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By guest contributors Andrew McGonigle, MD & Matthew Huy, MSc, authors of The Physiology of Yoga

Introduction from Jenni

I’m thrilled to feature this two-part educational article by two authors and yoga content creators whose work I value immensely! Both are longtime yoga teachers who teach anatomy and physiology in yoga teacher trainings worldwide. Andrew McGonigle, MD, is a medically-trained yoga teacher and educator, and Matthew Huy recently finished his master’s degree in Sport, Health & Exercise Science at Brunel University in London.

Andrew and Matt co-authored a new book, The Physiology of Yoga, that just released in June 2022 (published by Human Kinetics). I’m so excited about this new educational resource that’s now available for the yoga community!

This article here on my blog is adapted from Andrew and Matt’s new book. You can read more about their book (which I highly recommend!) below, after the conclusion of the article. And you can order your copy of The Physiology of Yoga right here!

One aspect I love about the book is the abundance of physiology and yoga myths and misconceptions that the authors thoughtfully and diplomatically question using insights from scientific research. In today’s article, we’ve selected three specific claims about yoga’s impact on physiology that we felt would be particularly relevant for my audience.

The three claims are as follows:

1. “Yin yoga targets the fascia.”

2. “Shoulder stand stimulates the thyroid.”

3. “Twists detoxify the liver.”

We hope you find the insights that Andrew and Matt have shared about these topics to be both interesting and educational. Enjoy reading, and please let us know what you think if you have a moment!

–Jenni

3 Controversial Claims About Yoga & Physiology

By Andrew McGonigle, MD & Matthew Huy, MSc

Claim #1: Yin Yoga Targets the Fascia

It is commonly claimed that yin yoga works on the connective tissues of the body, particularly the fascia, the thin casing of connective tissue that surrounds and invests all muscles as well as every other organ, blood vessel, bone, and nerve fiber. Yin teachers might talk about stressing the tissues of the body and distinguish yin yoga from restorative yoga. While restorative yoga is about resting in comfortable positions, yin should have some degree of discomfort or challenge to elicit change in the tissues of the body (Clark, 2012).

But does yin yoga, wherein poses are often held for three to five minutes, really target the fascia?

Participants in a yin class are often asked to release all muscular tension and relax into the pull of gravity, whereas participants in a vinyasa class might be asked to co-contract antagonist muscles or core muscles. No matter what instruction is given, a stretch is a stretch—and a stretch is a tensile force on a muscle. When a tensile force is applied to a muscle, it is also applied to the surrounding fascia and invested muscle fibers and bundles. The muscle and fascia (or myofascial units) are so interwoven that you cannot choose which one you are stretching by contracting or not contracting certain other muscles.

In vinyasa yoga, Standing Forward Fold is called Uttanasana. In yin yoga, it is called Dangling. Biomechanically and physiologically, there is little to no difference between the two as long as the duration of the stretch (or tensile force) is the same between the two poses. Additionally, engaging the antagonist muscle group (the quadriceps) has little to no effect on the hamstrings (Sharman et al., 2006). So, a standing forward fold, by any name, is a static stretch, and all stretching is a tensile force applied to a myofascial unit.

It should be noted that as tensile force is applied to tissues, they creep. Creep is the biomechanical term for the deformation of viscoelastic tissues. Once the tensile force is removed, tissues then recover and return to their original length, as long as they have not been elongated beyond their elastic capacity.

One study (Ryan et al., 2010) looked at creep in the muscle–tendon unit of living humans during a 30-second stretch, finding that the greatest amount of creep was measured to occur within the first 15 to 20 seconds. Beyond that, to our knowledge, no studies exist on yoga poses and creep, which means we do not know the ideal duration for stretching tissues or how long tissues take to fully recover from their creep.

So, yes, yin yoga does affect the viscoelastic fascia of our bodies – but not in isolation from muscles – and not any more than the same stretch performed in a different way (i.e., a more active way). Yin yoga, or any stretch held for three to five minutes, will affect the fascia, but the ideal frequency and duration remain a mystery.

