If you are going to exercise vigorously for more than an hour, you need to eat before you exercise or your muscles and liver will run out of sugar and you will tire earlier. Your brain gets more than 98 percent of its energy from sugar in your bloodstream. But there is only enough sugar in your bloodstream to last three minutes. So you liver has to constantly release sugar from its cells into your bloodstream. There is only enough sugar in your liver to last about an hour when you exercise vigorously. Eating before exercising can help you to exercise longer. If you do not eat before you exercise for more than an hour, and during exercise that lasts more than two hours, your liver will probably run out of sugar. Your blood sugar level can drop, and you will feel terrible fatigue and tiredness.
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 stomach cramps because their hearts are strong enough to pump blood to both their exercising muscles and their stomach muscles.
Another theoretical concern is that eating sugar before you exercise 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. 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.
Yes, I agree with the gluten intolerance causing stomach pain. If I eat gluten, in any quantity, it causes stomach pain, joint pain, migraines and, for a few days, I type words backwards and will have an inability to get my words out correctly.