<P> Long - distance athletes, such as marathon runners, cross-country skiers, and cyclists, often experience glycogen depletion, where almost all of the athlete's glycogen stores are depleted after long periods of exertion without sufficient carbohydrate consumption . This phenomenon is referred to as "hitting the wall". </P> <P> Glycogen depletion can be forestalled in three possible ways . First, during exercise, carbohydrates with the highest possible rate of conversion to blood glucose (high glycemic index) are ingested continuously . The best possible outcome of this strategy replaces about 35% of glucose consumed at heart rates above about 80% of maximum . Second, through endurance training adaptations and specialized regimens (e.g. fasting low - intensity endurance training), the body can condition type I muscle fibers to improve both fuel use efficiency and workload capacity to increase the percentage of fatty acids used as fuel, sparing carbohydrate use from all sources . Third, by consuming large quantities of carbohydrates after depleting glycogen stores as a result of exercise or diet, the body can increase storage capacity of intramuscular glycogen stores . This process is known as carbohydrate loading . In general, glycemic index of carbohydrate source does not matter since muscular insulin sensitivity is increased as a result of temporary glycogen depletion . </P> <P> When experiencing glycogen debt, athletes often experience extreme fatigue to the point that it is difficult to move . As a reference, the very best professional cyclists in the world will usually finish a 4 - to 5 - hr stage race right at the limit of glycogen depletion using the first three strategies . </P> <P> When athletes ingest both carbohydrate and caffeine following exhaustive exercise, their glycogen stores tend to be replenished more rapidly; however, the minimum dose of caffeine at which there is a clinically significant effect on glycogen repletion has not been established . </P>

The body stores glucose in which of the following forms