims to investigate the efficacy of body-oriented yoga in treating mental disorders and to study possible factors of influence. To this end, we conducted a systematic review and meta-analysis of randomized controlled studies.
The objectives, inclusion criteria, and methods of this review were pre-specified in a review protocol (PROSPERO Reg. No.: CRD42014015347).
The systematic review included studies that met the criteria listed in Table 1.
In order to identify relevant studies, searches were conducted in the electronic databases PubMed, Web of Science, PsycINFO, ProQuest, and by using the search engine Google Scholar (eTable 2). Furthermore, manual searches for additional relevant studies were undertaken for the specialized journals International Journal of Yoga, International Journal of Yoga Therapy, Journal of Yoga and Physical Therapy, and Indian Journal of Psychiatry (special issue vol. 55, suppl.3), as well as the reference lists of review articles published to date and the included primary studies.
Study selection and extraction of relevant information
Firstly, we checked whether titles and summaries/abstracts provided any information of whether the studies met the inclusion criteria. We then obtained the full text of relevant studies and, where appropriate, extracted relevant information from those studies.
Where important data were missing we contacted the authors. Where insufficient relevant statistical data were reported for the effect size calculation and these could not be obtained by contacting the authors, these were approximated by using diverse estimation procedures. Two assessors undertook the study selection and coding (RK, JR). Disagreements were resolved by consensus discussions.
Risk of bias in the individual studies
The risk of bias in the individual studies was assessed by applying the following quality criteria from the Cochrane Collaboration (e9): selection bias (randomization, allocation concealment), performance bias (blinding of patients and staff), detection bias (blinded collection of endpoints), attrition bias (handling missing data because of subjects dropping out of studies), and reporting bias (selective reporting of results). Furthermore, we collected data in potential conflicts of interest of the study authors.
Effect size estimates
We calculated adjusted mean differences (Hedges’ g) for each study endpoint (e10). In case of dichotomous endpoints we determined odds ratios and converted these into Hedges’ g (e11). If a study reported several effects for the same endpoint, these were pooled. All effect size estimates were reported with 95% confidence intervals (CI). Positive effects indicate an advantage in favor of the yoga intervention, negative effects in favor of the control intervention. The effect size Hedges’ g provides in small samples more precise estimates. In analogy to the conventions of Cohen’s d, values of 0.20 are interpreted as small effects, 0.50 as medium-sized effects, and 0.80 as large effects (e12). Furthermore, the effect sizes were transformed into numbers needed to treat (NNT, e13).
Synthesis of results
The effect sizes of individual studies were aggregated in a meta-analysis by using a random effects model and were weighted by using the inverse variance method. The heterogeneity of study results was estimated by using the method of DerSimonian and Laird (e14). Heterogeneity between studies was tested for significance by using Q heterogeneity tests (Cochran’s Q), and I2 was calculated as the common measure for heterogeneity. I2 values of >50% were interpreted as substantial, any generalizability of the results is consequently limited (e15).
We conducted separate meta-analyses for the comparison of the yoga intervention groups with the untreated control groups (waiting list) and the active control groups (psychotherapeutic standard treatment, attention control group, exercise).
Risk of bias across studies
In order to check for publication bias, funnel plots were assessed visually, and Egger’s test (e16) and the Trim and Fill procedure (Duval and Tweedie, e17) were conducted.
To compare the yoga intervention groups with untreated control groups, meta-regression analyses and subgroup analyses were undertaken, in order to study heterogeneity of study effects and potential factors of influence.
The software package Comprehensive Metaanalysis (CMA; version 3.0; Biostat Inc) was used to conduct all data analyses.
A total of 2644 references were identified during the literature search; of these, 25 studies with 27 comparisons were included (Figure 1).
