Fasting for just a few hours slows an athlete down and the longer he fasts, the slower he moves. A recent study from Denmark shows that after 72 hours of fasting, a person's muscles accumulate far more fat and glycogen (stored sugar) than after 10 hours of fasting (American Journal of Physiology, Endocrinology and Metabolism, January 2012). This slows a competitive athlete down because it keeps muscles from responding to insulin and using more sugar for energy.
Insulin drives sugar into muscles most effectively when muscles are low on sugar and fat. Filling muscles with sugar or fat blocks insulin and reduces the amount of sugar that can enter muscle cells. Remember that when you exercise for more than an hour, you need to keep on taking sugar. The more intensely you exercise, the greater percentage of sugar your muscles use for energy. Emptying your muscles of sugar causes sugar to enter muscles even faster.
LACK OF SUGAR LIMITS SPEED: The time it takes to get oxygen into muscles is the limiting factor for how fast an athlete can move in competition. If he can get more oxygen into muscles, he will move faster. Muscles use fat, sugar and (to a lesser degree) protein for energy. Since sugar requires the least oxygen, an athlete moves faster when his muscles burn a greater percentage of sugar. If he fasts, he gets almost all of the energy to drive his muscles from his own body fat. Using fat for energy requires more oxygen, so he has to slow down.
MUSCLES CAN STILL FILL WITH SUGAR DURING FASTING: How can muscles start to fill up with sugar after three days of fasting? With fasting, the body breaks down its own protein for energy. Protein is made up of chains of building blocks called amino acids. Some of the amino acids are called branched-chain amino acids. The liver can convert these amino acids into sugars (called gluconeogenesis) which then travel in the bloodstream to be stored in muscles as glycogen.
HOW TO EAT TO COMPETE: If you are competing in sporting events that require speed, you should eat a meal that contains carbohydrates closer than three hours before the start of your event. If the event lasts more than an hour, take some source of sugar during your event, such as sugared drinks, fruit, candy, grain bars or dried fruit paste (fruit leather).