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:’’ yama (universal ethics), niyama (individual ethics), asana (physical postures), pranayama (breath control), pratyahara (control of the senses), dharana (concentration), dyana (meditation), and samadhi (bliss). 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 psychologic 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 rennin 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-614 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. Researchers have noted significant reductions in low-frequency heart rate variability (HRV)—a sign of sympathetic nervous system activation—in depressed patients following an 8-week yoga intervention. 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. Galantino et al. published a systematic review of the effects of yoga on children. 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.
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 in the table 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), tantra yoga, 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 more benefits than Hatha 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,52,53 while intense exercise leads to proportional increases in cortisol.54 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.54 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. 55,56 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 vomiting58 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.
