Do you skip breakfast before early morning exercise because you say it upsets your stomach? From now on, don't skip it. Missing breakfast makes you feel tired sooner and keeps you from performing at your best.
For example, cyclists need to eat during competition to prevent "bonking," a sudden drop in blood sugar that can cause them to pass out. Marathon runners eat before races to prevent "hitting the wall," or running out of muscle sugar that causes the muscles to hurt or become hard to coordinate.
There is only enough sugar in your bloodstream to fuel your body for three minutes. To keep your blood-sugar level from dropping, your liver constantly releases sugar from its cells into your bloodstream. When your liver runs out of its supply of stored sugar, your blood-sugar drops and you will bonk.