Our gastrointestinal tract performs optimally at rest. Exercise interrupts the process via diverting the blood flow away from the gut, and alteration and secretion of various hormones, which act on the gut.
One strategy to minimise symptoms is to exercise on a relatively empty gut. For example, if you are competing in an endurance event any time before lunch, consider having nothing but fluids after dinner the night before. This will enable you to start the event with less work for the gut to perform in dealing with solid ingested material.
During the event, do not push the fluid replacement too aggressively. Follow the advice of Dr. Tim Noakes in Chapter 58 of Clinical Sports Medicine and drink according to thirst during the event. You should obviously aim to start well-hydrated. If you want to have any solid food then try a range of the proprietary sports bars or gels; in a training situation, find one that suits you.
If you are still experiencing GI symptoms, particularly if they occur at rest as well as with exercise, consider getting a check up from your doctor. It may be that you have intolerance of lactose or gluten, or need to consider going on a low FODMAP diet; FODMAP is an acronym for fermentable oligosaccharides, disaccharides, monosaccharides and polyols.
This diet can help reduce bowel spasm by reducing the presence of undigested carbohydrate in the colon. This, in turn, will mean there is not so much fluid that needs to be delivered to colon and it does not get stretched so far. Collectively, these changes can result in reduced bowel symptoms.
Do not attempt to self-diagnose and seek the assistance of your general practitioner, who may elect to involve a gastroenterologist as well. Obviously a sports dietician can be of tremendous help in working through some of these issues.
Dr Chris Milne is a sports physician practising in Hamilton, New Zealand. He has been NZ team doctor at four Olympic Games since 1996, and is currently Chairman of the Medical Commission for Oceania National Olympic Committees. He is a Past President of the Australasian College of Sports Physicians, and has a particular clinical interest in the medical aspects of sport and exercise.