Claim #2: Shoulder Stand Stimulates the Thyroid Gland

It is also commonly said in yoga classes that certain poses affect certain glands, the most common probably being that Shoulder Stand (Sarvangasana) stimulates the thyroid gland. The communal repetition of a claim, however, is not evidence that a claim is valid. The term availability cascade describes how a collective belief gains more and more acceptance simply through its increasing repetition in public discourse.

The claim of Shoulder Stand stimulating the thyroid gland has not been investigated scientifically and is based purely on speculation (Pierce, 2011). While some studies have been conducted on thyroid conditions and yoga in general, we authors cannot find any studies in the scientific literature looking specifically at how Shoulder Stand might affect the thyroid gland. Furthermore, the idea that this pose might stimulate the thyroid gland does not have sound physiological reasoning.

The workings of the endocrine system are much more complex than is suggested with this idea of applying manual pressure to create change. The endocrine system works through molecular and cellular processes in which one molecule initiates a cascade of events to create the desired outcome.

Situated at the base of the brain, the pituitary gland is the master that governs all other glands, and the pituitary is governed by the brain’s hippocampus. The hippocampus releases thyrotropin-releasing hormone (TRH), which is received by the pituitary, which then releases thyroid-stimulating hormone (TSH), which is then received by the thyroid to produce thyroid hormones. If the hippocampus’s ability to produce TRH were impaired or the pituitary’s ability to produce TSH were impaired, which might occur in the presence of a tumor, the thyroid would not be able to produce the needed thyroid hormones no matter how many Shoulder Stands were performed. Similarly, if your diet were deficient in iodine, which is rare in the United States as iodine is added to food, your thyroid would not be able to produce enough thyroid hormones, no matter which asanas were performed.

The reality is that there is no scientific evidence to support the notion that Shoulder Stand might directly affect thyroid function. Furthermore, does the thyroid need stimulating? Wouldn’t that depend on whether someone has an underactive versus overactive thyroid (hypothyroidism versus hyperthyroidism)?

On the other hand, just because there is no scientific evidence to support a claim does not mean that the claim is false. What we do know is that all moderate exercise will have some positive effect on the endocrine system, including the thyroid. General exercise has immense health benefits, such as improving sleep, bone density, and brain health while decreasing your chances of cancer, diabetes, and depression (Piercy et al., 2018), regardless of the notion of applying pressure to the thyroid. We also know that chronically elevated cortisol (the stress hormone) adversely impacts the thyroid, so taking time to relax, as we do in yoga, might also benefit thyroid function.

Claim #3: Twists Help the Liver Detoxify

B.K.S. Iyengar famously stated in his classes that deep twists cause a squeeze-and-soak effect on the body, especially the intervertebral discs and internal organs. Attempting to explain Iyengar’s idea, Natasha Rizopoulos (2017) wrote:

The theory is that twists cleanse the internal organs in much the same way that a sponge discharges dirty water when squeezed and can then absorb fresh water and expand again. The idea is that, when you twist, you create a similar wringing action, removing stale blood and allowing a freshly oxygenated supply to flow in. (para 2)

Whether or not Iyengar was the original source of this idea, the claim that twists detoxify the body has certainly become commonplace and is firmly planted in the minds of many yoga teachers. 

While Iyengar, who died of kidney failure in 2014, may have been well meaning in his assertions about detoxification, the squeeze-and-soak theory is inaccurate and has no scientific basis. Every second of every day, whether you are asleep or awake, your body is detoxifying itself, and this detoxification is a cellular process, not a mechanical one that requires the compressing, wringing, or stretching of internal organs.

Our bodies are regularly and inevitably exposed to toxins – poisonous substances produced by living organisms, including those produced in the body itself, such as lactic acid and microbial waste products in the gut. Your body removes these toxins through the liver, feces, and urine. The liver, specifically, alters toxic substances chemically, yielding them harmless and ready for excretion.

Even with our bodies’ integrated detoxification system, there are still some chemicals that cannot be easily removed through these processes, including persistent organic pollutants (found in pesticides), phthalates (found in hundreds of plastic products), bisphenol A (found in many food containers and hygiene products), and heavy metals (found in agriculture, medicine, and industry). Known to accumulate in the body and take a very long time, potentially years, to be removed, these chemicals may be linked to various chronic diseases including asthma, cancer, autism, and attention-deficit/hyperactivity disorder (Sears & Genuis, 2012).

