There are two energy-producing systems in the body that allow us to be in motion. One is for brief high-intensity activity, called the Anaerobic Energy System, and the other is for low-intensity endurance-type (or long and slow) activity, called the Aerobic Energy System.

The body, like a machine, needs energy to operate. This requires some type of fuel, in this case food in the form of protein, carbohydrates and fat, which is burned off into energy. You also have stored fuel sugar and fat.

There are two energy-producing systems in the body that allow us to be in motion. One is for brief high-intensity activity, called the Anaerobic Energy System, and the other is for low-intensity endurance-type (or long and slow) activity, called the Aerobic Energy System.

Under certain circumstances, such as in emergencies or times of sudden intense exertion, we need quick, explosive energy for a short duration. On other occasions, we need longer-lasting endurance-type energy to keep us going for extended periods.

From The 7 Principles of Fat Burning