The Scientist: Nancy Clark, M.S., R.D., a Boston-area sports nutritionist and author of “Nancy Clark’s Sports Nutrition Guidebook”
Symptoms are nearly twice as likely to happen while running than other high-endurance sports like cycling and swimming, which suggests that the jostling of the intestines plays a role. At the same time, while running long distances, blood flow is diverted from the intestines to the muscles at work. These factors could alter gut permeability, which can contribute to GI symptoms, though that link has not been fully substantiated. Then there’s dehydration; GI complaints are common in runners who have sweated off more than 4 percent of their body weight (that’s 5.6 pounds for a 140-pound athlete).
To keep the chance of tummy trouble to a minimum, reduce your intake of high-fiber foods, stay away from sugar-free candies that contain sorbitol—which is known to have a diuretic effect—and drink extra water to maintain hydration without overhydrating. You can also try exercising lightly before a race to help empty the bowels ahead of time.