Contribution of Amino Acids and Insulin to Protein Anabolism During Meal Absorption

The contribution of dietary amino acids and endogenous hyperinsulinemia to prandial protein anabolism still has not been established. To this end, leucine estimates ([1- 14C]leucine infusion, plasma α-ketoisocaproic acid [KIC] specific activity [SA] as precursor pool SA) of whole-body protein kinetics and fractional secretory rates (FSRs) of albumin, fibrinogen, antithrombin III, and immunoglobulin G (IgG) were measured in three groups of healthy volunteers during intragastric infusion of water (controls, n = 5), liquid glucose–lipid–amino acid (AA) meal (meal+AA, n = 7), or isocaloric glucose–lipid meal (meal-AA, n = 7) that induced the same insulin response as the meal+AA. The results of this study demonstrate that 1) by increasing (P < 0.01) whole-body protein synthesis and decreasing (P < 0.01) proteolysis, dietary amino acids account for the largest part (∼90%) of postprandial protein anabolism; 2) the ingestion of an isocaloric meal deprived of amino acids exerts a modest protein anabolic effect (10% of postprandial protein anabolism) by decreasing amino acid oxidation and increasing (P < 0.01) albumin synthesis; 3) albumin FSR is increased (∼20%) by postprandial hyperinsulinemia (meal–AA) and additionally increased (∼50%) by amino acid intake (meal+AA); 4) IgG FSR is stimulated (∼40%) by amino acids, not by insulin; and 5) fibrinogen and antithrombin III FSR are not regulated by amino acids or insulin.