There are no enzymes in the mouth acting on protein. In the stomach, the hydrochloric acid helps to make it soften and swell, and then pepsin begins its digestion. Protein, like fat and carbohydrate, can be subdivided into smaller and smaller portions, finally being reduced to a form which the body can absorb, namely, amino acids, of which there may be 17 or 18 kinds from a single protein.

The digestion in the stomach produces chiefly large fragments of the original protein, called proteoses. In the pancreatic juice is a powerful enzyme called trypsin, which digests proteins, first to fragments, next smaller than proteoses, called peptones, and finally breaks these peptones into amino acids. In the intestinal juice is another enzyme called erepsin, which also forms amino acids from proteoses and peptones, thus finishing any digestion of protein left incomplete by the trypsin.