Central characteristics of the included studies are summarized in eTable 3. In total, studies from nine different countries were evaluated that had been published between 1997 and 2014. Most of the studies came from India (n=7) or the United States (n=8). The total sample from all included studies amounted to 1339 subjects, of whom 656 were in the yoga intervention groups and 683 in the control groups. The dropout rate in the yoga intervention groups was 13.0% (standard deviation [SD]=13.1) and in the control groups 16.4% (SD=17.8; P=0.408). The participants’ mean age was between 22 and 59 years (median 36; interquartile range [IQR]: 30–43). The proportion of women in the total sample was 61.7%. The included studies investigated the efficacy of yoga in relation to nine different mental disorders, albeit exclusively axis-1 disorders. Only 11 primary studies reported the mean duration of illness, which was between five and 313 months (median: 86; IQR: 74.3–162.8).
In most of the studies (n=19) the intervention was directed by a yoga teacher. Yoga was conducted in groups in all studies. Group sizes varied between three and 25 participants (median:11.5; IQR: 5.8–15.0). The total duration of the yoga intervention was between five and 100 hours (median: 15; IQR: 10–32) over a time period of two to 24 weeks. In 17 of 25 studies, patients received a co-intervention (baseline treatment) in the intervention group as well as in the control group. Further characteristics of the included studies are described in eTable 4.
Risk of bias within studies
The way in which the risk of bias was assessed in the included studies by using the quality criteria of the Cochrane Collaboration is shown in eTable 5.
Altogether, the included studies were subject to comparably high bias risks, especially in relation to blinding patients/staff. In most studies, incomplete data resulted in a high risk of bias (completer analysis).
Results from individual studies and synthesis of results
Comparisons between yoga intervention groups and untreated control groups in terms of primary outcome variables at the end of treatment showed a significant, large effect for yoga; Hedges’ g=0.91; 95% CI [0.55; 1.28]; P<0.001; k=13; NNT=2.03 (Figure 2). However, the effects were subject to substantial heterogeneity (I2=69.8%). The comparison of yoga intervention groups and active control groups showed a small significant overall effect in favor of the yoga intervention; Hedges’ g=0.26; [0.09; 0.44]; P=0.004; k=12 (Figure 3). Although yoga showed small significant effects compared with exercise (g=030; [0.01; 0.59]; P=0.040; k=5; NNT=5.75) and attention control group (g=0.39; [0.04; 0.73]; P=0.027; k=4; NNT=4.55), comparable effects were seen for yoga and psychotherapeutic standard treatment (g=0.08; [–0.24; 0.40]; P=0.625; k=3; NNT=21.89) when used as complementary treatment with medication. The study effects were not or only very slightly heterogeneous (I2=15.6% for the comparison with exercise; I2=0% for all other comparisons). Comparable effects were seen for secondary outcome variables (eTables 6 and 7).
Risk of bias across studies
In the comparison of yoga intervention groups and untreated control groups the Trim and Fill analysis found two missing studies. The adjusted effect size (g=0.75; [0.36; 1.13]) did not differ substantially from the observed effect size; furthermore, Egger’s regression test did not reach significance (P=0.239). For the comparison of yoga intervention groups with active control groups, no indications of the existence of publication bias were seen, neither in the Trim and Fill analysis nor in Egger’s regression test (P=0.299) (eFigure).
Meta-regression analyses were conducted to gain clarity about the effects of the duration and frequency of the yoga intervention and of the publication year on effect size estimates. Only the effect of the publication year reached significance (β=–0.13; [–0.24; –0.02]; P=0.026), with more recent studies reporting lower effects. Furthermore, subgroup analyses were conducted, which showed a significant effect for the treatment context and the type of recruitment. In subjects who were being treated as inpatients or outpatients, yoga had a significantly lower effect on symptom-related outcomes than in subjects who were not currently therapeutically treated. Studies that had recruited subjects through the healthcare system had a significantly lower effect than studies in which recruitment had been realized by using different media. Furthermore, studies conducted in an Eastern cultural setting showed lower effects than studies from the Western cultural setting.
