The field of yoga research has boomed in the past decade. This means that we are learning far more about why and how yoga works (and doesn’t work) than we have ever had before. This is exciting news for the millions of yoga practitioners, educators and therapists interested in using the principles of yoga to attain health and wellbeing.
Several years ago when I wrote the article, Why Yoga Research Has A Long Way to Go, many yoga studies were low in quality, suffering from small sample sizes, low statistical power, weak research designs, poor measurement, and ill-defined interventions. As a result, it was difficult to be confident about the results. Although many of these factors persist, a new article published in Complementary Therapies in Clinical Practice,finds that the volume of high quality research continues to grow. Here is what we have learned so far.
Trends in Yoga Research
There has been a notable rise in the volume of yoga research since 2007, with approximately 200 studies being published each year since 2011. One analysis of yoga studies found 486 articles from 29 different countries including more than 28,000 participants published in 217 different peer-reviewed journals between 1967-2013.
More importantly, this analysis revealed that 45% of the studies included randomized controlled trials (RCTs). RCTs are preferable because they allow us to compare the outcomes of a yoga intervention group with those of a control group. We then can infer that any changes in the outcomes of interest are the result of the yoga itself, rather than purely by chance. Of those studies using comparison groups, the majority used a no-treatment control group, while the remainder included an active group that received either physical exercise (43%), relaxation/meditation (20%), or some form of education (16%) as alternatives to yoga.
Who Is Practicing Yoga and Why?
A 2016 survey noted that roughly 21 million Americans reported practicing yoga during the past year. Of those respondents, 66% indicated that their primary reason for practicing was to increase energy, followed by improving immune function (55%), and health and disease prevention (28%). Although the population of yoga students is becoming more demographically diverse, the majority of practitioners continue to be white, educated women. But given current trends, that picture is likely to shift as more men and women of all ages, sizes and backgrounds continue to adopt a regular yoga practice.
Who Is Being Studied and Why?
Most yoga studies continue to be conducted with adults (85%), with 34% of those individuals being elderly. To date, only 10% of studies include children. Generally speaking, yoga studies include participants with a target condition such as type 2 diabetes, depression, asthma or breast cancer, as opposed to a smaller number that included healthy individuals (27%). Of the published research, the most common targets for intervention include pain, stress, anxiety, depression, and the effects of cancer treatment.
What Types of Yoga Are Being Studied?
There continues to be considerable variability in the styles and traditions on which the interventions are based. For example, only 11% of studies used some form of Hatha yoga, with the remaining 89% employing one or more of 46 other styles of yoga. Because of this variability, and the lack of replication of these studies, it is presently impossible to understand which aspects of a yoga intervention are reliably related to change.
Overall Physical Benefits
The most commonly assessed physical benefits of yoga practice include flexibility, physical fitness, balance, muscular endurance, and spinal mobility.
In terms of flexibility, yoga was most often related to improvements in flexibility in the lumbar spine and hamstrings. For strength, evidence shows that postures like downward facing may strengthen abdominal musculature, whereas positions like warrior pose and chair pose enhance gluteus maximus muscle strength. Other studies suggest that both upper body and abdominal muscular endurance increase as a function of frequent yoga practice.
Overall, yoga appears to be associated with greater gains in flexibility and balance compared to other forms of exercise, and slow movements tend to be more effective in attaining greater flexibility than rapid postural shifts.
Although there are few studies of the cognitive benefits of yoga to date, emerging evidence from one systematic review and one meta-analysis suggest that regular yoga practice is linked to improvements in working memory, reaction time, attention, processing speed and overall executive function.
The greatest evidence of cognitive benefit comes from studies of experienced meditators, and those using meditation as a primary intervention. These studies consistently show that regular meditation is linked to increased volume and connectivity of the prefrontal cortex (executive functioning) and hippocampus (memory), and decreased volume of the amygdala (emotional reactivity). Further research is needed to assess the overall functional significance of these findings.
Effects on Stress, Emotional Wellbeing and Mental Health
The effect of yoga on physical and psychological stress continues to be one of the most widely studied outcomes in the yoga research literature. This is likely due to the fact that stress, particularly when chronic, has the potential to harm all of the major systems in the body (cardiovascular, respiratory, gastrointestinal, endocrine, immune, musculoskeletal etc.), and that yoga practices that emphasize relaxation appear to be uniquely effective in mitigating the stress response.
Yoga and Stress Reduction
In a review of the studies of yoga for stress reduction, researchers found that 12 of 17 studies showed positive physiological and psychological outcomes following a yoga intervention. Researchers are continuing to explore the impact of yoga on four major stress mechanisms: the posterior hypothalamus where cortisol is first released, overall cortisol levels in the bloodstream, and amounts of interleukin-6 and C-reactive protein(both markers of inflammation). This will likely be a promising direction for future yoga studies.
Yoga and Emotional Wellbeing
Generally speaking, yoga research on emotional wellbeing has focused on changes in positive and negative emotions. Studies consistently show that positive emotion tends to increase following participation in a yoga class or program. It is thought that these emotional benefits may be related to functional changes in brain connectivity in structures including the caudate, basal ganglia, and cortical-thalamic feedback loops.
