The gut is an important athletic organ because it mediates water and nutrient uptake during exercise. But upper- and lower-gastrointestinal complaints, common to many athletes during exercise, can negatively impact performance, and in severe cases put health at risk. The gut is sensitive to water and nutrient intake during exercise and to hypovolemia, hyperthermia, hypoglycemia, hypoxia, and ischemia. Most gastrointestinal complaints during exercise are mild and of no risk to health, but hemorrhagic gastritis, hematochezia, and ischemic bowel can present medical challenges. Proper training and nutrition minimize the risk of gastrointestinal discomfort during exercise by assuring rapid gastric emptying and absorption of water and nutrients and by maintaining adequate perfusion of the splanchnic vasculature.