The sum of the physical and chemical changes involved in the breakdown and synthesis of proteins in the body. Dietary proteins are digested to amino acids, which are absorbed by the small intestine and used to synthesize enzymes and the structural proteins that are essential for growth and repair of tissue. Amino acids in excess of protein synthesis requirements are deaminated; the NH2 group is removed and converted to urea, which is excreted by the kidneys. The remaining carbon chain may be converted to a simple carbohydrate and oxidized to produce energy.