Unfortunately, this area of research has been hampered by factors mentioned earlier such as small sample sizes, different styles of yoga interventions, lack of matched control groups, and variable doses of yoga interventions. More research will be needed to disentangle how and why yoga interventions may be associated with mood enhancement.
Yoga and Posttraumatic Stress Disorder (PTSD)
PTSD is one of the most severe forms of chronic stress, often leading to debilitating levels of anxiety and mood disturbance, and considerable negative effects on quality of life. Some studies find that yoga interventions can be beneficial for individuals diagnosed with PTSD. Results are not as conclusive when yoga has been compared to other forms of treatment, however.
In general, regular yoga practice has been linked to decreases in PTSD symptoms and depression, as well as reductions in substance use. Additional high quality studies will be needed to further explore yoga’s benefits for those with PTSD.
Yoga and Anxiety and Panic Disorders
In light of its ability to reduce the physiological and psychological effects of stress, yoga seems to hold great promise for the treatment of anxiety-related symptoms. Studies of individuals with mild, moderate and severe anxiety find that consistent yoga practice is associated with decreases in anxiety, as well as reductions in heart rate and systolic blood pressure.
Similarly, in studies with those diagnosed with panic disorder, a more severe form of anxiety, participants reported reductions in both anxiety, and panic-related bodily sensations, although these reductions were most pronounced for the group in which yoga was combined with cognitive-behavioral therapy (CBT).
Yoga and Depression
In recent years, the effects of yoga and meditation interventions have been compared to those of antidepressant medications. Although this research is still in the early stages, reviews suggest that yoga may be effective for reducing symptoms of depression, and that yoga may be uniquely beneficial due to its capacity to alleviate stress, which is often linked with mood disturbance.
In general, studies support the proposition that sustained yoga may provide a useful, adjunct treatment option for those with depression, and help to relieve associated symptoms including ruminative thoughts, anxiety, stress, and poor quality of life.
Yoga and Overeating
One of the mechanisms of overeating explored in yoga studies has been low distress tolerance. Those who have a low ability to manage distress tend to avoid emotional experiences, and engage in distracting experiences such as overeating to ameliorate their negative feelings.
In one study, women who were randomized to a twice-weekly, 8-week Hatha yoga group reported significant reductions in emotional eating compared to wait-list controls. In another study, adults who participated in a 10-day yoga intervention showed reduced weight and BMI, a smaller waist-to-hip ratio, and lower blood glucose and pro-inflammatory cytokine levels. These changes in weight loss and systolic blood pressure were maintained at a 30-day follow-up.
Yoga has become increasingly popular among expectant mothers hoping to relieve stress and increase wellbeing during their pregnancy. This has caused some concern about the safety of certain practices for both pregnant women and their unborn children.
In a study examining the intensity level of a prenatal yoga class for healthy pregnant women, researchers found that women’s energy expenditure was categorized as sedentary 93% of the time, with only 7% of classes being considered of moderate physical intensity. Physiological data collected suggested that there were no pre-to-post session differences in women’s heart rate, temperature, or fetal heart rate, and that there were no falls or injuries during the course of the intervention. These findings persisted as late as 38 weeks into gestation.
On the whole, studies show that yoga is beneficial for expecting mothers, relieving prenatal depression and anxiety, and enhancing fetal growth. A meta-analysis of 6, randomized controlled trials found that depression and anxiety were reduced in yoga group participants. This was particularly the case when practices included breathing and meditation.
A systematic review of 10 randomized controlled trials found that yoga participants had a lower incidence of prenatal disorders, higher relationship scores, fewer reports of pain and stress, and greater gestational age of their offspring. A single study also noted significantly increased fetal growth in mothers attending yoga during the 12th to 28th week of their pregnancy. Although the research is minimal, there are indications that yoga may help to reduce the symptoms of prenatal depression as well.
Is Yoga Safe?
The answer to this question is that it depends on a multitude of factors including practitioner age, health and physical condition, as well as the nature of the asana (physical postures). In general, research suggests that yoga is no more dangerous than physical exercise, however a significant number of individuals have reported that yoga classes are physically and mentally stressful.
In general, studies in which adverse events were reported tended to include elderly individuals, persons in poor physical health, and those with chronic musculoskeletal disease. Additional research is needed to better understand which types of practice are best suited for different ages and stages of life.
Barriers to and Motivators For Practicing Yoga
A number of studies have assessed barriers to practicing yoga, as well what motivates individuals to practice regularly. Among the obstacles are time, cost, stereotypes about the required flexibility, athleticism, and the appearance of yoga students, and a lack of pragmatic information about what yoga entails. Conversely, the motivators for practice included health promotion and emotional well-being, seeking relief from pain, athletic pursuit, and a sense of community.
In general, the research supports the proposition that yoga may be a safe and effective complementary approach to prevention and wellness for a variety of physical and psychological symptoms. In Part II we will examine what is known about the benefits of yoga for medical illnesses including heart disease, pain, auto-immune disorders, osteoporosis, dementia, and other aging-related conditions.
Field, T. (2016). Yoga Research Review. Complementary Therapies In Clinical Practice, 24, 145-161.