The idea that a certain yoga pose, a certain diet, or a certain product can remove these stubborn toxins from the body is a tempting one. However, there is little to no evidence that one pose, diet, or product can do just that. In a 2015 review of all studies to date examining the efficacy of detox diets, Klein and Kiat found that we have very little clinical evidence to support the use of detox diets despite a booming detox industry and product packaging that makes bold but unsubstantiated claims. Although a small number of studies have found commercial detox diets might enhance liver detoxification and eliminate persistent organic pollutants, these studies had flawed methodologies and small sample sizes, thus reducing their scientific credibility. Klein and Kiat concluded:

To the best of our knowledge, no randomised controlled trials have been conducted to assess the effectiveness of commercial detox diets in humans. This is an area that deserves attention so that consumers can be informed of the potential benefits and risks of detox programs. (2015, p. 1)

As for yoga, there has been no research, as far as we are aware, to show that it improves the body’s natural detoxification system. This might be because our body already does a fantastic job of detoxification on its own. If you were not able to remove toxins from your body, you would know, as a plethora of symptoms would occur, even fatal ones.

This is not to say that yoga (or exercise in general) has no effect on our body’s ability to detoxify, but the benefits likely come from exercise’s ability to decrease inflammation and increase vascularization, or blood supply. While some inflammation is necessary in recovering from a wound or infection, chronic inflammation is hard on the body and weakens many systems. Long-term inflammation underlies many serious diseases, including cardiovascular disease and diabetes, while cancer, obesity, osteoporosis, Alzheimer’s disease, and cardiovascular disease have all been linked to increased biomarkers of inflammation (Tabas and Glass, 2013).

It is well established that exercise creates an anti-inflammatory response in the body – one of its most potent benefits (Flynn, McFarlin, and Markofski, 2007). When inflammation is reduced, all systems of the body can work more efficiently, including those of digestion and detoxification. Another well-known benefit of exercise is increased circulation, so more blood, which contains essential nutrients and oxygen, is available for digestion and detoxification. In these ways among other benefits, exercise can certainly help optimize our body’s detoxing processes.

The few studies that have examined yoga and inflammation have suggested that yoga also has the same anti-inflammatory benefits as other forms of exercise (Pullen et al., 2008; Pullen et al., 2010). Even though the research on yoga is much sparser, it is reasonable to assume that a physical yoga practice would provide many of the same exercise benefits such as decreased inflammation and increased circulation. If you are doing a lunge, your body does not know if you are in a yoga class or a personal training session; the physiological effects would be largely the same. Interestingly, one small study also found that a nonphysical practice of mindfulness decreased systemic inflammation (Ng et al., 2020), so more research is needed to fully understand the mechanisms behind decreasing inflammation.

So, do twists detoxify your body? Not in the way Iyengar suggested. Neither yoga nor diet do anything that our bodies cannot do on their own. However, yoga can optimize the body’s natural detoxification system, not through one specific pose such as a twist, but through the powerful benefits of decreasing inflammation and increasing circulation that come with exercise. Trust that your body is fully equipped to handle toxins and other unwanted substances.

Another thing we can do to improve our detoxification process is to simply reduce our exposure to toxins. While detox diets do not work in the way they are often advertised to, nearly all diets encourage healthful, balanced eating, which reduces your exposure to harmful products, including artificial ingredients found in junk food. We can also reduce our exposure to persistent organic pollutants by eating organically grown food. We can reduce the burden on our liver by drinking only moderate amounts of alcohol. Finally, keeping active with moderate exercise, yoga or otherwise, will help the body do what it does very well.

Conclusion

It is tempting to believe that one pose, one style of yoga, or one miraculous food will sort out our ailments and make us thrive. But physiology is incredibly complex with many factors including activity level, stress, hormones, age, biological sex, and genetics – among countless others – affecting our health. Nonetheless, exercise, sleep, and a balanced diet are some of the best things we can do for overall health and recommend to others.

