Stomach cramps at kilometre 30. Nausea just before the finish line. The unexpected detour to the portable toilets during a half marathon. Anyone who has run endurance races knows these situations, or has at least feared them. Gastrointestinal problems are among the most common reasons why runners abandon a race or finish well below their potential.
What many don’t know: the gut can be trained, just like the legs.
How common are gastrointestinal problems in running?
Studies cite figures between 30 and 90%, a range explained mainly by different definitions of “symptom”. If every mild bout of bloating is counted, values quickly reach the higher end. A realistic figure for first-time marathon runners sits between 30 and 50%. That means roughly one in two marathon runners experiences some form of discomfort during a race, ranging from mild nausea to cramps. Some of it is harmless; some of it costs minutes or the finish itself.
The most common symptoms are nausea, cramps, bloating and diarrhoea, often grouped together under the term “Runner’s Stomach”.
Why do gastrointestinal problems occur in the first place?
The main culprit is a mechanism that, at first glance, seems to have nothing to do with the digestive system: the redistribution of blood flow.
During intense running, the muscles, heart and lungs need enormous amounts of oxygen. The body responds by diverting blood away from the digestive tract and redirecting it to the periphery. After just ten minutes of intense effort, up to 80% of blood flow is redirected away from the abdominal area. The intestinal lining is temporarily deprived of oxygen, which damages intestinal cells, increases the permeability of the gut wall, and impairs the absorption of fluids, electrolytes and carbohydrates.
Mechanical factors also play a role. The repeated vertical impact of running shakes the entire digestive tract, which explains why runners suffer from gastrointestinal complaints far more often than cyclists.
The key discovery: the gut can be trained
The gastrointestinal system is highly adaptable, just like the heart muscle or the quadriceps. And as with running training, the same principle applies: the body adapts to what it is regularly exposed to.
The principle is straightforward. Anyone who wants to train their gut must eat and drink while running, even if it feels uncomfortable at first. In practice, this means taking in carbohydrates regularly during long runs – gels, bars, bananas or sports drinks – and gradually increasing the amount over the course of weeks. The gut learns to work under load.
Three things happen simultaneously in the body:
- More transport proteins: The intestine produces more SGLT1 and GLUT5 transporters, which absorb carbohydrates more efficiently. Without them, sugars remain in the gut and draw in water, causing bloating and diarrhoea.
- Greater gastric capacity: Repeatedly eating and drinking during runs gradually expands the stomach’s capacity. Over time, larger amounts trigger less feelings of fullness.
- Nervous system adaptation: The enteric nervous system – the so-called “second brain” – gets used to the fact that eating while running is normal, and becomes less sensitive to the combination of exercise and food intake.
What the science says: in a randomised study from Monash University, two weeks of targeted gut-training were enough to reduce gastrointestinal complaints by 60 to 63% and improve running performance by around 5%. A 2023 systematic review summarising eight studies confirmed this: on average, symptoms decrease by 47% and carbohydrate malabsorption by 45 to 54%. Initial adaptations often begin after just a few days.
Which sessions can I use to train my gut?
Runners following a dynamic training plan can deliberately integrate gut-training into long runs and tempo runs. Sessions lasting more than 60 minutes are ideal.
Conclusion
The gut responds to stimuli, just like the muscles. Those who train it systematically can, according to research, significantly reduce gastrointestinal problems while also improving their energy supply during a race.
Realistically, after two weeks of consistent gut-training, a noticeable improvement in tolerance can be expected. After 6 to 10 weeks, the gut is ready for the demands of competition: higher carbohydrate loads, greater fluid volumes, fewer surprises.
What doesn’t work is last-minute gut-training. Anyone who swallows their first gel on race day is risking exactly what they were trying to avoid. The gut needs to be trained before it’s needed – just like the legs.
Sources
- de Oliveira, E.P. et al. (2014). Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports Medicine, 44(Suppl 1), 79–85. PMC4008808
- Pfeiffer, B. et al. (2012). Frequency of chronic gastrointestinal distress in runners. PubMed 28253039
- van Nieuwenhoven, M.A. et al. (2008). Prevalence of GI complaints in runners: observational study in 1281 subjects. PubMed 18777440
- van Wijck, K. et al. (2012). Physiology of splanchnic hypoperfusion during exercise. AJP Gastrointestinal and Liver Physiology. DOI
- Peters, H.P.F. et al. (1999). Gastrointestinal symptoms in runners, cyclists, and triathletes. Am J Gastroenterology. PubMed 10364027
- Costa, R.J.S. et al. (2017). Gut-training: two weeks repetitive gut-challenge during exercise. Applied Physiology, Nutrition, and Metabolism, 42, 547–557. PubMed 28177715
- Jeukendrup, A.E. (2017). Training the gut for athletes. Sports Medicine, 47(Suppl 1), 101–110. PMC5371619