Does caffeine cause stomach problems when running?

High doses, above roughly 400 mg, meaningfully raise the risk of gastrointestinal distress during a race — and running already stresses the gut on its own.

Gastrointestinal problems affect a large share of marathon runners regardless of what they take, because running reduces splanchnic blood flow and mechanically jostles the gut. Caffeine adds to that: it increases gastric acid secretion and colonic motility, effects that are harmless sitting down and considerably less so at 30 km.

The dose relationship is the useful part. Below roughly 400 mg, most runners tolerate caffeine well. Above it, the incidence of nausea, cramping and urgency rises noticeably, which is one more reason the 3–6 mg/kg window has an upper bound rather than being open-ended.

Combination effects matter too. Caffeine taken alongside a high carbohydrate intake rate, in a runner whose gut is not trained for either, compounds two independent stressors. The gels most runners use late in a race deliver both at once, which is exactly when splanchnic blood flow is lowest.

The prevention is the same as for every other fuelling problem: practise the exact dose, product and timing in long runs before you rely on it. A caffeine strategy you have never rehearsed is an experiment scheduled for the least convenient possible moment.

Glossary: GI Distress, Caffeine