What is glycogen, and why does it run out?
Glycogen is the stored carbohydrate that fuels marathon running — roughly 400–600 g in muscle and 90–120 g in the liver, which is not quite enough to cover 42 km at race pace.
Glycogen is glucose stored in long branched chains, held in skeletal muscle and in the liver. Muscle glycogen fuels the muscle it sits in and cannot be shared; liver glycogen is the body's way of keeping blood glucose steady, which matters for your brain as much as for your legs. A trained runner carries roughly 400–600 g in muscle and another 90–120 g in the liver.
At about 17 kJ per gram, a full tank holds somewhere between 8,300 and 12,200 kJ. A marathon costs roughly 1 kcal per kilogram per kilometre, so a 70 kg runner spends around 2,950 kcal — about 12,400 kJ — over the distance. Those two numbers are close enough that the tank runs low near the end, which is exactly why the marathon has a wall and the half marathon does not.
Two things change the arithmetic. Running slower shifts a larger fraction of the energy toward fat, which is effectively unlimited, so the glycogen lasts longer. And eating during the race adds carbohydrate from outside the tank, which is why fuelling strategy is a pacing decision as much as a nutrition one.
Glycogen is stored with water — roughly 3 g of water per gram of glycogen — which is why a carbohydrate-loaded runner weighs more on race morning and why the first days of a very low-carbohydrate diet produce dramatic, misleading weight loss.
Key numbers
| Muscle glycogen | 400–600 g | trained runner, fully loaded |
|---|---|---|
| Liver glycogen | 90–120 g | maintains blood glucose |
| Energy density | 17 kJ/g | carbohydrate |
Work it out for yourself
Substrate & time-to-exhaustion calculator — Model your own glycogen depletion at any pace and carbohydrate intake rate.
Frequently asked questions
How much glycogen can a runner store?
Roughly 400–600 g in skeletal muscle and 90–120 g in the liver, so about 500–720 g in total. Training increases storage capacity, and carbohydrate loading in the days before a race fills it toward the top of that range.
Does carbohydrate loading actually work?
It reliably increases stored glycogen, and the effect is largest for events lasting longer than about 90 minutes — which includes essentially every marathon. It is not free: each gram of glycogen is stored with roughly 3 g of water, so expect to be 1–2 kg heavier on race morning.
Sources
- Areta & Hopkins (2018). Skeletal muscle glycogen content at rest and during endurance exercise. Sports Medicine.
- Rapoport (2010). Metabolic factors limiting performance in marathon runners. PLoS Computational Biology.
Related explainers
Glossary: Glycogen, CHO Oxidation