Whenever your stomach fills with food, its muscles contract and require large amounts of blood. When you exercise vigorously, your heart pumps large amounts of blood to your skeletal muscles. If your heart is not strong enough to pump blood to both your stomach and your skeletal muscles, blood is shunted from your stomach muscles, the muscles lack oxygen, lactic acid builds up in muscles and they start to hurt. However, most people can exercise after eating without suffering cramps because their hearts are strong enough to pump blood to both their exercising muscles and their stomach muscles.
Some researchers believe that you shouldn’t eat sugar before you exercise because it will cause your blood sugar level to rise and your pancreas to release insulin, which will cause your blood sugar to drop too low so you will feel tired during exercise. However, the major cause of tiredness that you feel in your muscles during exercise is lack of stored sugar in muscles. Taking any extra calories before and during exercise helps to preserve the sugar that is stored in muscles and help you to exercise longer. If you are going to exercise for more than an hour, eat or drink anything you like before and during your exercise. Most people will not get stomach cramps while exercising, no matter what or when they eat.