Protein foods must always furnish a part of the day's energy, from 10 to 15 per cent, not because the protein is needed for the energy which it furnishes but because it has value as building material for the body. The energy which it gives is a byproduct. Although protein gives energy, it should not be used as a main source of fuel, since it is more scarce and, therefore, more expensive than other fuels, and large amounts may overtax the powers of the body to care for it.