It also helps insulate the body and keep it warm. Primarily, however, the body uses fat for energy. Fat contains more than twice as much energy per gram as do carbohydrates. Fat is also the primary fuel of choice during slow aerobic exercise, while glucose is used during fast aerobics or anaerobic exercise. Proteins can also be used for energy, but primarily, they are used as the building blocks of the body.
Muscle tissue is composed of proteins.
A protein matrix is also contained in the integument and the linings of organs. If the body runs out of glucose and fat during exercise, it will burn protein. The amount of energy contained in food is typically measured in calories . Technically, a dietary calorie is actually a kilocalorie using the chemical definition. A calorie is the amount of heat energy that is required to increase the temperature of 1 gram of water by 1 degree Celsius.