Amino Acid and Peptide Absorption in Man

An intestinal perfusion technique has been used in man to study absorption of glycine and L-alanine from the dipeptide glycyl-L-alanine and the tripeptide L-alanylglycylglycine. Glycine was absorbed faster from both peptides than from their equivalent free amino acid mixtures, and in the dipeptide experiments the differential in absorption rate of glycine and L-alanine observed when the equivalent mixtures of free amino acids were perfused was not observed during the perfusions with glycyl-L-alanine. These findings suggest that peptide-bound and free amino acids are absorbed from the normal human intestine by separate transport processes. Absorption of individual amino acids from two partial enzymic hydrolysates of casein which consisted predominantly of small peptides was then studied. Absorption of α-amino nitrogen was greater from the casein hydrolysates than from their two equivalent mixtures of amino acids. There was considerable variation in absorption of individual amino acids during perfusion of the two free amino acid mixtures, but this was much reduced when the enzymic hy-drolysate solutions were perfused. This was so because a number of individual amino acid residues that were poorly absorbed from the free amino acid mixtures were absorbed significantly faster from the enzymic hydrolysates. These results suggested that there could be an advantage in using mixtures of peptides rather than free amino acids as the nitrogen source in elemental diets. To test this hypothesis, plasma α-amino nitrogen increments and extent as well as site of absorption of amino acid residues were compared after two liquid test meals containing a protein hydrolysate or its equivalent amino acid mixture had been administered to normal human subjects. The finding of less secretion of fluid into the intestinal lumen, and higher concentrations of α-amino nitrogen in plasma after the protein hydrolysate meal, and the fact that after the two meals amino acid residues were absorbed to the same extent and at the same location, suggest that it would be worth considering the use of peptide mixtures as the nitrogen source in elemental diets.