The gut’s seal is built from living cells
The intestinal lining is not an impermeable wall. Its cells absorb water and nutrients while tiny protein connections between neighboring cells—often called tight junctions—help keep microbes and their components inside the gut. When prolonged, strenuous exercise reduces blood flow to this tissue, the cells can become stressed and those connections may loosen.
Heat and dehydration can add to the strain. During an ultramarathon, the body must balance competing demands: active muscles need circulation, while the gut also needs oxygen and fluid to maintain its lining. The longer that imbalance lasts, the more vulnerable the barrier may become.
From the intestine to an immune response
If the barrier becomes more permeable, bacterial components such as endotoxin can pass from the intestine into nearby tissue or the bloodstream. The immune system recognizes these materials as warning signals and may respond with inflammation. That response is part of the body’s defense, but in a race it can contribute to gastrointestinal distress and, in more serious cases, broader illness.
This is not the same as saying that every runner with an upset stomach has toxins circulating at dangerous levels. Gut symptoms during endurance events can have several causes, and barrier disruption varies from person to person. Still, the mechanism helps explain why the digestive tract can become a limiting part of an event that seems to be mainly about muscles and stamina.
Why recovery matters
When the intense effort ends and circulation returns toward normal, the gut lining can begin repairing itself. But pushing through severe or worsening symptoms—especially when heat stress or dehydration is also present—can make the situation more concerning. In an ultramarathon, the body’s capacity to keep moving does not mean every organ is coping equally well.
