Intestinal absorption of protein hydrolysis products: a review.

Many experimental techniques have allowed researchers to probe the fate of hydrolysis products from proteins in the small intestine. An overview of amino acid and peptide absorption from the small intestine is presented with attention given to historical perspectives that have led to current concepts. Speculation about nutritional significance of these processes is offered. Species differences exist in site of amino acid absorption. Numerous mechanisms are available for the transport of amino acids, including Na+-dependent carriers (energy-requiring), Na+-independent carriers and diffusion. The relative contribution each transport system makes to absorption is dependent on substrate concentration. Individual amino acids are not absorbed with equal efficiency; methionine usually is absorbed in the greatest proportion. There are interactions among amino acids for transport by specific transport systems. Small peptides (mostly di- and tripeptides) are absorbed from the small intestine more rapidly than are free amino acids; peptides are transported by systems independent of those responsible for transporting free amino acids. Evidence exists that the active transport of these peptides is via a proton gradient. Although the concept that peptides are absorbed intact into the circulation is not universally accepted, evidence supporting the possibility of tissue utilization of these small peptides is accumulating. Nutritional implications imposed by peptide absorption remain largely unexplored.