A protein is a complex material which is used by the body for the formation, replacement, or repair of such diverse materials as hair, skin, muscle, blood vessels, nerves, soft parts of bone, the brain, and body organs of which the heart may be taken as an example.
Proteins are made up of the same simpler materials, but not always in the same proportions. Different proteins have been likened to different types of buildings, in which the proportions of building materials vary.
The simple building materials must be present in a protein foodstuff in the right proportion or obtained from a mixture of protein foodstuffs, which when broken down in digestion afford the necessary building materials for body proteins.
Proteins can also be used by the body for the production of fats and energy. There are, however, classes of foodstuffs from which the human being can obtain fats and energy more cheaply than from proteins.