Abstract
Objectives: Exercise is considered an acceptable method for improving and maintaining physical and emotional health. A growing body of evidence supports the belief that yoga benefits physical and mental health via down-regulation of the hypothalamic–pituitary–adrenal (HPA) axis and the sympathetic nervous system (SNS). The purpose of this article is to provide a scholarly review of the literature regarding research studies comparing the effects of yoga and exercise on a variety of health outcomes and health conditions.
Methods: Using PubMed® and the key word “yoga,” a comprehensive search of the research literature from core scientific and nursing journals yielded 81 studies that met inclusion criteria. These studies subsequently were classified as uncontrolled (n=30), wait list controlled (n=16), or comparison (n=35). The most common comparison intervention (n=10) involved exercise. These studies were included in this review.
Results: In the studies reviewed, yoga interventions appeared to be equal or superior to exercise in nearly every outcome measured except those involving physical fitness.
Conclusions: The studies comparing the effects of yoga and exercise seem to indicate that, in both healthy and diseased populations, yoga may be as effective as or better than exercise at improving a variety of health-related outcome measures. Future clinical trials are needed to examine the distinctions between exercise and yoga, particularly how the two modalities may differ in their effects on the SNS/HPA axis. Additional studies using rigorous methodologies are needed to examine the health benefits of the various types of yoga.
Introduction
Yoga is an ancient discipline designed to bring balance and health to the physical, mental, emotional, and spiritual dimensions of the individual. Yoga is often depicted metaphorically as a tree and comprises eight aspects, or “limbs.” Long a popular practice in India, yoga has become increasingly more common in Western society. In a national, population-based telephone survey, 3.8% of respondents reported using yoga in the previous year and cited wellness (64%) and specific health conditions (48%) as the motivation for doing yoga.
A growing body of research evidence supports the belief that certain yoga techniques may improve physical and mental health through down-regulation of the hypothalamic–pituitary–adrenal (HPA) axis and the sympathetic nervous system (SNS). The HPA axis and SNS are triggered as a response to a physical or psychologic demand (stressor), leading to a cascade of physiologic, behavioral, and psychological effects, primarily as a result of the release of cortisol and catecholamines (epinephrine and norepinephrine). This response leads to the mobilization of energy needed to combat the stressor through the classic “fight or flight” syndrome. Over time, the constant state of hypervigilence resulting from repeated firing of the HPA axis and SNS can lead to dysregulation of the system and ultimately diseases such as obesity, diabetes, autoimmune disorders, depression, substance abuse, and cardiovascular disease.
Numerous studies have shown yoga to have an immediate downregulating effect on both the SNS/HPA axis response to stress. Studies show that yoga decreases levels of salivary cortisol, blood glucose, as well as plasma renin levels, and 24-hour urine norepinephrine and epinephrine levels. Yoga significantly decreases heart rate and systolic and diastolic blood pressure. Studies suggest that yoga reverses the negative impact of stress on the immune system by increasing levels of immunoglobulin A as well as natural killer cells. Yoga has been found to decrease markers of inflammation such as high sensitivity C-reactive protein as well as inflammatory cytokines such as interleukin-6 and lymphocyte-1B.
These studies suggest that yoga has an immediate quieting effect on the SNS/HPA axis response to stress. While the precise mechanism of action has not been determined, it has been hypothesized that some yoga exercises cause a shift toward parasympathetic nervous system dominance, possibly via direct vagal stimulation. Regardless of the pathophysiologic pathway, yoga has been shown to have immediate psychologic effects: decreasing anxiety and increasing feelings of emotional, social, and spiritual well-being.
Several literature reviews have been conducted that examined the impact of yoga on specific health conditions including cardiovascular disease, metabolic syndrome, diabetes, cancer, and anxiety. These reviews have contributed to the large body of research evidence attesting to the positive health benefits of yoga. Many of the studies compared yoga to other treatment modalities, most commonly to exercise, meditation, and traditional medicine. However, little has been written about what distinguishes yoga from other treatment modalities. The purpose of this article is to present a comprehensive review of the literature regarding the impact of yoga compared to exercise on a variety of health outcomes and conditions.
