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Millions of Americans are serious about fitness since we have long been told of its benefits. It is well established that consistent exercise promotes long-term health by lowering blood pressure, improving glucose tolerance, and assisting in weight management. Most people who exercise report an additional positive; physical activity improves their mental outlook. Recent reports go even further. Regular exercise may delay the onset of dementia for those who are prone.1

However, those are not the only systems that are affected. Modern research has documented many substantial changes that occur with exercise in our gut microbiome, which is being reshaped as a result of the metabolic changes that incur from regular moderate exercise.2 Exercise is associated with an increase in gut microbial diversity, which is associated with an improved metabolic profile and a strengthened immune system.3 This shift is believed to be the result of an increase in a particular bacterial strain, Bifidobacteria, that is essential for optimum health and whose levels tend to decline with age. Exercise provides a valuable boost to that population in our gut.


Extreme exercise

Although the great majority of those who exercise do so in the moderate range, there are increasing numbers of recreational and elite performance athletes who engage in very high levels of physical exercise in order to enhance performance. Extreme exercise is associated with a number of challenges, including a 10-20% increase in heart size and an increased potential for cardiac arrhythmias.4 More recent evidence suggests that there is also a general immune deficiency associated with prolonged exertion.5 In fact, it is common for elite athletes at the peak of training fitness during preparation for major sporting events to be prone to respiratory illnesses and a range of gastrointestinal symptoms. For example, it has been documented that among ultra-marathoners, there is an increased risk of upper respiratory tract infection during periods of heavy training and the 1- to 2-week period after marathon-type race event.6

Previously, it had been supposed that the depressed immune function coincident with strenuous exertion and heavy training was a by-product of dietary deficiencies in protein, carbohydrates, or specific micronutrients. However, there is new evidence for a surprisingly different common denominator for this set of problems among endurance athletes which is centered within the gut microbiome. Extreme exercise is associated with gut related immune deficiency from increased permeability of the intestinal cells that line our gut.7 This disruption leads to the common symptoms of nausea, bloating, cramping, pain, diarrhea, and even bleeding. Under the chronic stress of exercise-induced muscle fatigue and dehydration, the intestinal cells cannot sufficiently counteract and detoxify the free radicals that are being produced by extreme exertion. In these circumstances, some of the less desirable products of on-going gut microbial metabolism can cross the gut lining barrier, a condition known as endotoxemia. As a result, extreme endurance training has been shown to be associated with a rise in inflammatory markers in the body and a breakdown of gut integrity. These are known pathways towards chronic disease.8


Pre- and probiotic supplements

Fortunately, there is a way to counteract these negative effects. A number of studies have suggested a benefit from prebiotic and probiotic supplements for serious recreational and elite athletes.9 For example, prebiotics, such as oligo-fructose enriched inulin, increase the number of the beneficial Bifidobacteria and Lactobacillus in the colon. These bacteria protect against pathogens and antioxidants and also stimulate the immune system. A specific by-product of Bifidobacteria metabolism, butyrate, is an essential metabolite in our colon that helps maintain the energy supply of the body cells that line the colon and protects gut integrity.10 It is now known that increasing the proper microbes in your gut enhances these protective effects.11

The good news for recreational and endurance athletes is that there are convenient means to prevent the most common non-muscular complications of strenuous exercise. Certainly, the best advice for your optimum health is to exercise in moderation and support those efforts with a balanced and nutritious diet. But, if you insist on pushing towards even higher levels of athletic performance, part of your exercise regimen should include a daily supplement to boost your gut microbes, which are among your most important athletic partners.

1. Larson EB, Wang L, Bowen JD, McCormick WC, Teri L, Crane P, Kukull W. Exercise Is Associated with Reduced Risk for Incident Dementia among Persons 65 Years of Age and Older Exercise, Aging, and Risk for Incident Dementia. Annals of internal medicine. 2006.
2. Clarke S, Murphy E, O’Sullivan O,et al. Exercise and associated dietary extremes impact on gut microbial diversity.Gut 2014. doi:10.1136/gutjnl-2013-306541.
3. Hold GL. The gut microbiota, dietary extremes and exercise. Gut. 2014 Dec 1;63(12):1838-9.
4. Eijsvogels TM, Fernandez AB, Thompson PD. Are there deleterious cardiac effects of acute and chronic endurance exercise?. Physiological reviews. 2016 Jan 1;96(1):99-125.
5. Mach N, Fuster-Botella D. Endurance exercise and gut microbiota: A review. Journal of Sport and Health Science. 2016 May 10.
6. Marinkovic D, Minic RND, Andjelkovic M, Kostic-Vucicevic M, et al. Lactobacillus helveticus Lafti®L10 supplementation reduces respiratory infection duration in a cohort of elite athletes: a randomized double-blind placebo-controlled trial. Applied Physiol.
7. Pires W, Veneroso CE, Wanner SP, Pacheco DA, Vaz GC, Amorim FT, Tonoli C, Soares DD, Coimbra CC. Association between exercise-induced hyperthermia and intestinal permeability: a systematic review. Sports Medicine. 2016 Dec 10:1-5.
8. Van Houten JM, Wessells RJ, Lujan HL, DiCarlo SE. My gut feeling says rest: Increased intestinal permeability contributes to chronic diseases in high-intensity exercisers. Medical hypotheses. 2015 Dec 31;85(6):882-6.
9. Roberts JD, Suckling CA, Peedle GY, Murphy JA, Dawkins TG, Roberts MG. An Exploratory Investigation of Endotoxin Levels in Novice Long Distance Triathletes, and the Effects of a Multi-Strain Probiotic/Prebiotic, Antioxidant Intervention. Nutrients. 2016.
10. Brown CT, Davis-Richardson AG, Giongo A, Gano KA, Crabb DB, Mukherjee N, Casella G, Drew JC, Ilonen J, Knip M, Hyöty H. Gut micrometer metagenomics analysis suggests a functional model for the development of autoimmunity for type 1 diabetes. PloS one. 201.
11. Rastall, R.A.; Gibson, G.R. Recent developments in prebiotics to selectively impact beneficial microbes and promote intestinal health. Curr. Opin. Biotechnol. 2015, 32, 42–46.


  1. An excellent question. How much is too much? I wish that there was an absolute formula, but there is none, since we are discussing an effect that is mediated through the gut microbiome which differs among individuals. However, as a good rule of thumb, and now speaking as someone who has been running continuously for over 50 years, I can say that every person who exercises knows when they have reached their actual natural limit of fatigue, exhaustion, dehydration, etc. For conditioned athletes, I would offer this rough definition of extreme. Vigorous continuous daily exercise over 45 minutes begins a practical threshold of extreme exercise when it comes to the bodies metabolic accommodations. Below that, general benefit. Over that amount, and depending on the individual, things can begin to deteriorate. I can state with certainty that there is no further practical beneficial cardiorespiratory conditioning beyond that amount (focusing that metric on longevity, insulin sensitivity, etc.) Beyond that are differing levels of extreme conditioning. Everyone who engages at the extreme level will experience some of these unintended effects, subject to normal wide human variability.
    I am not speaking out against extreme exercise for those who love it. It is worth some risk. But it pays to understand that there are risks beyond musculoskeletal ones, even if small for some.
    Hope this helps.


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