The gut plays an important role in the absorption of nutrients while preventing infections, inflammation and other problems. The barrier function of the intestine, comprising of a single layer of cells, prevents large molecules and germs from passing from inside the bowel into the bloodstream. However, it can become less effective and "leaky," potentially causing gastrointestinal (GI) problems in athletes. In this article, we explore whether exercise-induced heat or hypoxia may cause a leaky gut.

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A Healthy Gut and a Leaky Gut During Exercise

During exercise, the muscles, heart, lungs, and gut are affected. GI symptoms such as vomiting, diarrhea, or bloating may result. Research has been carried out to understand how exercise affects the gut, uncovering mechanisms behind exercise-related GI symptoms and intestinal permeability. Two such mechanisms are reduced splanchnic blood flow and an increase in core temperature.

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Increased Muscular Activity and Permeability

Exercise generates heat, resulting in an increase in core temperature. This rise in temperature has been associated with an increase in permeability of cells within the GI tract, allowing molecules and bacteria contained within the GI tract to enter the bloodstream. Endotoxemia may occur if these molecules and bacteria cannot be cleared fast enough, leading to potentially life-threatening inflammatory responses associated with heat exhaustion and heat stroke.

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More Heat, More Permeability?

A 2017 review article by Pires and colleagues suggested a sliding scale relationship between intestinal permeability and core temperature, with an increase in permeability occurring in some athletes when core temperature reaches 38.5°C. However, when core temperature reaches and goes above 39°C, increased GI permeability is universal. Exercise also causes a redistribution of blood flow away from the gut towards the working muscle, reducing the delivery of blood (and thus oxygen) to the gut.

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Two New Studies

Two recent studies aimed to investigate the relationship between heat and hypoxia and their contribution to increased GI permeability. Participants were heated by being submerged in a warm bath set at 40°C for an hour. In one trial, core temperature was maintained at 38.5°C, while in the other trial, it was between 39 - 39.5°C. Participants ingested a drink containing two or more sugars to measure permeability.

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Study Findings

A mild increase in the plasma ratio of sugars was observed in all trials with no difference between conditions, including the control condition, where participants rested in a laboratory at 22°C. This indicates that the increase in plasma ratio represented the normal rate of digestion of sugars across the GI tract. The participants reported no GI symptoms but showed symptoms associated with heat illness, such as light-headedness and nausea.

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Can Glutamine Prevent a Leaky Gut?

The second study aimed to measure the effects of glutamine, which has previously shown to have an attenuating effect on GI permeability and core temperature, on resting human participants exposed to heat stress. The study found that the supplementation of glutamine, 0.5 [g.kg](http://g.kg/) body mass two hours before the warm bath, had a possibly small reducing effect on I-FABP, a marker of GI injury. However, the effect of glutamine on GI permeability in resting human participants exposed to heat stress remains unknown.

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Conclusion

Exercise-induced heat or hypoxia may cause a leaky gut, leading to GI symptoms and potentially life-threatening inflammatory responses. The relationship between heat and hypoxia and their contribution to increased GI permeability requires further investigation. The use of supplements such as glutamine to prevent a leaky gut is still under scrutiny.

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Scientific Reference

1. Pires, W., Veneroso, C.E., Wanner, S.P., Pacheco, D.A.S., Vaz, G.C., Amorim, F.T., Tonoli, C., Soares, D.D. and Coimbra, C.C. (2017) Association Between Exercise-Induced Hyperthermia and Intestinal Permeability: A Systematic Review. _Sports Medicine_, 47 (7), 1389-1403.
2. Pugh, J.N., Sage, S., Hutson, M., Doran, D.A., Fleming, S.C., Highton, J., Morton, J.P. and Close, G.L. (2017) Glutamine supplementation reduces markers of intestinal permeability during running in the heat in a dose-dependent manner. _European Journal of Applied Physiology_, 117 (12), 2569-2577.
3. Soares AD, Costa KA, Wanner SP, Santos RG, Fernandes SO, Martins FS, Nicoli JR, Coimbra CC, Cardoso VN. Dietary glutamine prevents the loss of intestinal barrier function and attenuates the increase in core body temperature induced by acute heat exposure. Br J Nutr. 2014 Nov 28;112(10):1601-10.