The Effects of Yoga on Physiologic Stress Parameters

Clinical and Research Findings

Birdee and colleagues reported results from a U.S. survey, stating that “a majority of yoga users reported yoga as important for maintaining their health.” ¹ Research provides insights into why increasing numbers of people are drawn to a yoga practice, and suggests that yoga is linked to improved metabolic, HPA axis, and nervous-system regulation. Specifically, the research literature has linked yoga practice to improvement in medical conditions, such as anxiety and depression, heart disease, high blood pressure (BP), insomnia, and fatigue, as well as to improvements in quality of life. ² While yoga has become well-known as a stress-reducing activity among the general public and the integrative medicine community, now, research affirming the physiologic correlates of how yoga acts as a stress reducer is also emerging.

One review explored the effects of yoga on sympathetic nervous system (SNS) and HPA axis regulation measures, including BP, heart rate, cortisol, peripheral cytokine expression, and/or structural and functional brain measures. ³ The review, which included 25 randomized controlled studies, concluded that a yoga practice leads to better regulation of the SNS and HPA axis and may decrease depressive and anxious symptoms in a range of populations. The researchers commented: “Despite methodological limitations, the large majority of the studies provide some evidence that yoga is associated with biological changes in blood pressure, heart rate, cortisol or cytokine levels.” ³ The researchers stated that “yoga may affect mood via SNS and HPA axis regulation,” ³ and advised that further studies are needed to “explore the effects of yoga on the neural correlates in the context of mood changes” and to “facilitate implementation in clinical settings.” ³

Timothy McCall, MD—the founder and director of Yoga as Medicine Seminars and Teacher Trainings, and author of Yoga as Medicine, ⁴ commented: “Clinically, in yoga therapy, the beneficial effects are clear. Certain breathing techniques can shift the autonomic nervous system (ANS) to parasympathetic dominance within a few breaths. Also, if self-reports of stress levels are the best way to measure stress, people clearly find that yoga reduces stress and increases stress resilience.” With regard to yoga's unique effects on specific stress parameters, McCall noted:

Lorenzo Cohen, PhD—a professor and the director of the Integrative Medicine Program at The University of Texas, MD Anderson Cancer Center, in Houston, Texas, and a distinguished clinical professor at Fudan University Shanghai Cancer Center, in Shanghai, China—commented on the yoga literature regarding cortisol findings: “Several studies looking at the effects of yoga in people with cancer have shown that yoga and/or meditation lead to improved cortisol regulation, ^6,7 including a study that my colleagues and I conducted.” ⁸ In that study, women with breast cancer scheduled for radiotherapy (XRT) were randomly assigned to yoga or stretching three times per week for 6 weeks during XRT or to a wait-list control. The patients' cortisol levels were measured five times per day for 3 consecutive days at baseline; at the end of treatment; and at 1, 3, and 6 months as a measure of stress arousal. Diurnal changes in circulating cortisol levels during waking hours were assessed by examining the steepness of the cortisol slope.

Cohen and colleagues indicated in that study article:

Although there was a blunting of the cortisol slope by the end of XRT [for all groups], participants in the yoga group had a significantly steeper cortisol slope than the other groups. Although the clinical significance of this finding is unclear, it does suggest the positive effects of yoga on the stress hormone cortisol. There is evidence that a blunted cortisol slope is associated with tumor progression and decreased survival in patients with breast cancer, so maintaining a sustained steep cortisol slope may therefore have prognostic implications. ⁸

Cohen added, however, that “cortisol is, of course, not the only relevant outcome to measure HPA access activation and the effects of yoga on stress. Heart rate variability (HRV) and vagal activity, for example, are important to assess and examine the benefits of yoga.” McCall affirmed this, and commented: “Studies have shown that yoga improves HRV and baroreceptor sensitivity—two markers of a healthy ANS.” He explained: “I measure pulse rate variability as an easy surrogate for HRV when I work with students. I have worked with people who are paradoxical or reverse breathers and who are anxious, restless, and sleeping poorly.” He added: “For [people] who appear to have no variability in pulse rate, I have utilized a supported backbend (reclined Cobbler's pose) with simple belly breathing, breathing evenly for 5 or 10 minutes. Checking the pulse again, I have found that HRV returns. With daily practice, symptoms may improve rapidly.”

An article by Streeter and colleagues on the effects of yoga on the ANS and on various medical conditions reported that stress leads to an imbalanced ANS, underactivity of gamma amino-butyric acid (GABA), and increased allostatic load. ⁹ These researchers hypothesize that yoga might help restore balance to patients undergoing these stress-related occurrences present in various disease conditions and ease symptoms. Specifically, the researchers stated that “neurophysiological, neuroanatomical, and clinical evidence converge” to support the theory that therapies such as yoga may “stimulate an underactive parasympathetic nervous system and increase the inhibitory action of a hypoactive GABA system in brain pathways and structures that are critical for threat perception, emotion regulation, and stress reactivity.” ⁹

Streeter and colleagues commented: “Depression, epilepsy, post traumatic stress disorder (PTSD), and chronic pain exemplify medical conditions that are exacerbated by stress, have low heart rate variability (HRV) and low GABAergic activity, respond to pharmacologic agents that increase activity of the GABA system, and show symptom improvement in response to yoga-based interventions.” ⁹

Conclusion

As there is no single modality and certainly no single pill that relieves the symptoms and effects of chronic stress fully, an integrative approach is imperative. As experts and the research demonstrate, yoga is one practice that should be at top of one's mind when considering therapies to restore balance to a stressed system. Many researchers on the topic comment that integrating yoga into a treatment plan for relieving stress and translating the current science into accessible interventions in clinical settings should be a priority. Streeter and colleagues said that the current findings of the effects of yoga on the ANS have “far-reaching implications for the integration of yoga-based practices in the treatment of a broad array of disorders exacerbated by stress.” ⁹ ▪