Glycogen — Stored Carbohydrate Fuel
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Glycogen is the body's stored carbohydrate, held in muscle and liver; it powers higher-intensity exercise and its depletion is a major cause of endurance fatigue.
Detail
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 Carbohydrate 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.
Key facts
- 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.
Connections
- Carbohydrates — Types, Glycemic Index, and Requirements — dietary source of glycogen.
- Carbohydrate Loading — maximizing stores before competition.
- Carbohydrate Timing for Endurance — intra- and post-exercise carbohydrate.
- Lactic Acid (Glycolytic) System — glycogen breakdown for ATP.
- Lactic Acid (Glycolytic) System — depletion as a fatigue mechanism.
Sources
See how this applies in practice, with honest limits on what the plan changes.
Educational content only — not medical advice. Always consult a qualified professional for individualized guidance, especially around injury, pregnancy, or medical conditions.