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Gastrointestinal (GI) symptoms affect as many as 65% of long-distance runners.1 Exercise-related complaints for young athletes can include abdominal pain "stitch in the side," diarrhea "runner's trots." Exercise-related ischemic colitis can lead to GI blood loss in young endurance athletes under extreme conditions. Constipation, gastroesophageal reflux, and gastroenteritis can affect young athletes. Traveler's diarrhea can affect young athletes that travel for competition. Exercise in can uncover underlying conditions such as irritable bowel syndrome or inflammatory bowel disease.

Exercise is associated with reduced gastric emptying, malabsorption of water and nutrients, delayed transit time, and a decrease in splanchnic blood flow. Although small bowel transit time may be decreased, there is no clear effect on overall gastrointestinal (GI) transit time. Esophageal peristalsis is altered by exercise, but there is no consensus of effect on motility (Table 17-1).

Table 17-1. Regional Physiologic GI Tract Effects of Exercise

Gastric emptying has been shown to decrease with increasing intensity of activity. Gastric emptying is impeded by exercise intensity of greater than 70% VO2max.2 Consumption of a high-carbohydrate load (>7%) can slow gastric emptying as well.2 Dehydration and hyperthermia will both further impair gastric emptying. Slowed gastric emptying may lead to symptoms such as nausea vomiting, reflux, heartburn, side ache, and chest pain.

A direct mechanical effect of exercise has been suggested. Muscle hypertrophy, for example, of the psoas muscle may alter GI motility directly.3 The role of physical activity on motility is not clear. Only heavy activity may have an impact on motility. No significant change in motility has been found because of transmitted movements with running.3 However, increased intra-abdominal pressures related to exertion of lifting and increased activity of the abdominal muscles can lead to reflux of gastric contents.

Overall, there is no change in small colon transit time or absorption related to exercise. Small bowel transit time and propulsion decline with increasing exercise intensity.3 Exercise may have no effect on large bowel transit time. Therefore, it is likely that exercise has no overall effect on GI motility. One study showed small bowel and colonic transit times were similar in trained and sedentary subjects at the rest and with exercise. The diarrhea seen in this study did not result from accelerated colonic transit.4 Small colon transit time has been shown to increase in some studies, decrease in other studies. No total reduction in absorption in spite of decreased blood flow, is probably because of length of colon. Small colon distension may feedback to stomach to decrease emptying.


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