When you work out after a big meal, you may feel sluggish or have cramps or an upset stomach. That's because your muscles and your digestive system are competing with each other for energy. On the flip side, if you don't eat before you exercise, especially right away in the morning, you may feel weak, faint or tired. To strike the right balance, eat a light snack before exercising. After a big meal, wait a few hours before hitting the gym.