During a marathon, your body redirects up to 80% of its blood flow away from the stomach to power your legs.

This physiological response is called splanchnic hypoperfusion. During intense aerobic exercise like a marathon, your sympathetic nervous system triggers vasoconstriction in the gastrointestinal tract. Research published in Sports Medicine shows that blood flow to the gut can drop by as much as 80%, ensuring skeletal muscles receive the oxygen and glucose they need to maintain pace.This blood flow redistribution is a survival mechanism that prioritizes movement over digestion. While a resting adult directs about 25% of cardiac output to the digestive organs, this drops dramatically as heart rate climbs. The resulting lack of blood flow, or ischemia, weakens the intestinal barrier and slows gastric emptying—explaining why long-distance runners frequently experience gastrointestinal distress during or after races.A 2014 study found that between 30% and 90% of endurance athletes experience at least one symptom of GI distress. Symptom severity typically correlates with exercise intensity and ambient temperature. When your body struggles to cool itself, even more blood is diverted to the skin for thermoregulation, further reducing blood supply to the stomach. Fortunately, proper training and nutrition strategies can help your body adapt to these extreme circulatory shifts over time.