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The Many Health Benefits of Yoga: What Does the Research Say? Part II
According to a comprehensive review of yoga research reviews and meta-analyses published in Complementary Therapies in Clinical Practice, the field of yoga research has grown exponentially in the past 5 years, with nearly 200 studies being published annually. As we continue to gain evidence into how yogic principles and practices can be used to maintain and restore health, we will be more equipped to use these technologies in our daily lives, and incorporate them into mainstream health care.
In Part I of this yoga research article, we examined current trends in yoga research, as well as who is practicing yoga, how, its effectiveness for relieving symptoms of stress and psychological illness, and its overall safety for the general public, and for pregnant women. In Part II, we will explore the research on the benefits of yoga for a breadth of physical ailments and illnesses.
Yoga and Heart Health
As we reviewed in Part I, one of the greatest known benefits of yoga is its potential to relieve stress. Decades of research points to chronic stress as a key risk factor for physical and psychological illness. When it comes to the heart, stress is linked to the onset and progression of most cardiovascular conditions such as heart attack and stroke, as well as precursors of heart disease like hypertension, high cholesterol, and Type II Diabetes.
Yoga and Prehypertension
Hypertension, also known as high blood pressure, is one of the leading causes of heart attack, stroke, and premature death. Hypertension has reached epidemic proportions in the United States, with approximately 20.1% of Americans being diagnosed with the disease in 2011-2012. Prehypertension is a condition characterized by elevated blood pressure (systolic BP 120-139 and diastolic BP 80-89). It is often the precursor to full blown hypertension, and is the target of aggressive intervention.
In a randomized controlled trial comparing yoga to lifestyle modification (healthy diet, exercise etc.) and yoga plus lifestyle modification in adults with prehypertension, participants in the yoga plus lifestyle group showed significant decreases in systolic blood pressure (BP) compared to the yoga only group, which experienced a significant decrease in diastolic BP. This suggests that yoga may contribute to lifestyle modifications aimed at reducing cardiovascular disease risk.
Yoga for Hypertension
Considerably more research has addressed the effects of yoga interventions on hypertension (high BP). A systematic review of 17 randomized controlled trials found 11 studies that linked regular yoga practice to significantly greater reductions in systolic BP compared to pharmacotherapy, breath awareness, health education, no treatment, or usual care. Conversely, 5 studies found that yoga had no effect on systolic BP compared to dietary modification, enhanced usual care, relaxation or physical exercise.
Of these studies, 8 trials showed that yoga was linked to greater reductions in diastolic BP when compared to pharmacotherapy, no treatment or usual care, however the other 8 studies reported no effects of diastolic BP relative to control conditions. As such, the evidence of yoga as an ancillary treatment for hypertension is encouraging, but inconclusive.
Yoga and Cholesterol
High cholesterol is also considered a significant risk factor for heart disease. Similar to the findings on yoga for hypertension, the scientific evidence is encouraging but mixed.
In a systematic review and meta-analysis of 44 randomized controlled trials that included over 3,100 participants, those participating in a yoga intervention showed improved HDL, VLCI, triglycerides and insulin resistance - all markers of cholesterol - compared to usual care or no treatment. Yoga participants also showed superior declines in HDL.
Results from this meta-analysis also revealed improved systolic and diastolic BP, and lowered heart and respiratory rates, as well as decreased waist circumference in the yoga groups compared to no intervention controls. Similarly a review of 37 RCTs showed that yoga participants had significantly better results on measures of HDL, LDL, total cholesterol, triglycerides, systolic BP and heart rate compared to non-exercise controls. While this is encouraging, authors of the study suggest interpreting results with caution due to the lack of high-quality research in some cases.
Yoga for Pain Reduction
A great deal of yoga research has focused on pain syndromes including arthritis, neck and back pain, headaches, and the underlying mechanisms of pain reduction in yoga. Although there is much more to be learned, initial findings are cause for optimism.
Yoga for Arthritis
Roughly 21% of American adults suffer from some form of arthritis, making it one of the leading causes of chronic pain and disability. In a systematic review studies in which yoga was used to relieve the pain of arthritis, 6 of 9 studies reported positive physiological and psychological outcomes. Another recent review of 12 trials involving 589 participants found that regular yoga practice was associated with decreased pain, stiffness and swelling. Unfortunately, these studies were hampered by a high degree of variability in approach, dosage, and inconsistent measurement.
