protein metabolism, the processes whereby protein foods are used by the body to make tissue proteins, together with the processes of breakdown of tissue proteins in the production of energy. Food proteins are first broken down into amino acids, then absorbed into the bloodstream, and finally used in body cells to form new proteins. Amino acids in excess of the body’s needs may be converted by liver enzymes into keto acids and urea. The keto acids may be used as sources of energy via the citric acid cycle, or they may be converted into glucose or fat for storage. Urea is excreted in urine and sweat. Growth hormone, insulin, and androgens stimulate protein formation, and adrenal cortical hormones tend to cause breakdown of body proteins. Diseases affecting protein metabolism include homocystinuria, liver disease, maple sugar urine disease, and phenylketonuria.
