Scientific Research on Yoga
By Thaís Salles Araujo, MD, LAc
Nowadays, the electronic library of global medical reports known as PubMed shows thousands of papers related to Yoga and this number has tripled only over the last decade (1).
Challenges in Yoga research
Before we start to explore the updated studies of Yoga, it is important to point out some challenges in Yoga research. There are several considerations about the over-generalization of the scientific results on Yoga that can be problematic (2):
- The exact definitions of what Yoga is in modern scientific studies have proved controversial and are widely variable.
- It is often reported that studies of Yoga suffer from methodological weaknesses which include improper randomization of participants in control and intervention groups, small numbers of participants, failure to compare Yoga with well-established interventions and inappropriate control groups (3), (4).
- Yoga is a complex multicomponent intervention. For a rigorous study, adherence to a defined intervention must be assured (e.g. dose, delivery, duration, specific style, components, and sequence; instructor training and skill level).
- It is difficult to separate Yoga’s specific effects from nonspecific effects (e.g. benefits from expectation, therapeutic and interpersonal skills of the Yoga teacher, placebo effects) (5).
- To be considered high-quality evidence and prevent biased results, scientific studies shall “blind” participants, which means to not inform them if they are part of the intervention group or part of the control group. In Yoga research this is virtually impossible.
- Most Yoga studies involve healthy participants and it may be difficult to assess improvements in these groups.
- Different Yoga styles have been investigated in different studies. This becomes a challenge to compare this different studies in meta-analysis* or systematic reviews†.
- Dose-response – Randomized controlled trials‡ could not demonstrate a relationship between the number of Yoga classes and the outcome measures (6).
Despite the presented challenges, the quality of Yoga research is improving and new scientific evidence on the topic is being introduced and/or incorporated into the conventional medical education, research community and clinical practices.
Current scientific evidence of Yoga
This overview of the research on Yoga is based on meta-analysis and randomized controlled trials (RCTs) which offer the most scientific evidence on the topic. For our purpose here, “Yoga” has been defined as a body-mind practice which includes four main components: postures (āsanas), breathing techniques (prānāyāma), relaxation and meditation (dhyāna). In order to better organize our findings, the scientific evidence on Yoga will be divided in different layers as follows: physical, physiological, psychological and spiritual levels, along with the effects of Yoga on full manifested diseases.
I. Physical level
Studies were conducted to evaluate the effects of Yoga on physical fitness, strength, flexibility, balance and mobility.
Physical Fitness: Energy expenditure in Yoga is usually insufficient to improve cardiovascular fitness (7). In a 2016 meta-analysis of 17 studies, Yoga was found to be mostly light to moderate in aerobic intensity which means less than 3 METs§ to up to 6 METs (8). A study also found that energy expenditure with Yoga was less than other forms of exercise at equivalent heart rates (9). The majority of studies that compare Yoga and aerobic exercise found a significantly greater improvement in the maximum rate of oxygen consumption (VO2 max) and anaerobic threshold in aerobic exercise than in Yoga (10).
Flexibility: Trials have demonstrated that practicing Yoga increases flexibility in both healthy individuals and patients with common musculoskeletal problems comparable with those achieved with regular stretching. Randomized trials comparing Yoga with untreated control groups have demonstrated increases in flexibility in healthy subjects of various ages, comparable with those achieved with regular stretching (11). Increases in flexibility have also been reported in controlled trials of Yoga in patients with common musculoskeletal problems.
Strength: One study concluded that strength gain with Yoga was equivalent to strength training in sedentary older adults. These findings have clinical implications as Yoga is a more amenable form of exercise than strengthening exercises (12).
Balance and Mobility: A 2016 meta-analysis from The University of Sydney evaluating Yoga practice in healthy older adults found moderate improvements in physical mobility and small improvements in balance (13).
II. Physiological level
Studies were conducted to evaluate the effects of Yoga on inflammatory markers, immunity response, blood pressure, glucose and cholesterol levels, pain control (labor, osteoarthritis, back pain), sleep quality and fatigue.
Inflammatory Markers and Immune functioning: Yoga and other mind and body practices, especially meditation, may decrease markers of inflammation and increase virus-specific immune responses to vaccines according to a study (14).
Blood Pressure, Glucose, and Cholesterol Levels: Numerous systematic reviews and meta-analyses have found promising evidence that Yoga may reduce some risk factors for cardiovascular disease (blood pressure, glucose, lipids) when compared with inactive control groups (15). A meta-analysis and systematic review of six trials found that Yoga lowered systolic blood pressure 5 mm and diastolic blood pressure 4 mm on average (16).
Pain Control: A 2012 meta-analysis of 16 studies including 1007 participants suggested that Yoga had a moderate effect on pain associated disability in a number of different conditions and may be a useful supplement to pain medications (17). A Cochrane review reported low to moderate-quality evidence suggesting Yoga relieved low back pain when compared with untreated controls but that data was inconclusive in comparison to other exercises (18). Yoga also appears to be one of a number of non-pharmacologic treatments effective for labor pain (19).
III. Psychological and Spiritual Level
Studies were conducted to evaluate the effects of Yoga on stress levels, anxiety and cognitive function.
Stress and anxiety: Yoga has shown promising results as therapeutic interventions for stress and anxiety (20). In small randomized studies, Yoga has been found more effective than walking or no treatment (21), (22). A study from the University of Southern Mississippi proposing to compare an integrated Yoga practice to a simple exercise Yoga has indicated greater effects in reducing anxiety-related symptoms and decreased salivary cortisol from the beginning to the end of the study (23).
