The primary function of dietary protein is to supply nitrogen and amino acids for the synthesis of body protein, replenishment of tissue proteins and other nitrogen containing substances. The food protein are digested by proteolytic enzymes in the gastrointestinal tract, producing free amino acids and small peptides.

For 100 gram of food proteins ingested per day, only about 10 gram is eliminated in the feces, although an additional 50-70 g of endogenous proteins are secreted daily. This mechanism provides optimal amounts and proportions of amino acids for the synthesis of body proteins.

Muscular proteins constitute calories reserves that can be used during fasting or other periods of caloric deprivation. When the protein intake is low, and the food calories supply is adequate, the breakdown of body proteins is minimal. This adaptation mechanism complicates the evaluation of daily protein requirements.

Muscular protein turnover increases when there is a deficiency in dietary protein calorie, the need of Insulin and some other hormones, but decreases with age. Protein deficiency also decreases the hepatic synthesis, the blood level and the catabolism of plasma proteins.