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You started training for a race.
In her book, Yoshida mentions a study that found that 71 percent of runners—almost three-quarters of them!—experience lower-GI problems like flatulence, bloating and diarrhea, and she adds that other studies have confirmed GI problems in endurance athletes. She speculates that intense exercise may slow the speed at which food moves through the body, and also make the gut more sensitive. Suddenly amping up your workouts, which can result in injuries to your extremities, can also wreak havoc on your insides. Play it safe by gradually increasing the intensity of your training.