We propose that hyperinsulinemia stimulates protein synthesis when postabsorptive plasma amino acid (AA) concentrations are maintained. During a euglycemic hyperinsulinemic clamp, many AA, notably the branched-chain amino acids (BCAA), decline markedly. Therefore, we tested whether individual plasma AA could be maintained within the range of postabsorptive concentrations to assess the effects of insulin, infused at 40 mU/m(2) x min on whole-body protein and glucose metabolism, using [1-(13)C]-leucine and [3-(3)H]-glucose methodology. Validation studies of background [(13)C] enrichment and breath (13)CO(2) recovery factors were performed in a subset of 6 subjects. In 10 healthy, young men, infusion rates of an AA solution were based on fluorometric determinations of total BCAA every 5 minutes. All 21 plasma AA remained in the target range; 15, including the BCAA, alanine, and glycine were within 13% of baseline, and only 6 (Thr, His, Arg, Asn, Cit, Tyr) varied more (18% to 42%). Notably, both leucine flux and nonoxidative leucine R(d) (protein synthesis) increased with insulin (2.36 +/- 0.06 to 2.81 +/- 0.10 and 1.79 +/- 0.05 to 2.18 +/- 0.10 micromol/kg fat-free mass (FFM) x min, respectively; P <.0005) while leucine oxidation only tended to increase (P =.05) and endogenous leucine R(a) (protein breakdown) decreased by 18% (2.36 +/- 0.06 to 1.94 +/- 0.09 micromol/kg FFM x min; P <.0005), resulting in a marked elevation of net protein synthesis (-0.57 +/- 0.02 to 0.24 +/- 0.02 micromol/kg FFM x min; P <.0000001). Thus, in vivo protein anabolism was induced when maintaining postabsorptive plasma amino acid concentrations during hyperinsulinemia through a suppression of whole-body protein breakdown, no significant change in oxidation and an elevation of synthesis compared with postabsorptive conditions.