No effect was confirmed for diagnosis, standardization, and disorder-targetedness of the yoga intervention (Table 2). Neither did risks of bias have a moderating influence on effects (eTable 8).
The comparison of yoga intervention groups and untreated control groups found medium-sized significant effects in favor of yoga for primary and secondary outcomes. However, study effects for disorder-specific symptoms were characterized by substantial heterogeneity. Compared with exercise and attention control group, yoga was found to have small, significant effects. Furthermore, the findings indicate equal efficacy for yoga and psychotherapeutic standard treatment when used complementary to medication treatment. Additionally, other studies have shown that the efficacy of yoga combined with psychotherapeutic treatment—for example, cognitive behavioral therapy—exceeds the individual effects of the interventions (40).
The findings of this systematic review are comparable to those of existing meta-analyses, most of which reported moderate to large positive effects of yoga in disorder-specific symptoms (1, 8, 9, 11, 12). For the comparison of yoga intervention groups with untreated control groups, different factors were identified that influenced the efficacy of yoga. Among other factors, the year of publication has a role: more recent studies found smaller effects than older studies. This effect was also seen in other meta-analyses (e18, e19). Among the possible explanations that have been discussed for this finding are larger numbers of endpoints and their increasing standardization in more recent studies.
Furthermore, the treatment context was found to be a significant factor of influence. Patients who were being treated on an inpatient or outpatient basis benefited less from yoga than subjects who were not currently receiving treatment because of their psychological symptoms. These results possibly reflect the fact that the severity of the disorder may be a relevant factor of influence on the efficacy of yoga, something that one meta-analysis has already confirmed (12).
The fact that the way in which subjects are recruited has an influence possibly reflects selection bias: subjects that were recruited through media campaigns may have a more positive attitude towards yoga and may therefore benefit more from it. The marginal significant difference in efficacy related to subjects’ cultural setting can possibly be explained by the fact that yoga represented a lower added value for patients from an Eastern cultural setting because it is very popular in these settings, and many people practice it.
Comparing yoga intervention groups with untreated control groups identified substantial heterogeneity of the study effects, which can be explained only partly in moderator analyses. The relevant findings can therefore be generalized only to a limited degree. Compared with untreated control groups, indications were found of publication bias, but the adjusted effects do not differ substantially. Furthermore, the risk of bias within the studies is altogether to be considered as high, and for many studies the relevant assessment was not possible because of lacking data. Lacking information in the studies further makes it difficult to assess central study characteristics, such as characteristics of the interventions or patients (for example, group size, components of the yoga intervention, affinity of recruited subjects for yoga). This often leads to a situation whereby these studies cannot be considered in analyzing factors of influence, which ultimately reduces statistical power or renders relevant analyses completely impossible. The results of the available analyses should therefore be interpreted with caution. It also should be noted that they relate exclusively to axis-I disorders.
In spite of the listed limitations, the results of this meta-analysis provide indications that body-oriented yoga with the central components asanas and pranayama represent a promising complementary approach to the treatment of mental disorders. Further high-quality studies are needed in order to support these findings.
Our meta-analysis included 25 studies and provides a comprehensive overview of the efficacy of yoga in mental disorders. We were able to analyze a large proportion of studies that had not previously been included in systematic reviews. Additionally, potential factors influencing the efficacy of yoga were studied for the first time. The results provide indications for the differential efficacy of yoga, which future studies should investigate in a targeted fashion.
Conflict of interest statement
The authors declare that no conflict of interest exists.
Manuscript received on 22 September 2015, revised version accepted on
16 December 2015.
PD Dr. phil. Jenny Rosendahl
Institut für Psychosoziale Medizin und Psychotherapie
07743 Jena, Germany
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Institute of Psychology, Department of Research Synthesis, Intervention and Evaluation,
Friedrich-Schiller-University Jena: Dipl.-Psych. Pabst, Prof. Dr. phil. Beelmann
AHG Psychosomatische Klinik Bad Pyrmont: Dipl.-Psych. Pabst
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