Digestion of proteins begins in the stomach. The HCL of the stomach denatures the protein. Pepsinogen (activated form of pepsin) then begins to break the links of the large protein molecules into polypeptides. Complete digestion of proteins occurs mainly in the small intestine. Proteolytic enzymes produced and stored in the pancreas hydrolyze the denatured protein strands from the stomach. These enzymes work to break polypeptides into smaller dipeptides, tripeptides and individual amino acids. The individual amino acids are absorbed into the bloodstream. Some of the dipeptides and tripeptides are absorbed into the cells lining the small intestine where additional cleaving occurs prior to absorption into the bloodstream. The healthy body is extremely efficient at dismantling large proteins and absorbing amino acids at rates optimal for the body’s use.
Once the amino acids enter the bloodstream they are carried to the liver where they may be used for protein synthesis, an energy source, or circulated to other cells in the body. Unlike the brain and muscles that primarily use glucose for energy, the intestines and liver use amino acids as their primary energy source.