Similarly, randomized controlled trials in which yoga was used to alleviate the pain of osteoarthritis in the knee have shown larger improvements in walking pain, knee disability, joint tenderness, swelling and crepitus, as well as greater range of motion when compared to therapeutic exercises or a combination of transcutaneous stimulation and ultrasound treatment.
A randomized controlled trial examining the long-term effects of yoga for those with rheumatoid arthritis, likewise found that those in the yoga group outperformed waitlist controls on measures of walking, flexibility and quality of life following 8 weeks of twice weekly, 60 minute classes, as well as 9 months later. More high-quality studies will be needed to better understand why and how interventions that incorporate yogic movements can benefit individuals with arthritic conditions.
Yoga for Relieving Neck Pain and Headaches
Chronic neck pain is also a debilitating condition that affects millions of adults worldwide. In a study comparing groups receiving either 12 sessions of yoga or Pilates, researchers found that both were comparable in decreasing disability and pain. These studies used postural modifications to assure the safety of participants, meaning that the approach varied by individual, which makes it difficult to ascertain how these benefits were achieved.
There is also some preliminary evidence that regular yoga practice may help to alleviate chronic migraine pain. In one randomized controlled study, individuals were assigned to either a medication or a medication plus yoga group. Those receiving yoga training reported significant decrease in headache frequency and severity compared to controls. Correspondingly, declines in reported headache intensity and frequency were more pronounced in yoga participants in a study evaluating the difference between conventional care and conventional care plus yoga. The yoga group also evidenced an increase in vagal tone, and decreased sympathetic nervous system activity – both indicators of reduced physiological stress.
Yoga for Low Back Pain
The lifetime prevalence of low back pain in industrialized countries is estimated at 60% to 70% of the population, making it one of the most pervasive forms of chronic pain. A global study of over 290 physical conditions ranked low back pain as the number one cause of disability. As a result, researchers are beginning to pay greater attention to it as a target of yoga intervention.
Results of a 2013 meta-analysis of yoga for low back pain found that individuals who participate in regular yoga practice show significant reductions in short- and long-term low back pain, and back pain-specific disability compared to controls. Similarly, a 2011 meta-analysis found that yoga is beneficial for those suffering from chronic low back pain.
In a randomized controlled study, adults with chronic low back pain were trained in 29 different Iyengar yoga poses. Their pain ratings were then compared to a conventional exercise group. Iyengar poses were held for lengthy periods to assure that they would be equivalent to exercise in their intensity. Both groups showed significant declines in pain and increases in self-reported quality of life, however, the yoga group reported nearly 2 times more pain reduction than the exercise group controls.
Yoga for Autoimmune Conditions
Recent studies have examined the effectiveness of yoga as an alternative for a number of known autoimmune conditions including asthma, chronic obstructive pulmonary disease (COPD), Type II diabetes, multiple sclerosis, irritable bowel syndrome (IBS), and chronic fatigue syndrome.
Yoga for Asthma and COPD
Asthma is characterized by a decreased lung capacity due to constriction of the airways as a result of inflammation. Outcomes of interest typically include forced vital capacity (FVC), peak expiratory flow (PERF), and forced expiratory volume (FEV).
A systematic review and meta-analysis of 14 randomized controlled trials in which yoga iwas used to relieve asthma symptoms found that yoga groups showed significant improvements in PEFR and FVC compared to usual care controls. Improvements were also superior in the yoga groups for biochemical variables including a greater decrease in total leukocyte count in one study after 6 months of yoga practice. Yoga groups also performed better than psychological interventions on measures of PEFR.
Fewer studies have examined the benefits of yoga for those with COPD. One systematic review and meta-analysis of 5 randomized controlled trials found that yoga significantly increased forced expiratory volume in 1S and 6 minute walking distance, which suggests that yoga practice may enhance exercise capacity and lung function.
Yoga and Type-II Diabetes
Type II Diabetes is a common target for yoga interventions because of the risk that it poses for a variety of complications including heart and blood vessel disease, nerve damage (neuropathy), kidney disease, hearing impairment, and Alzheimer’s disease.
A systematic review of 25 yoga trials that included 2170 participants found that those practicing yoga experienced numerous benefits including improved lipid levels and glycemic control, reduced oxidative stress and blood pressure, and better pulmonary and autonomic function. This may have been related to the reduced need for medication and improved sleep also found among those assigned to yoga programs.
