The major component of food is protein. The proteins ingested as part of our diet are not the same proteins required by the body, nor can large molecules be absorbed from the gut. Therefore, these proteins are digested and their component amino acids are absorbed into the blood stream. Amino acids which are not required by the tissues are excreted in the urine as urea. The remaining amino acids are used for tissue repair and body building.

Digestion breaks protein down to amino acids. If amino acids are in excess of the body's requirements, they are metabolized to glycogen or fat and subsequently used for energy metabolism.