One of the principal factors involved in the onset of exhaustion or fatigue is a depletion of muscle glycogen stores. The carbohydrate in sport drinks becomes an available source of glucose to working muscle. It was once thought that the carbohydrate in a sport drink might slow the rate of glycogen breakdown and thus prolong endurance exercise. However, research has shown that the carbohydrate in a sport drink actually becomes an increasingly more important carbohydrate source for working muscle as glycogen stores wane. This contribution seems to be significant enough to push back the onset of fatigue. This could be the difference in finishing strongly during a marathon or fatiguing in the last couple of miles.
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