We often picture yoga practitioners as being young, slender, and supple, like the beautiful people that grace the covers of Yoga Journal—every one of them perfect pictures of presumably perfect health. However, the population of yoga lovers is growing and there are, in fact, a wide variety of people with less than perfect bodies and less than perfect health who now practice yoga.
Some of those people have or are at risk of becoming insulin resistant because of their genes and because they have gained weight in all the wrong places. That means that there will be lots of people with the complications of insulin resistance, such as high blood pressure, abnormal lipids, glucose intolerance, type 2 diabetes, and cardiovascular disease, that regularly roll out their yoga mats, control their breathing, and stretch their bodies into poses of varying degrees of difficulty.
Does yoga protect us: A systematic review of the scientific literature
There is a belief among many yoga teachers and practitioners that yoga provides protection against heart disease…and, indeed, it may. But we should take a critical look at what the medical literature shows before reaching a firm conclusion. Dr. Kim Innes and colleagues did just that in a paper published in the Journal of the American Board of Family Practice last year (Volume 18, number 15, pages 491-519).
The authors performed a search of the medical literature, published in English, in journals from the West and in journals from India, where a lot of studies have been conducted. In order to understand the conclusions of this review, it is essential to understand how scientists go about proving that a treatment or other intervention causes an outcome. Failure to do that often leads to false conclusions on the part of clinicians and non-clinicians alike.
The best study designs to prove causality
The best study designs to prove that something, say yoga, is causally linked to an outcome, say the reduction in heart disease, is the randomized controlled trial (RCT). In this type of study, individuals are randomly assigned to get the intervention (the intervention group) or not get the intervention (the control or comparison group). Ideally, the two study groups are similar to each other in all other ways. Any difference in outcome between the two groups is likely to be because of the intervention. Less rigorous study designs may suggest causality, but usually, those results need to be confirmed by RCT before we can be confident that the link is causal.
So…what did the researchers find?
Although Dr. Innes and her colleagues found a lot of studies looking at the impact of yoga on cardiovascular risks related to insulin resistance, very few of them were RCTs—most of them were either nonrandomized controlled clinical trials or uncontrolled clinical trials. Nonrandomized controlled trials may be flawed because of selection bias. For example, what if the people who practiced yoga ate better diets than the comparison group and the diet, not the yoga, led to the reduction in heart disease. Uncontrolled trials do not have a comparison group so you do not know if there was something else going on in the general population that accounts for the observed improvement. For example, a powerful new drug was introduced that is widely used and is responsible for the decline in heart disease.
The authors also note that there were a lot of other study design problems in the papers they reviewed:
- Small numbers of people in the study
- Poor description of the intervention
- Short study duration
In the end, the researchers analyzed 71 studies (22 RCTs, 22 nonrandomized controlled trials, 26 uncontrolled trials, and 1 observational study). They concluded that yoga may reduce insulin resistance syndrome-related risk factors for cardiovascular disease (e.g., blood pressure, abnormal lipids, glucose intolerance). Further, yoga may improve clinical outcomes and may be a useful adjunct in the management of cardiovascular disease and insulin resistance. The recommend that additional high-quality RCTs be performed to “confirm and further elucidate” the effects of yoga on cardiovascular risk.
So, there you go. Yoga may be helpful. If you have or are at risk for insulin resistance and you practice yoga, great. Keep on doing it…but, also be sure you have regular check-ups, “know your numbers” (blood pressure, glucose, and lipid levels), eat right, and maintain a weight that keeps you healthy.