Methods
A comprehensive search for research articles focusing on yoga interventions was completed from September until December 2008. The articles were identified using PubMed®, the online database of biomedical journal citations produced by the United States National Library of Medicine (NLM®). Using the key word “yoga” and limiting the search to core clinical and nursing journals published in English, 183 articles published after 1970 were identified. Although meditation, one of the eight limbs of yoga, and yoga interventions such as cleansing exercises arguably could be included in a scholarly review of yoga literature, studies solely focusing on these modalities were excluded. Articles were eliminated if they were editorials, anecdotal or single case studies, or of extremely poor quality.
Studies were included in the review if they were of reasonably good quality and involved yoga asana as the primary intervention modality. Quality of studies was determined using the criteria outlined by Greenhalgh. Eighty-one (81) studies met the inclusion criteria and were available at the NLM. Of these, more than half (n=46) were published outside of the United States, with 29 of these published in Indian journals. These 81 studies examined a wide range of outcome measures and included numerous healthy and diseased populations. The studies were separated into three categories: uncontrolled studies, controlled studies, and comparison group studies. Thirty (30; 37.0%) of the studies were uncontrolled quasi-experimental studies typically comparing pretest and post-test scores on a variety of outcome criteria following a yoga intervention. Sixteen (16; 19.8%) were wait list or nonintervention controlled studies, of which 12 were randomized controlled trials. The remaining 35 studies (43.2%) compared yoga to some other treatment modality. These 35 studies subsequently were classified according to the type of intervention being compared to yoga. The following categories of interventions were created: exercise, relaxation response, usual medical treatment, psychotherapy/cognitive interventions, and “other.”
The single largest category (n=12) of comparison studies involved the effects of yoga being compared to exercise, and it is this category that is the focus of this article. Several studies seemed to span multiple categories, such as usual cardiac care, which often utilizes an exercise component. However, for the purposes of this article, only studies that listed exercise as the primary intervention were placed in that category.
Results
In the 12 studies that compared the effects of yoga and exercise, yoga interventions yielded positive results in both healthy and diseased populations. However, with the exception of the studies by Oken et al., no group of researchers has sought to compare the effects of yoga and exercise in a systematic fashion with variety of patient populations. Nevertheless, the evidence presented suggests that yoga interventions appear to be equal or superior to exercise in nearly every outcome measured except those involving physical fitness.
Nearly every study reviewed utilized a combination of different yoga therapies including vigorous physical asanas, gentle restorative poses, breath work, and meditation. This raises an important question that has not been adequately addressed in the literature. Just as there are different specialties in the practice of medicine, there are several different styles of yoga, each with distinctive challenges and varying levels of difficulty. Some types of yoga may be gentle and meditative (Integral, Svaroopa), vigorous (Ashtanga, Power Yoga), or both (Iyengar, Kundalini). Some forms involve changes in the environment such as using heaters and humidifiers (Bikram). Iyengar yoga frequently is used for therapeutics and incorporates the use of props such as ropes, straps, and chairs to enable students to achieve poses that might not be accessible otherwise. Each style of yoga differs in the emphasis placed on the various components of yoga such as asana, pranayama, or meditation. The relative effects of these different types of yoga on the HPA axis and SNS in response to acute and chronic stress have not been adequately examined.
Only one study could be found comparing the various styles of yoga. In a convenience sample of 16 volunteers, only Ashtanga yoga resulted in significantly higher heart rates than either Hatha or gentle yoga. This study examined only heart rate as an outcome variable. Clearly, additional studies are called for, using larger sample sizes and better research methodologies that compare the effects of the various types of yoga on a variety of outcome measures in a variety of populations, both healthy and diseased.
