Glycogen Depletion and Endurance Exercise
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 enough energy consumption. This phenomenon is referred to as "hitting the wall". In marathon runners, it normally happens around the 20-mile (32 km) point of a marathon, depending on the size of the runner and the race course.
Glycogen depletion can be forestalled in four possible ways. First, during exercise carbohydrates with the highest possible rate of conversion to blood glucose per time (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 training, the body can be conditioned to burn fat earlier, faster, and more efficiently, sparing carbohydrate use from all sources. Third, by consuming foods low on the glycemic Index for 12–18 hours before the event, the liver and muscles will store the resulting slow but steady stream of glucose as glycogen, instead of fat. This process is known as carbohydrate loading.
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 4-5hr stage race right at the limit of glycogen depletion using the first 3 strategies.
A study published in the Journal of Applied Physiology (online May 8, 2008) suggests that, when athletes ingest both carbohydrate and caffeine following exhaustive exercise, their glycogen is replenished more rapidly.
Read more about this topic: Glycogen, Clinical Relevance
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