"Intense exercise seems to take its toll on the immune system by compromising the integrity of the intestinal barrier leading to “leaks” and increased permeability. Any type of leak, whether under your sink or in your intestines, is not good.
In the case of your intestines, leaks mean that toxins and other undesirables make their way into your bloodstream. When the bad bugs overtake the good ones, illness usually results.
In this study, after 14 weeks of supplementation with probiotics, zonulin concentrations – a modulator of intestinal barrier function – decreased from slightly above normal to normal range."
Precision Nutrition » Research Review: Good bacteria, gut health and exercise