In brief: Many athletes think that their performance is helped by a high-sugar snack shortly before competition. To test this theory, the authors studied the effect of a preexercise snack on the duration to exhaustion and the changes that occur to blood glucose and blood lactate. Five college distance runners rode a bicycle ergometer to exhaustion—once after a glucose drink and once after drinking an equal volume of sugar-free, caffeine-free cola. Exercise time to exhaustion was 25% longer without the sugar drink than with it.