Tiffany Field, of the Touch Research Institute, University of Miami School of Medicine and the Fielding Graduate University, reviewed recent empirical research of yoga’s impact on psychiatric and neurological conditions, cardiovascular health, immune conditions, and pregnancy. She then assessed yoga’s physiological effects, including decreased heart rate and blood pressure, and proposed mechanisms for its efficacy, focusing on increased vagus nerve activity and reduced cortisol levels.
1. Yoga Research
1.1. Origins and Practice
Field begins by placing yoga in context by noting that the Indian physician Patajali is traditionally held to have written the Yoga sutras approximately 5000 years ago. She mentions yoga’s general method as incorporating stretching exercises with diaphragmatic breathing to increase flexibility, balance, and muscle tone. Field then indulges a rudimentary description of yoga’s typical modern practice, with mats and bare feet, and an emphasis on form both during and between poses.
1.2. Psychological Effects
Field notes that at least two studies have demonstrated significant increase in mindfulness, while several others have measured reduction in job stress in both office and fire station settings. Studies of yoga’s effects on anxiety are common, with significant series and single session effects on measures of stress, anxiety, fatigue and depression, well-being and vigor. She then cites several studies in which measures of depression decreased after extended practice (~2 months). Field also reviewed sleep studies focused on yoga’s effects on insomniac, pregnant, geriatric, and pain syndrome groups. In all studies, yoga was found to have increased sleep efficiency, total sleep time, number of awakenings, and sleep quality.
1.3. Pain Syndromes
For those suffering from pain syndromes, such as lower back pain, headaches, osteoarthritis, and rheumatoid arthritis, Field notes findings of significant pain reduction and less analgesic and opiate use in yoga than control groups, and that these findings held regardless of gender or age differences among participants.
1.4. Cardiovascular Conditions
Field describes several studies addressing coronary artery disease and hypertension. In each, yoga was found to significantly improve cholesterol and serum low-density lipid levels. Yoga groups also had fewer anginal episodes, improved exercise capacity, decreased body weight, and lowered triglyceride levels than control groups. Blood pressure and blood glucose were reduced, and self-reported well-being and quality of life were increased.
1.5. Immune Conditions
Field notes that for immune (and autoimmune) conditions such as asthma, diabetes, multiple sclerosis, lymphoma, and breast cancer, yoga has been associated with several beneficial effects. For example, in studies of diabetes, daily yoga decreased blood glucose levels, including fasting levels, glycosylated hemoglobin levels, heart rate, systolic and diastolic blood pressure, and oxidative stress markers. Increased energy and decreased stress were also observed among diabetic groups. Lymphoma and cancer patients, too, Field reports, benefited from many of the same effects already mentioned, including lower sleep disturbance scores, reduced anxiety, pain, and fatigue, and increased relaxation.
Pregnancy-specific conditions, such as hypertension, preterm labor, and labor pain, as well as more general measures of stress and vagal activity in the context of pregnancy, have been studied in association with yoga. Field discusses several of these studies’ findings. In one, complications such as pregnancy-induced hypertension, preterm labor, and prematurity were significantly less frequent in the yoga than control walking group. In another study, Field reports significant improvements in both subjects’ perceived stress levels and their adaptive autonomic responses to stress following daily yoga throughout pregnancy. Overall labor duration, as well as labor pain, both during and following delivery, was found to be lower for yoga than control groups by another study.
2. Physiological Effects of Yoga Practice
Field reiterates the physiological effects documented across the studies reviewed, noting heart rate, blood pressure, EEG, pulmonary function, and oxygen consumption, as well as physical effects such as weight loss and increased balance and flexibility. Field goes on to cite studies focused on these physiological effects. For example, prolonged yoga training caused a decrease in exercise-induced heart rate in three mentioned studies, while another study found that baseline heart rate and lowest heart rate during a 6-minute exercise period were both significantly reduced in yoga versus walking groups. Vagus nerve activity, as measured by parameters associated with heart rate and heart rate variability, was significantly increased in yoga groups, as was oxygen consumption and breath volume. Contradictory evidence was found in another study mentioned by Field in which oxygen consumption was found to be lower following a yoga session than following rest alone. Yoga has also been found to reduce overall food consumption, eating speed, and food choices. Long-term yoga practice was also associated with lower weight gains, especially – notes Field – among overweight participants.
3. Potential Mechanisms of Action
Field proposes that yoga’s positive effects may be due to increased vagus nerve activity and decreased cortisol levels. She cites studies that have found significant increases in vagal activity in yoga over control groups. Field speculates that pressure receptors stimulated during yoga activity may activate afferent fibers connected to the limbic system (specifically hypothalamic) structures involved in cortisol secretion. She cites anatomical studies connecting baro- and mechanoreceptors of the dermis to vagal afferent fibers, functional studies associating direct, artificial vagal stimulation with reduced cortisol levels, and finally, two studies connecting yoga to reduced cortisol levels.
Overall, Field’s review of the literature is inexhaustive, but informative nonetheless. She notes the similarities of benefit between yoga and massage therapy, concluding that yoga may be considered a kind of self-massage. Here conclusions include a discussion of the limitations in the body of literature addressing yoga’s psychological and physiological effects, and ends with recommendations for future yoga research designs and directions.
Title: Yoga Clinical Research Review
Author: Tiffany Field
Journal: Complementary Therapies in Clinical Practice
Citation: Field T. Yoga clinical research review. Complementary Therapies in Clinical Practice. 2011;17(1):1–8. Available at: http://www.sciencedirect.com/science/article/pii/S1744388110000794. Accessed February 18, 2012.
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