Train your gut like a muscle
GI distress is a training adaptation problem. It is fixable.
Most athletes treat stomach issues during exercise as bad luck or personal sensitivity. The research says something different. The gut is trainable tissue, and the athletes who get this right outperform those who do not by a meaningful margin on race day.
What happens to the gut during exercise
During intense exercise, blood is redirected away from the gut and toward working muscle. Splanchnic blood flow can fall by 60 to 80 percent at high intensities. The result is slower gastric emptying, reduced nutrient absorption, and the bloating, cramping, or nausea that derails races. Research by Peters et al. documented this clearly, showing GI complaints correlate directly with reduced gut perfusion, not with what the athlete ate.
This is physiologically normal. The body is making a sensible trade-off: fuel working muscle over digesting food. The problem is that most training programmes do not prepare the gut to function under these conditions, so race day is the first time it is asked to absorb meaningful carbohydrate at intensity.
Why the gut can be trained
The intestine contains transport proteins, primarily SGLT1 and GLUT5, that move glucose and fructose across the gut wall into the bloodstream. These transporters are upregulated by consistent exposure. Jeukendrup's gut training protocols show that 10 to 28 days of carbohydrate consumption during exercise can substantially increase absorption capacity, even at race intensities.
The practical implication: the fuelling protocol that works on race day needs to be practised in training, not introduced on the start line. Your gut will not perform at a level it has not been asked to reach.
concept//fuel builds your intake schedule around this principle. The targets shown for race efforts are where you need to get to, not where you start.
How to build gut tolerance
- //Start with 30 to 40g of carbohydrate per hour in your first few long sessions.
- //Add 10g per hour each week until you hit your target intake.
- //Use a mix of glucose and fructose sources from the beginning, not just gels.
- //Practise at the same intensity you plan to race at, not just on easy long runs.
- //Accept some discomfort in the first few sessions. It is the system adapting, not failing.
Peters et al. (2000) Eur J Appl Physiol; Jeukendrup (2017) Sports Medicine; van Wijck et al. (2012) Am J Physiol.
Put it into practice
Apply this to your own training.