Stomach issues are bound to happen during exercise. They could be due to a number of different reasons, such as your food intake or electrolyte imbalance, and can result in nausea, heartburn, or gastrointestinal bleeding. Runners, especially endurance athletes, are particularly at risk to these negative effects which is why a plan is needed for dealing with the gastrointestinal stress of exercise. If left untreated, an unhealthy gut can impair recovery, cause inflammation, throw immunity out of balance, or decrease energy levels.