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.