Glycogen — Stored Carbohydrate Fuel

Glycogen is a branched chain of glucose molecules — the body's storage form of carbohydrate. It is held in two pools:

• Muscle glycogen (~300–500 g in a trained adult): fuels the working muscle that stores it; cannot be released back into the bloodstream.
• Liver glycogen (~80–110 g): broken down to glucose and released to maintain blood sugar for the brain and other tissues.

Total stores hold roughly 1,500–2,000 kcal, enough for about 90–120 minutes of sustained moderate-to-high-intensity exercise. Each gram of stored glycogen binds about 3 grams of water, which is why glycogen loading and depletion cause noticeable swings in body weight.

When muscle glycogen runs low during prolonged exercise, intensity must drop — the endurance phenomenon athletes call "bonking" or "hitting the wall." Strategies to manage glycogen include carbohydrate loading before long events (see carb-loading), consuming carbohydrate during exercise lasting over ~60–90 minutes, and post-exercise refueling to restore stores. The ISSN notes aggressive refeeding of roughly 1.2 g/kg/hour of higher-glycemic carbohydrate is warranted when there are fewer than ~4 hours between sessions.

• Storage form of glucose; muscle (~300–500 g) and liver (~80–110 g) pools.
• Total ~1,500–2,000 kcal; ~90–120 min of higher-intensity exercise.
• Stores ~3 g water per gram of glycogen (affects body weight).
• Muscle glycogen stays local; liver glycogen maintains blood glucose.
• Rapid refuel ~1.2 g/kg/h carbohydrate when <4 h between sessions.

• carbohydrates — dietary source of glycogen.
• carb-loading — maximizing stores before competition.
• carbohydrate-timing — intra- and post-exercise carbohydrate.
• lactic-acid-system — glycogen breakdown for ATP.
• lactic-acid-system — depletion as a fatigue mechanism.