It takes a lot of energy to run, cycle or ski long distances; to play three sets of tennis or a pickup game of basketball; or to walk for several hours. During intense exercise, your muscles draw sugar from your bloodstream at a rapid rate. Your liver can run out of its stored sugar and your blood sugar level can drop. You can markedly improve your performance in any of these events by eating shortly before your event begins, and by starting to eat and drink as soon as you start exercising.
The energy for your brain comes almost exclusively from the sugar in your bloodstream. When blood sugar levels drop, you feel tired and have difficulty coordinating your muscles. However, there is only enough sugar in your bloodstream to last three minutes at rest. To maintain blood sugar levels, your liver has to release sugar into your bloodstream. But there is only enough sugar in your liver to last around twelve hours at rest and far less than that when you exercise. When muscles run out of their stored sugar supply, it hurts to exercise and the muscles become difficult to control. Most people who exercise for more than an hour will improve their endurance if they start replenishing energy reserves as soon as they start to exercise.
Hunger during exercise is a very late sign of not getting enough calories. You can increase your endurance by starting to eat anything or to drink fluids that contain sugar as soon as you start to exercise. This will give you far greater endurance than waiting to take food after an hour of exercise or when you feel hungry.
