Digesting dietary proteins is the function of metabolism.
Good sources of protein include egg whites, legumes, soy products, milk whey, fish and of course red meat.
There is an abundance of protein in the American diet and a deficiency is rare,
There are a few strict vegetarians however who may not be eating balanced meals and therfore will be missing some of the essential amino acids.
Dietary proteins supply the body with all the essential amino acids it cannot manufacture or produce in adequate amounts. These proteins must first be degraded by a group of proteolytic enzymes called proteases.
Proteases catalyze the hydrolysis (adding of water) of peptide bonds. These bonds connect one amino acid to another.
Proteins are much more readily degraded by proteases if they are denatured first. Denaturing occurs during cooking or when a protein is exposed to the strongly acidic conditions of the stomach.
The stomach lining contains parietal cells that secrete an acid, hydrogen chloride (HCl) in the stomach. This acid denatures the protein, unfolding it to expose cleavage points where the proteolytic enzymes can work.
The actual digestion and active absorption of proteins occurs in the villi of the small intestine. The structure of the small intestine is ideally suited to this important function.
Numerous folds or finger-like villi increase the surface area of the small intestine. The amino acids are thus able to diffuse across the brush border of the villi and enter the circulation. The metabolism of amino acids is very complex with different pathways allowing them to be converted from one to another. The branch-chained amino acids (leucine, isoleucine and valine) are capable of being oxidized for energy during periods of intense activities