Protein Digestion and Bioavailability

Proteins are the principal nitrogenous constituents of the protoplasm of all animal and plant tissue, and it is estimated that almost half of the dry weight of animal cells is composed of proteins. The basic structural units of proteins are the amino acids. Dietary proteins are the main sources for the essential amino acids, which cannot be synthesized by humans. The purpose of digestion is to hydrolyze proteins to small peptides and amino acids so that these can be absorbed. The digestion of proteins begins in the stomach by the actions of pepsins and continues in the intestines. Although the final end product of protein digestion is amino acids, small peptides are the dominant form of entry of amino acids into enterocytes, where they are further hydrolyzed into amino acids and absorbed into the bloodstream. The digestibility of a protein is a measure of the amount of protein available from it for absorption after digestion; this is usually obtained from estimates of dietary nitrogen and fecal and urinary nitrogen.