The adjective metabolic describes anything relating to the processes in the body that change food into energy. The high metabolic rate of an endurance athlete requires them to eat enormous meals.
Metabolic processes are incredibly complicated, and they happen at the cellular level in human (and non-human) bodies. Turning calories from food into energy which keeps a body moving is just the simplest way to think about metabolic activity. When you consider the change that occurs (a peanut butter and jelly sandwich transforming into energy you use to play a soccer game), the root of metabolic makes total sense: it's the Greek metabole, "a change."