Our daily food choices resupply the potential energy, or fuel, that the body requires to continue to function normally. This energy takes three forms: carbohydrate, fat, and protein. (See table 2.1, Estimated Energy Stores in Humans.) The body can store some of these fuels in a form that offers muscles an immediate source of energy. Carbohydrates, such as sugar and starch, for example, are readily broken down into glucose, the bodyâs principal energy source. Glucose can be used immediately as fuel, or can be sent to the liver and muscles and stored as glycogen.