The Glycemic Index measures how high blood sugar levels rise 30 to 120 minutes after eating a particular food or combination of foods. A study from Loughborough University in England shows that athletes in sports events lasting more than a couple hours may benefit from a pre-competition meal that has a low glycemic index.
How long you can exercise a muscle without hurting depends on how much sugar you can store in that muscle and how long you can keep that sugar in the muscle during competition. Just about everyone agrees that taking extra carbohydrates for two or there days prior to an endurance competition can help fill your muscles maximally with stored sugar and therefore increase endurance. A well-trained athlete can also fill his muscles maximally with sugar just by cutting back on workouts for a few days prior to competition, no matter what he eats.
Since it takes up to 24 hours to fill your muscles maximally with sugar, the pre-race meal is not used for that purpose. This new study showed that a low-glycemic index meal taken three hours prior to competition may help an athlete to exercise longer by causing muscles to use more fat, and less sugar, for energy. While nobody really knows why, the most likely explanation is that when blood sugar levels rise too high, the pancreas releases huge amounts of insulin. Insulin drives sugar into cells and causes cells to burn more sugar. This uses up sugar more quickly. On the other hand, the low-glycemic meal does not cause a high rise in insulin, so muscle burn more fat, preserve their stored sugar supply and can be exercised longer.