To prepare for a long-distance race, it is essential to train your digestive system ('gut training') to assimilate energy during exertion, a practice presented as being as crucial as muscular training of the legs to avoid dropping out.

noli's read

Digestive issues are indeed one of the main barriers to performance during endurance events, making this topic very relevant. Research strongly supports this concept: a consensus review by researcher Asker Jeukendrup (2017) shows that the stomach and intestines adapt within a few weeks to better tolerate volumes of fluids and carbohydrates during exertion. Furthermore, a literature review by Dr. Ricardo Costa's team (2017) confirms that this targeted training significantly reduces intestinal discomfort in runners. The claim that this preparation is 'at least as important as training your legs' is, however, a provocative but physiologically exaggerated statement, as cardiovascular and muscular adaptation remains the primary limiting factor in running. Nevertheless, digestive adaptation is now validated as a major optimization strategy for athletes of all levels.

the sources

No named source held this one up — that absence is part of the verdict.