Serious athletes often develop gastrointestinal (GI) disorders during training and competition that include cramps, diarrhoea, and nausea. Cramps and diarrhoea are a reflection of over activity of the lower intestinal tract or colon, and are much more common in runners than in cyclists.

Studies have demonstrated a reduced blood flow to the digestive system during vigorous exercise - an 80% reduction after 1 hour cycling at 70% VO2max. Plus there was a direct relationship in that individuals with the most severe symptoms had the greatest decrease in blood flows. The type of exercise also plays a role, and it is speculated that the mechanical trauma (a jostling effect) to the abdominal organs may explain why runners have more symptoms than cyclists. Changes in GI hormone levels have been noted with vigorous exercise, but a cause and effect relationship to symptoms has not been proven. Stress factors are probably more important as a cause of pre competition symptoms such as nausea, vomiting, and diarrhoea.

Heartburn (or oesophageal reflux) is more frequent when exercising within 2 hours of eating. The current feeling is that this increase in reflux is related to a combination of meal effects (especially fats) on the oesophageal sphincter pressure (which prevents reflux of stomach contents into the oesophagus), the increased volume of food and acid in the stomach available to reflux, and the mechanical jostling that occurs.  This is usually a minor problem for cyclists and is best handled by delaying exercise after eating.

Exercise delays stomach emptying, and the more vigorous the exercise, the greater the delay. Running once again appears to have a greater effect than cycling, presumably because of the mechanical jostling of the stomach as well as other abdominal organs. In addition to the increase in oesophageal reflux the delay in stomach emptying can cause a sensation of fullness and nausea as well as limiting the immediate availability of Calories from the food eaten. 

An increase in small and large intestinal activity is the cause of abdominal cramps and is reflected in an increase in the frequency of defecation as well. It has been speculated that there might be changes in digestive hormones associated with exercise that then stimulate the colon. But it is more likely that once again the mechanical factor of jostling the bowel is a more important factor. A fibre rich, pre race meal can also play a role. In a recent post race survey, almost all the triathletes who had eaten a high fibre meal suffered from cramps. Minimizing cramps requires a focus on:

Most of these issues are more problematic for runners (and thus triathletes) than cyclists. Except for competitive cyclists, the effects of exercise on the GI tract are minimal.

So let's review the tips to decrease GI problems: