[Note: Much of the data for this post was drawn from a review article in Sport Medicine1]
Gastrointestinal (GI) complaints are very common among endurance enthusiasts and may be the most common cause of underperformance in endurance events.
Bill Rodgers, a marathoner with four victories in both the Boston and New York City marathons said, "More marathons are won or lost in the porta-potties than at the dinner table."
Generally, GI problems appear to affect 30-50% of endurance event participants. The research numbers, however, vary widely. One study of long-distance triathletes in extreme conditions reported 93% with GI symptoms. Another study of Ironman participants found 4% complaining of GI problems during the running race, but as high as 32% during cycling. In ultra-marathons over 40 miles, 37-89% of the runners reported GI problems ranging from nausea and vomiting to abdominal cramping and diarrhea.
There seem to be three main causes of gastrointestinal symptoms:
During strenuous exercise, blood is shunted away from your gut toward your active muscles, cardiopulmonary system, and skin. The reduction in blood flow (splanchnic hypoperfusion) can lead to abdominal distress and may impair your ability to absorb much needed nutrients—such as carbohydrates and protein, both during and after an activity. Besides the symptoms mentioned above, reflux during exercise may be a result.
The impact-related mechanical causes of GI problems seem to be more common among runners than cyclists. The repetitive high-impact and jostling nature of running can cause damage to the lower GI, resulting in flatulence, diarrhea and urgency.
Posture can have an effect on GI symptoms. Cycling position, especially when "aero," puts pressure on the upper GI. "Swallowing air," as a result of harder, faster breathing and drinking from water bottles, can also result in mild to moderate stomach distress.
In general, the only way to reduce these mechanical causes of GI discomfort is through training.
Nutrition, both during training or an event, and afterwards, during recovery, can also influence GI distress. Research has linked fiber, fat, protein and fructose with a greater risk of developing GI problems. Dehydration can make GI problems worse. Also, beverages with too high a carbohydrate concentration (osmolality >500 mOsm/L) can increase the frequency and severity of symptoms. It is not simply carbohydrate intake that may cause GI problems, but the type and acidity of the beverage (see Carbohydrates—How Much Is Right For You).
If you’re struggling with GI distress during your training or events, or during recovery, try these tips:
1. De Oliveira, E. P., Burini, R. C., & Jeukendrup, A. (2014). Gastrointestinal Complaints During Exercise: Prevalence, Etiology, and Nutritional Recommendations. Sports Medicine (Auckland, N.z.), 44(Suppl 1), 79–85. http://doi.org/10.1007/s40279-014-0153-2