Glycogen and glycogen depletion
Evidence: strong
Glycogen is the body’s limited store of fast carbohydrate fuel, enough for roughly 90 to 120 minutes of hard running. Running it down is “the wall”. Loading raises it, in-race carbohydrate extends it, and training teaches the body to spare it.
Glycogen is carbohydrate stored for quick use, held in the muscles and the liver. It is the fuel that matters most for running at race intensities, because although the body holds far more energy as fat, fat cannot be broken down fast enough to fully power a hard pace. At anything above an easy jog, carbohydrate supplies a large and rising share of the energy (energy systems).
The stores are small
Total glycogen stores come to roughly 400 to 500 g, mostly in muscle with a smaller liver reserve, around 1,600 to 2,000 kcal of usable carbohydrate. That is enough for somewhere in the region of 90 to 120 minutes of hard running before stores run low, which is why fuelling becomes decisive in events beyond about that duration (Burke et al. 2011). Each gram of glycogen is stored with around 3 g of water, which is why carbohydrate loading adds a kilogram or two of water weight.
Depletion: “the wall”
When muscle glycogen runs low, pace collapses, the marathoner’s “wall” or “bonk”, typically around 30 km in a marathon run too fast or fuelled too little. Muscle glycogen depletion forces a drop to a pace that fat oxidation can sustain, which is much slower, while liver glycogen depletion lowers blood glucose and starves the brain, adding light-headedness, heaviness and a sudden loss of drive on top of the muscular slowdown. This depletion is also a large part of the late-race deterioration captured by durability.
Managing it
It is managed on several fronts, each with its own page:
- Raise the stores. Carbohydrate loading roughly doubles muscle glycogen before long races.
- Top them up during the race. In-race carbohydrate spares and supplements glycogen, the main reason fuelling extends how long pace can be held.
- Spare it through training. Endurance training raises the capacity to burn fat at a given pace, which preserves glycogen, and the long run rehearses running as stores fall.
- Pace sensibly. Starting too fast burns glycogen faster, since carbohydrate’s share of fuel rises with intensity, bringing the wall forward.
Training low, deliberately
Some training is done with low glycogen on purpose, “train low”, because the low-fuel state amplifies the molecular signals for adaptation. The signalling is real, but periodised carbohydrate restriction has not translated into clear performance gains, so it is a tool to use sparingly rather than a default (Hawley et al. 2018). For refuelling between sessions, rapid carbohydrate intake matters mainly when the next hard effort is within about eight hours (Kerksick et al. 2017).