Glycogen

Glycogen is how the body stores carbohydrate — chains of glucose kept mainly in the muscles and liver, ready to be broken down for energy. Muscle glycogen fuels the muscle it's stored in; liver glycogen helps maintain blood sugar.

It matters because glycogen is the primary fuel for hard, high-intensity and prolonged exercise; running low contributes to fatigue and 'hitting the wall'. Each gram of glycogen also holds water, which is why carbohydrate intake affects how full or flat muscles look and short-term scale weight.

In practice, carbohydrate around training tops up glycogen, and the water shifts that come with changing carbohydrate intake explain much of the rapid weight change people see when starting or stopping a low-carb diet.