Regardless of what you're eating, whether it's a gel or a whole food, it's common to experience some gastrointestinal distress , especially later on in a race, Chag says. "This is due to physiological, mechanical, and nutritional reasons, such as blood flow redirected to the muscles and away from the intestines, repeated impact during running, and eating foods higher in fiber or fat, respectively," she says. Some people on Reddit say that energy gels can cause diarrhea , because the nutrients are so highly concentrated. To avoid this, it's best to drink plenty of water , and fuel early on in the run. And hey, even if you do have to stop for the bathroom in the middle of a race, it's not a big deal at all — Des Linden did, and she won the Boston Marathon