Multiple Sclerosis (MS)
Multiple sclerosis (MS) is the most common autoimmune disease of the central nervous system, impacting roughly 2.3 million people worldwide. Symptoms include imbalance, impaired mobility, fatigue, chronic pain, cognitive, visual and speech impairment, sensory disturbance, depression, bowel and bladder dysfunction and spasticity.
A systematic review and meta-analysis of 7 randomized controlled trials comparing yoga with usual care showed greater improvements in the yoga group on measures of depression and fatigue compared to controls. When yoga was compared to walking, however, both groups yielded similar improvements in mood scores, with the exception of a study of yoga compared to treadmill walking, in which yoga participants showed greater improvements on reaction time during a cognitive challenge. Additional, high-quality studies will be needed to better understand these results.
Yoga for Irritable Bowel Syndrome (IBS)
IBS is the most widespread gastrointestinal disorder, with prevalence estimates running around 10-15% globally. Although yoga interventions for IBS have not been widely studied, research suggest that yoga may be associated with significant reductions in pain, constipation and nausea, as well as reductions in symptom severity. Sadly, studies of yoga for IBS have been plagued by high drop out rates, making it difficult to assess whether this type of intervention may be applicable and beneficial for those with IBS. Further studies are needed to solve this mystery.
Yoga for Chronic Fatigue Syndrome
To date there is only one study examining the effectiveness of yoga for chronic fatigue syndrome symptom reduction. In the study, fatigue ratings decreased in the yoga group compared to waitlist controls following biweekly, 20-minute sessions and in-home practice for approximately 2 months.
Yoga and Cancer Symptom Reduction
Cancer has long been a focus of yoga researchers. Given its diverse etiology, treatment, and symptoms, and clinical side-effects, findings to date are mixed.
Yoga for Breast Cancer
Breast cancer is among the most widely studied form of illness. Randomized controlled trials, in which a yoga intervention is compared with either a passive (usual care) or active (psychotherapy, exercise, stretching, education etc.) control group point to generalized improvements in symptoms including sleep disturbance, depressed mood, cortisol levels, physical functioning, and lymphedema.
Studies in which outcomes following a yoga intervention were compared to active exercise and stretching conditions have yielded mixed data, with results being similar between the 2 groups.
Colorectal Cancer and Yoga
To date there is one published study of a yoga intervention for colorectal cancer. Following a 10-week, 90-min weekly yoga program, participants noted fewer sleep disturbances. Unfortunately, this study suffered from high dropout rates, making its utility and benefit questionable. In another small study using “laugher yoga”, adults with colorectal cancer reported decreased chemotherapy-related stress.
Yoga for Aging Conditions
A great deal of emphasis has been placed on the use of yoga to remedy the effects of aging including loss of balance, reduced bone density, cardiopulmonary disease, and dementia and related disorders including Alzheimer’s and Parkinson’s Disease.
Yoga and Improved Mobility and Balance
A single study assessed the pre- and post-intervention effects of a yoga intervention on postural control, gait speed, and mobility following 12-weeks of bi-weekly yoga classes. By the end of 12 weeks improvements were found in all 3 outcomes.
One systematic review of 6 randomized trials noted improved mobility and balance among adults participating in yoga interventions. Several randomized controlled trials have also addressed fall reduction among older adults. In one study, adults were randomized to either a yoga program or a no treatment control group. Yoga participants should improvements in balance and fewer falls compared to controls.
Other research has shown that participants in a yoga group performed similarly in measures of balance and postural stability as those in a tai chi control group, with both mind-body modalities being associated with fewer falls.
Yoga for Osteoporosis
The evidence for the effects of yoga for osteoporosis is slowly building, however most studies have been riddled methodological problems, lack of intervention specificity, and mixed results. In one study of postmeopausal women with osteoporosis, 6 months of a combination of an assortment of weight-bearing and non-weight-bearing poses, meditation and breathing exercises was associated with improvements in DEXA scan bone density readings at the end of training.
In another study comparing 8 months of 60-minute yoga sessions twice per week to usual care controls, yoga participants showed small effects on bone formation, but no effects on bone resorption.