Cognitive function and impairment: A meta-analysis concluded Yoga practice is associated with moderate improvements in acute and chronic cognitive function (24). A study found Yoga comparable to memory enhancement training in patients with mild cognitive impairment (25).
Sense of spirituality: Many Yoga participants cite spirituality as a reason for continuing to practice Yoga (26).
IV. Full Manifestation of Diseases
Studies were conducted to evaluate the effects of Yoga on specific diseases such as asthma, carpal tunnel syndrome, rheumatic disorders, PTSD, fibromyalgia and prenatal depression. All have shown significative positive outcomes for patients.
Rheumatic disorders: Yoga practices were considered helpful for patients with osteoarthritis and other rheumatological disorders (27).
Asthma: Breathing techniques designed to prolong exhalation and decrease minute ventilation have been studied as non-pharmacologic therapies for asthma (28) and data from systematic reviews and randomized controlled trials provide evidence of these benefits (29), (30). Similarly, Buteyko breathing exercises (designed to simulate prānāyāma breathing) were developed based on the theory that a reduction in minute ventilation might improve asthmatic control (31). Patients who used Buteyko breathing reported fewer symptoms and a decrease in the use of inhaled bronchodilators (32).
Fibromyalgia: A randomized controlled trial evaluated 53 fibromyalgia patients who were compared with waitlisted standard care. Those receiving the Yoga program demonstrated greater improvements in pain levels, fatigue, mood, acceptance and other coping strategies (33). Follow-up results showed that patients sustained most of their post-treatment gains with the functional scores¶ 21.9 percent improved at three months. Yoga practice rates were good and more practice was associated with more benefit for a variety of outcomes (34).
Carpal tunnel syndrome (CTS): Limited evidence suggests that Yoga may be beneficial for pain control in patients with CTS. A preliminary assessor-blinded controlled trial randomly assigned 42 patients with CTS to eight weeks of treatment with Yoga or wrist splinting. The Yoga intervention consisted of 11 Yoga postures designed for strengthening, stretching, and balancing each joint in the upper body along with relaxation given twice weekly (35). Patients in the Yoga group had statistically significant pain reduction compared with patients in the wrist splint group.
Cancer: There is some evidence from randomized trials that Yoga may improve sleep quality and fatigue – four of the five trials have enrolled cancer survivors (36), (37). In the largest trial, 410 patients with breast cancer who reported moderate or severe sleep disruption within 2 to 24 months after completing treatment were randomly assigned to standard care monitoring with or without Yoga (38). In this specific study, the Yoga intervention consisted of breathing exercises, 18 gentle and restorative postures and meditation performed twice weekly. After four weeks, Yoga participants demonstrated greater improvements compared with baseline in global sleep quality as the primary endpoint. Furthermore, the Yoga group demonstrated a greater reduction in daytime dysfunction and reduced use of sleep medications, in contrast to the control group, in which the use of sleep aids increased.
Post-traumatic Stress Disorder (PTSD): According to a randomized controlled trial Yoga significantly reduced PTSD symptomatology with effects comparable to well-researched psychotherapeutic and psychopharmacological approaches. The study concluded that Yoga may improve the functioning of traumatized individuals by helping them to better endure physical and sensory experiences associated with fear and helplessness to increase emotional awareness and tolerance (39).
Health Conditions Associated with Pregnancy: Yoga is widely practiced during pregnancy. Meta-analyses and systematic reviews have found low-quality evidence that Yoga may decrease pain during labor (40). Meta-analyses also found that Yoga decreased prenatal depression and increased satisfaction with the childbirth experience (41), (42). A randomized study reported Yoga more effective than walking in improving utero-fetal-placental circulation in high-risk women (43).
The Future of Yoga Research
The research of the healing effects of Yoga is a relatively new field of exploration in the West and is still evolving. Yoga is a whole healing system while the dominant scientific model is reductionist and fragmented. Despite the holistic approach of Yoga, there is still an attempt to fit this practice into separate boxes of direct causes and effects.
The challenge for future researches is to expand the view of the individual as a complex interconnected human being whose achievement of health and wellness depends on a combination of external and internal factors.
With the emerging field of integrative health and interdisciplinary research, increasing interest in innovative study models may expand in the boundaries of our comprehension of Yoga as medicine.
- *Meta analysis: A subset of systematic reviews; a method for systematically combining pertinent qualitative and quantitative study data from several selected studies to develop a single conclusion that has greater statistical power. Source: The Himmelfarb Health Sciences Library
- †Systematic review: A document often written by a panel that provides a comprehensive review of all relevant studies on a particular clinical or health-related topic/question. The systematic review is created after reviewing and combining all the information from both published and unpublished studies (focusing on clinical trials of similar treatments) and then summarizing the findings. Source: The Himmelfarb Health Sciences Library.
- ‡Randomized controlled trials: A study design that randomly assigns participants into an experimental group or a control group. As the study is conducted, the only expected difference between the control and experimental groups in a randomized controlled trial (RCT) is the outcome variable being studied. Source: The Himmelfarb Health Sciences Library.
- §METs stands for Metabolic Equivalent of Task (MET) It is a physiological measure expressing the rate of energy consumption during a specific physical activity.
- ¶Functional Score Questionnaire for Fibromyalgia measures physical functioning, work status (missed days of work and job difficulty), depression, anxiety, morning tiredness, pain, stiffness, fatigue, and well-being over the past week.