Studies suggest that 30–50 % of endurance athletes can experience gastrointestinal symptoms during exercise which are mild and of no risk to health. In some cases, however, it can be more serious and need medical intervention. In both cases, race performance will be affected and in extreme cases the athlete will have to retire from the race.
A number of nutritional strategies have been proposed to minimise gastrointestinal distress. It must be noted that these are based on limited research. Nevertheless, anecdotally they 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 fibre will help keep the bowel regular. However, before race day increased bowel movements are not desirable and can accelerate fluid and electrolyte loss. Depending on the type of fibre, it may also cause undesirable gas production and bloating! Highly refined and processed white & beige foods although not usually a feature of a healthy diet, have their place leading up to an event. Pasta, white rice, white bread and bagels will all reduce the likelihood of GI problems on race day.
Avoid foods which you suspect you might have an intolerance to, (e.g wheat, dairy etc). These can GI problems which will be exacerbated problems during exercise.
Avoid aspirin and Non Steroidal Anti Inflammatory Drugs (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, particularly for athletes with a history of gastrointestinal problems.
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.
For athletes who compete in endurance events, an interesting observation is that when a beverage is consumed that contains multiple transportable carbohydrates such as glucose and fructose maltodextrin and fructose, gastrointestinal symptoms seem to be reduced compared with the consumption of the same (large) amount of a single carbohydrate (glucose).
Avoid dehydration. As dehydration can exacerbate gastrointestinal symptoms, it is important to prevent dehydration. Start the race (or training) well hydrated.
Practise new nutrition strategies & train your gut. Make sure to experiment with the pre-race and race-day nutrition plan many times before the race day. This will allow you to work out what does and doesn’t work for you, and reduces the chances of causing GI problems. Another advantage of experimenting is that your gut will adapt to the foods it is regularly exposed too and so if you eat these during a race, they are less likely to cause GI distress.