Physical activity in the form of exercise requires the metabolism of bodily fuel reserves to provide energy for muscle contraction. Under normal circumstances, very little protein is metabolized to provide the energy for muscle contraction. At rest and at low exercise intensities, the metabolism of fat provides a considerable proportion of the energy for resting metabolic processes and muscle contraction. However, at exercise intensities at which athletes train and compete, the metabolism of bodily carbohydrate reserves (eg, blood glucose and liver and muscle glycogen) provides the predominant fuel for muscle contraction. Furthermore, when these substrates reach critically low amounts or are decreased by some amount, fatigue occurs. There is a significant body of literature examining the effects of ingestion of various types of sugars at various times during exercise and during recovery from exercise on carbohydrate fuel reserves and on physical performance.