CATABOLISM DOMINATES THE FIRST-PASS INTESTINAL METABOLISM OF DIETARY ESSENTIAL AMINO-ACIDS IN MILK PROTEIN-FED PIGLETS

To investigate the extent of first-pass intestinal metabolism of dieta ry amino acids, seven female pigs (28 d old, 8.0 kg) were implanted wi th arterial, venous, portal and gastric catheters and with an ultrason ic portal blood flow probe. The pigs were fed a milk-based diet once h ourly and infused intragastrically with [(UC)-C-13]algal protein. On a verage, 56% of the essential amino acid (EAA) intake appeared in the p ortal blood. However, the net portal balance of methionine (48% of int ake) and threonine (38% of intake) tended (P = 0.08) to be lower than the mean of all EAA. The net portal balance (expressed as a percentage of intake) of alanine (205%), tyrosine (167%) and arginine (137%) exc eeded their intake. Net portal outflow of ammonia accounted for 18% of total amino acid nitrogen intake. As a percentage of the enteral trac er input, there was substantial first-pass metabolism of lysine (35%), leucine (32%), phenylalanine (35%) and threonine (61%). However, only 18, 21, 18 and 12% of the total first-pass metabolism of lysine, leuc ine, phenylalanine and threonine, respectively, were recovered in muco sal protein. We conclude that roughly one third of dietary intake of E AA is consumed in first-pass metabolism by the intestine and that amin o acid catabolism by the mucosal cells is quantitatively greater than amino acid incorporation into mucosal protein.