Number one question we are asked is how to avoid stomach issues. Great article by Asker Jeukendrup at www.mysportscience.com.
In order to prevent gastrointestinal distress, a few guidelines can be provided. However, it must be noted that these are based on limited research. Nevertheless, anecdotally, these guidelines seem to be effective:
Avoid high-fiber foods in the day or even days before competition. For the athlete in training, a diet with adequate fiber will help keep the bowel regular. Avoid dairy, high fat and high protein foods the day before.
Avoid aspirin and NSAIDs such as ibuprofen. Both aspirin and NSAIDs have commonly been shown to increase intestinal permeability and may increase the incidence of gastrointestinal complaints. The use of NSAIDs in the pre-race period should be discouraged, mainly for athletes with a history of gastrointestinal problems.
Make training with carbohydrates part of your weekly routine and train your race nutrition.
Avoid high-fructose foods (in particular drinks that are exclusively fructose). However, interestingly, a fructose and glucose combination may not cause problems and may be better tolerated.
Avoid dehydration. As dehydration can exacerbate gastrointestinal symptoms, it is important to prevent dehydration. Start the race (or training) well hydrated.
Ingest carbohydrates with sufficient water or choose drinks with lower carbohydrate concentrations to prevent very high concentrations and osmolalities in the stomach.