Gastrointestinal Ischemia: A Hidden Risk of Endurance Cycling

Ever experience severe abdominal pain, persistent diarrhea, or bloody stools during or after an ultradistance cycling event?

The majority of gastrointestinal complaints during or after endurance exercise are harmless but occasionally may reflect something more serious: a reduction in blood flow that deprives the bowel of oxygen (intestinal ischemia).

A study presented at the 2026 American College of Gastroenterology meeting found that endurance athletes had nearly three times the odds of ischemic colitis (poor blood flow to the colon) compared with controls. The association was particularly strong among ultradistance cyclists, whose adjusted odds were nearly four times higher. Although this retrospective study cannot establish cause and effect or determine the absolute risk among all cyclists, it reinforces earlier clinical reports linking prolonged strenuous exercise to ischemic bowel injury.

During exercise, the body redirects blood flow away from the gastrointestinal tract toward working muscles and the skin. At rest, approximately 20% to 25% of cardiac output supplies the gastrointestinal organs through the splanchnic circulation. As exercise intensity increases, sympathetic nervous system activation constricts these blood vessels, redirecting blood toward working muscles and the skin for heat dissipation. During vigorous exercise, splanchnic blood flow can decrease (hypoperfusion) by 70% to 80%. This is a normal physiological response, but prolonged reductions in oxygen delivery can injure the intestinal lining.

The intestinal mucosa is particularly vulnerable to a lack of oxygen as its lining cells have high metabolic demands. Prolonged hypoperfusion can damage these cells. Dehydration, heat stress, and NSAID use can further increase the risk.

But does this reduction in intestinal blood flow actually injure the bowel? Controlled cycling studies suggest that it does. In a 2011 study, healthy men cycling for 60 minutes at 70% of maximum workload demonstrated reduced intestinal perfusion and injury to the lining cells, leading to leakage of intestinal fatty acid-binding protein (I-FABP), with a doubling of blood concentrations from 309 to 615 pg/mL. The degree of injury correlated with reduced intestinal blood flow. A separate 2021 study demonstrated that intestinal injury increased with cycling intensity, with significantly greater I-FABP elevations at 80% compared with 40% of VO₂max. Interestingly, in another comparison, cycling produced greater elevations than running, although the cyclists also experienced greater cardiovascular and perceived exertional stress.

There is, however, an important distinction between transient intestinal epithelial injury and clinically significant ischemic colitis. An elevated I-FABP indicates intestinal epithelial injury, not necessarily clinically significant bowel ischemia. The changes observed during controlled cycling were generally transient, and these studies did not demonstrate progression to ischemic colitis. Furthermore, I-FABP primarily reflects small intestinal epithelial injury, whereas ischemic colitis involves the colon. These findings establish a plausible biological mechanism linking vigorous exercise to intestinal injury, but they do not establish how frequently this injury progresses to clinically significant bowel damage.

The recent 2026 ACG report provides additional epidemiological evidence, but its reliance on hospital diagnostic codes limits the conclusions. Investigators could not establish when the ischemic episode occurred relative to exercise or determine the absolute incidence of ischemic colitis among endurance athletes. Nevertheless, the combination of clinical case reports, epidemiological associations, and controlled physiological experiments makes exercise-associated intestinal ischemia a credible concern.

For cyclists, the practical implications are straightforward: maintain adequate hydration, avoid NSAIDs during prolonged rides, reduce intensity when heat stress becomes excessive, and seek prompt medical evaluation for severe abdominal pain, especially when accompanied by bloody diarrhea.

A final note of caution. Exercise-associated ischemic colitis is uncommon, and new gastrointestinal symptoms in an endurance athlete should not automatically be attributed to exercise. Rectal bleeding or a persistent change in bowel habits requires medical evaluation, even when symptoms are first noticed after a long ride. Colorectal cancer is increasingly being diagnosed in younger adults, with incidence rising approximately 3% annually among Americans ages 20 to 49. Being young, fit, and physically active does not eliminate that risk.

Remember the old medical maxim: “When you hear hoofbeats, think of horses, not zebras.” Exercise-induced ischemic colitis is a possibility, but colorectal cancer and other common causes of bleeding must not be overlooked.