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Glycogen
Rating
Glycogen is the storage form of carbohydrates in the muscles and liver, the main "fast" fuel for intense physical work, including running.
In detail
Glycogen stores last on average 90-120 minutes of intense work (longer for trained athletes, due to a larger storage capacity and the ability to use it more economically). When glycogen runs out, you hit "the wall" — a sharp drop in performance, because fat as an alternative fuel provides energy noticeably more slowly.
How to use it
For races longer than 75-90 minutes, replenish carbohydrates during the race (gels, isotonic drinks) — waiting until glycogen is fully depleted to start eating is already too late.