Yoga for Parkinson’s and Alzheimer’s Disease
Emerging research suggests that yoga and meditation may be beneficial for those with Parkinson’s and Alzheimer’s Disease. To date, studies of adults with Parkinson’s have been limited, however in a single study comparing “high-speed yoga” with “power training” and a no activity control group, both active groups showed significant improvements in balance, postural sway, and single leg standing following 12 weeks of training. No differences were detected between the yoga and power training groups.
In one study of 9 adults with moderate to severe Alzheimer’s, all participants exhibited consistent and statistically significant improvement in balance by the end of a chair yoga program.
The findings are even more impressive for studies that combine yoga and meditation. In one study of adults with mild cognitive impairment, 20 minutes of daily Kirtan Kriya meditation per day, and one hour of Kundalini yoga per week outperformed a targeted memory training control group on measures of visuo-spatial memory, depression, anxiety, and resilience to stress.
Similarly, in a 6-month study of 53 adults who were randomly assigned to either a meditation training program, or a music listening class, members of both groups showed marked improvements in perceived stress, psychological wellbeing, and quality of life - all risk factors for dementia. These findings were particularly pronounced in the meditation group.
Yoga for Sleep Quality
Sleep quality is one of a multitude of variables considered in studies looking to reduce the risk of cognitive decline in elderly adults. As such, there are very few studies that specifically examine the benefits of yoga on sleep.
In one study, older adults with insomnia participated in 12 weeks of twice weekly yoga classes. At the end of the intervention they reported better sleep quality and efficiency, longer sleep latency and duration, and reduced depression, anxiety, stress and anger scores compared to a no treatment control group.
How Yoga Works – Mechanisms of Change
Reviews of the literature suggest that yoga practice can impact multiple systems with in the body to affect wellness and increase resilience. The strongest evidence in both quality and quantity is related to yoga’s positive effects on hormone regulation, including modulating salivary cortisol – an indicator of stress - elevating mood through increased serotonin production, and released oxytocin and metatonin, both of which are related to improved sleep quality and immunity.
Metabolically, studies suggest that regular yoga practice is linked with improved glucose tolerance and insulin sensitivity, and clinically significant changes in fasting plasma glucose (FPG) and postprandial plasma glucose (PPPG) levels, all important indicators of diabetes management.
In terms of nervous system function, yoga is commonly believed to directly influence sympathetic and parasympathetic nervous system activity. Specifically, breathing exercises (pranayama), visualization and calming techniques, and physical postures (asanas) increase parasympathetic activation. Yoga has also been found to elevate levels of gamma-aminobutyric acid (GABA), and regulate the hypothalamic-pituitary-adrenal (HPA) axis. The latter are related to decreased stress, anxiety and depression and improved subjective well-being.
We’ve long known that yoga is mind-body “medicine”, affecting individuals holistically rather than changing discrete elements of their anatomy, physiology, or mind. Although there is considerably more to learn via high quality yoga studies, this review suggests that a regular yoga practice has myriad benefits for overall health, wellness, disease prevention, and symptom reduction.
Field, T. (2016). Yoga Research Review. Complementary Therapies In Clinical Practice, 24, 145-161.
B Grace Bullock, PhD, E-RYT 500 is a psychologist, research scientist, educator, yoga and mindfulness expert and author of Mindful Relationships: Seven Skills for Success - Integrating the Science of Mind, Body and Brain. Her mission is to reduce stress, increase health and well-being and improve the quality of relationships. She offers classes, workshops, writing and research that combine the wisdom of applied neuroscience, psychophysiology, psychology and contemplative science and practice. Her goal is to empower individuals, groups, leaders and organizations to reduce chronic stress and increase awareness, attention, compassion, mindfulness and effective communication to strengthen relationships, release dysfunctional patterns and unlock new and healthy ways of being. Dr. Bullock is also the Founding Director and Principal Consultant of the International Science & Education Alliance, an organization devoted to exceptional research, program evaluation, assessment design, strategic planning and capacity building to support equity, programmatic diversity and scientific integrity, and promote effective leadership, decision-making and social change. Bullock is a Certified Viniyoga Therapist and Faculty at the Integrated Health Yoga Therapy (IHYT) Training program. She is the former Senior Research Scientist at the Mind & Life Institute and former Editor-in-Chief of the International Journal of Yoga Therapy. For more information see www.bgracebullock.com.