It is possible that the differences in fitness outcomes found in the comparison studies of yoga and exercise might not have been found if exercise were compared to the more vigorous forms of yoga. The differences that have been found between yoga and exercise interventions may be a result of how the two differ in their effects upon the SNS and HPA axis. Different levels of intensity of exercise have been shown to affect the HPA axis response to acute stress differently. Low-intensity exercise repeatedly has been shown to lower cortisol levels, while intense exercise leads to proportional increases in cortisol. The critical level of intensity that leads to release of cortisol is approximately 60% VO2max, with the greater the exercise intensity, the greater the cortisol release. Perhaps this explains why yoga, involving slow and often nonstrenuous activities, positively affects the HPA axis response to stress. Exercise stimulates the SNS, raising plasma epinephrine and norepinephrine. Yoga on the other hand, has been shown to lower sympathetic stimulation, significantly lowering levels of plasma norepinephrine and epinephrine.
Given that the eight limbs of yoga are so multidimensional and include aspects of exercise (Asana), breath work (Pranayama), concentration (Dharana), and meditation (Dyana), it is not surprising that researchers have found positive results regarding yoga in so many diverse areas. In three studies comparing yoga with meditation techniques such as progressive relaxation, yoga was found to be equal or superior to progressive relaxation in lowering blood pressure and in improving perceptions of mood and anxiety. Yoga, when compared with supportive psychotherapy in randomized trials involving patients with cancer undergoing chemotherapy, has been shown to be significantly better at decreasing levels of nausea and vomiting and strengthening the immune system. While the previously discussed exercise comparison studies involving yoga’s effects on cognitive function led to nonsignificant results, yoga clearly appears to have multidimensional effects on brain chemistry and this warrants further inquiry.
Given the fact that clear evidence exists regarding the efficacy of both exercise and yoga interventions in alleviating symptoms and improving outcomes of patients with coronary artery disease, it is somewhat surprising that researchers have not discriminated more clearly between the effects of the two interventions in this population. Exercise has been recognized as a key component in cardiac rehabilitation. Yoga, when added to the components of usual cardiac care in randomized trials, has been shown to be significantly better than usual cardiac care at improving blood lipid levels, decreasing markers of inflammation and in reducing the number of revascular procedures.
It is possible that yoga might be not only an acceptable additive to care, but an effective, feasible, and acceptable alternative to exercise in heart disease populations and in other populations that have traditionally benefited from exercise such as diabetes and obesity. This is a potentially rich area for research for a variety of reasons. First, strong evidence in the form of the Whitehall epidemiological studies suggests that there is a dose-response relationship between obesity and stress. Evidence also suggests that chronic stress leads to changes in food-seeking behavior, including increased consumption of foods high in sugar and fat, which may eventually lead to obesity. As yoga seems to provide many of the benefits typically associated with exercise and also strongly influences the SNS/HPA axis response to stress, it is possible that yoga might be a particularly useful weapon in the arsenal against obesity. In a recent population-based telephone survey involving 11,211 Americans, 57.4% of the 372 respondents who admitted using complementary and alternative medicine during the past year reported using yoga as a form of weight control. Research is needed to examine the efficacy, feasibility, and acceptability of yoga interventions for the prevention and treatment of obesity in both healthy and ill populations.
Conclusions
Overall, the studies comparing the effects of yoga and exercise seem to indicate that, in both healthy and diseased populations, yoga may be as effective or better than exercise at improving a variety of health-related outcome measures including HRV, blood glucose, blood lipids, salivary cortisol, and oxidative stress. Furthermore, yoga appears to improve subjective measures of fatigue, pain, and sleep in healthy and ill populations. However, future clinical trials are needed to further examine the distinctions between exercise and yoga, particularly how the two modalities may differ in their effects on the SNS/HPA axis. Additional studies are needed to distinguish between the different types of yoga and their various techniques. All of these studies need to use rigorous study methodologies, including the use of larger sample sizes, randomized samples, and blinding of researchers. These studies need to be replicated in a variety of populations, both sick and well, as the effects may vary depending upon the health status of the population.