Tune in next time for part 2 of this series in which we’ll explore the claims that yoga boosts the immune system, heavy sweating during hot yoga detoxes the body, and inversions bring more blood to the brain.

This article is adapted from the book The Physiology of Yoga by Andrew McGonigle and Matthew Huy, published by Human Kinetics in June 2022.

In this book, Andrew and Matt look at the anatomy and physiology of the systems of the body (from the musculoskeletal to the digestive and everything in between) and explore the latest research on how yoga affects these systems.

They examine topics such as whether yoga can reduce blood pressure, whether yoga can help treat diabetes, and whether breathwork improves mental health. They also examine whether some common claims heard in the yoga studio have any scientific basis.

Order your copy of The Physiology of Yoga here!

**Read Part 2 of this article series here!

About the Authors:

Andrew McGonigle, MD is a medically trained yoga anatomy teacher empowering yoga teachers to teach from a more science-informed perspective.

Website: www.doctor-yogi.com / Instagram: @doctoryogi

Matthew Huy, MSc is an American-born, UK-based senior yoga teacher who also teaches anatomy and physiology on several teacher training programmes in the UK and abroad. He holds a Master's in Health & Exercise Science and is the co-author of The Physiology of Yoga, out in June 2022 (published by Human Kinetics).

Website: www.matthewhuy.com / Instagram: @yogawithmatt

References

Clark, B. 2012. The Complete Guide to Yin Yoga: The Philosophy and Practice of Yin Yoga. Vancouver: Wild Strawberry Productions.

Flynn, M.G., B.K. McFarlin, and M.M. Markofski. 2007. “The Anti-Inflammatory Actions of Exercise Training.” American Journal of Lifestyle Medicine 1 (3): 220-235.

Klein, A.V., and H. Kiat. 2015. “Detox Diets for Toxin Elimination and Weight Management: A Critical Review of the Evidence.” Journal of Human Nutrition and Dietetics 28 (6): 675-686.

Ng, T.K.S., J. Fam, L. Feng, I.K-M. Cheah, C.T-Y. Tan, F. Nur, S.T. Wee, et al. 2020 “Mindfulness Improves Inflammatory Biomarker Levels in Older Adults With Mild Cognitive Impairment: A Randomized Controlled Trial.” Translational Psychiatry 10 (1): 1-14.

Pierce, S. 2011. “Integrative Treatment of Hypothyroidism.” Pearls for Clinicians. University of Wisconsin Integrative Medicine.

Piercy, K. L., Troiano, R. P., Ballard, R. M., Carlson, S. A., Fulton, J. E., Galuska, D. A., ... & Olson, R. D. (2018). The physical activity guidelines for Americans. Jama, 320(19), 2020-2028.

Pullen, P.R., S.H. Nagamia, P.K. Mehta, et al. 2008. “Effects of Yoga on Inflammation and Exercise Capacity in Patients With Chronic Heart Failure.” Journal of Cardiac Failure 14 (5): 407-413.

Pullen, P.R., W.R. Thompson, D. Benardot, et al. 2010. “Benefits of Yoga for African American Heart Failure Patients.” Medicine & Science in Sports & Exercise 42 (4): 651-657.

Rizopoulos, N. 2017. “Get Twisted.” Yoga Journal. Updated April 12, 2017.

Ryan, E.D., T.J. Herda, P.B. Costa, A.A. Walter, K.M. Hoge, J.R. Stout, and J.T. Cramer. 2010. “Viscoelastic Creep in the Human Skeletal Muscle-Tendon Unit.” European Journal of Applied Physiology 108 (1): 207-211.

Sharman, M.J., A.G. Cresswell, and S. Riek. 2006. “Proprioceptive Neuromuscular Facilitation Stretching: Mechanisms and Clinical Implications. Sports Medicine 36 (11): 929-939.

Sears, M., and S. Genuis. 2012. “Environmental Determinants of Chronic Disease and Medical Approaches: Recognition, Avoidance, Supportive Therapy, and Detoxification.” Journal of Environmental and Public Health, Article ID 356798.

Tabas, I., and C.K. Glass. 2013. “Anti-Inflammatory Therapy in Chronic Disease: Challenges and Opportunities.” Science 339 (6116): 166-172.

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