Dietary Protein Digestion

Most of the nitrogen in the diet is consumed in the form of protein, typically amounting to 70–100 g/day in the American diet. Proteins are generally too large to be absorbed by the intestine. [Note: An example of an exception to this rule is that newborns can take up maternal antibodies in breast milk.] Therefore, proteins must be hydrolyzed to yield di- and tripeptides as well as individual amino acids, which can be absorbed. Proteolytic enzymes responsible for degrading proteins are produced by three different organs: the stomach, the pancreas, and the small intestine (Fig. 1).

Figure 1:  Digestion of dietary proteins by the proteolytic enzymes of the gastrointestinal tract.