Previous reviews have reported that yoga is beneficial for people with cancer in managing symptoms such as fatigue, insomnia, mood disturbances and stress, and improving quality of life [33
]. However, to our knowledge, until now the size of the effect has not been quantified. We conducted the first meta-analysis of studies investigating yoga interventions for patients with cancer. Data were extracted from 10 RCTs with a total of 762 participants with cancer. The results of our meta-analysis suggest that yoga may have positive effects on psychological health of cancer patients.
Many cancer patients experience cancer-related psychological symptoms, including mood disturbances, stress, and distress [33
]. The results of our study revealed the efficacy of yoga on psychological health for cancer patients and are consistent with the result of a meta-analysis conducted by Ledesma and Kumano. [46
], which indicated that mindfulness-based stress reduction programs may indeed be helpful for the mental health of cancer patients. Also, Carlson et al. showed that MBSR significantly improved the overall symptoms of stress in cancer patients and these improvements were maintained over a year of followup [47
]. Thus, yoga may have long-term psychological effects for patients with cancer.
However, the present findings do not address whether the psychological health benefits were attributable directly to yoga as a whole or the specific components of yoga, such as meditation and attention, in patients with cancer. Given that several yoga programs included meditation and relaxation with imagery, the positive results on psychological health might be obtained from these. Nevertheless, because of the nature of yoga interventions, it is impossible to control for placebo effects in investigations.
Although most RCTs reported anxiety, depression, and stress as outcome measures, the assessment tools used to measure their outcomes were inconsistent, which limits the generalization of the pooled results. Future research should focus on higher-quality trials with larger sample size in order to provide more precise estimates of the effects of yoga as a treatment.
Three of 10 RCTs reported quality of life outcomes, and the results showed a borderline difference between two groups. In contrast to the results of previous studies [48
], our results only showed a trend toward a small positive effect of yoga on quality of life. This may be due to the small sample size and the limited number of studies available for analysis. Moreover, different QOL measures were used across the studies, future study should develop and use standardized measures agreed by the research community.
Our results showed that the overall effects for physical health outcomes were statistically nonsignificant. According to the previous review [34
], no significant differences were observed on the measure of physical health. Because of the limited number of studies and different measurement tools, the effects of yoga on physical health in people with cancer remain unclear. Furthermore, the studies included in the analysis used only the subscale of subjective questionnaires to report on physical health, meaning that our conclusions should be taken cautiously. Only one study [23
] examined the effects of yoga on physical fitness; therefore, future study could include outcome measures that not only include subjective feelings in questionnaires but also include physical performance tests such as the 6-minute walk test, physical strength, endurance, and flexibility.
Moreover, our results did not show positive effects of yoga on fatigue. According to Sood's review [50
], insufficient data exist at present to recommend any specific complementary and alternative medicine modality for cancer-related fatigue. Future studies with more participants and with a randomized clinical trial design should be conducted to investigate the effects of CAM interventions, including yoga, on cancer-related fatigue of cancer patients.
All studies included in the meta-analysis investigated participants with a diagnosis of cancer; however, the types of cancer varied among studies. Of the 10 included studies, 7 investigated breast cancer, 2 recruited mixed cancer populations, and 1 included patients with lymphoma. The result of Cohen's study on lymphoma [25
] showed no significant differences between groups in terms of anxiety, depression, distress, or fatigue; thus, it has little influence on our result. Therefore, since the majority of studies focused on breast cancer, future research needs to examine the use of yoga among male cancer patients and female nonbreast cancer patients.
In addition, various factors are associated with the execution of the intervention such as yoga styles and treatment doses that may influence effect size. Four different styles of yoga were used among the included studies: restorative, integrated, hatha, and Tibetan. Treatment dose, including duration and frequency, and the adherence to yoga intervention and home practice may also affect treatment outcome. According to Carson's study on yoga for women with metastatic breast cancer [51
], patients who practiced yoga longer on a given day were much more likely to experience less pain and fatigue and greater invigoration, acceptance, and relaxation on the next day. Future study needs to report adherence with the intervention protocol and the home practice to scrutinize the “dose-response” relationship between frequency and duration of yoga programs and changes on health outcomes.
All studies had inevitable limitations such as that it was not possible to blind subjects or therapists from group allocations in this type of empirical study. Therefore, the highest possible score that each study could get would be 8 out of 10 when using a PEDro scale or other types of quality criteria.
Considerable heterogeneity existed (I2
> 70%) when the effects of yoga on anxiety, depression, and stress were compared with control groups. Several potential sources of clinical heterogeneity are to be considered when interpreting the results. These include population sample studied, treatment maneuver, and study design and methods [44
]. Therefore, the random effect model was used as it addresses the variability that exists among studies.
The sensitivity analysis suggested that the effect of yoga was consistent across the intervention format and the types of cancer patients. As the literature search in this study was restricted to articles published in Chinese and English, this may introduce publication and language bias. Furthermore, this meta-analysis is limited due to the possibility of missing eligible unpublished or non-English studies and the fairly homogeneous studies included in the analysis. However, given the small number of studies included, the assessment of the heterogeneity or publication bias was difficult in this exposure.