Ultra-endurance runners on a long-term low-carbohydrate diet exhibit markedly higher fat oxidation but no difference in resting muscle glycogen levels or the extent of glycogen depletion during and after a 3-hour run compared to ultra-endurance runners on a high-carbohydrate diet.

When a person eats very little carbohydrate for a long time, their muscles learn to burn fat more efficiently by making more mitochondria and special proteins that pull fat into them. This lets the body use fat as its main fuel during long exercise, so it doesn't need to break down as much stored sugar. Even though fat burning goes up, the amount of stored sugar in the muscle at rest and how much gets used during a long run stays the same because the muscle still has access to its full sugar supply and uses it at the same rate as before.