Regulation of carbohydrate and fat metabolism during and after exercise

During vigorous exercise, carbohydrate, in the form of muscle glycogen and blood glucose, is the primary energy source, whereas fatty acids play a secondary, carbohydrate-sparing role. In the postabsorptive state, nearly all of the carbohydrate used during exercise comes from muscle and liver glycogen. The size of these glycogen stores plays a major role in determining how long vigorous endurance exercise can be performed if other causes of fatigue, such as dehydration or heat exhaustion, are avoided. In a "fight or flight" situation, individuals whose muscles are glycogen depleted are helpless, as they are unable to either run or fight. The same is true of someone who has become markedly hypoglycemic as a result of liver glycogen depletion.