Protein and amino acids
Protein is the only major nitrogen-containing component of the diet. It is composed of long chains of 20 different amino acids that are linked together by peptide bonds. Dietary proteins are digested to their constituent amino acids prior to absorption in the small intestine. Surplus amino acids generated from protein breakdown are used as an energy source; in adults, almost all of the dietary protein is used as an energy source either directly or when body protein is catabolised. The nitrogen-containing amino group is removed to leave a moiety called the keto acid. A negative balance indicates that body protein is being depleted because more nitrogen is being lost than is taken in the diet. Injury, illness, starvation or inadequate protein intake per se may lead to negative nitrogen balance. Studies using radioactively labelled amino acids indicate that around 300 g of protein is normally broken down and re-synthesised in the body each day, the protein turnover.