The influence of hyperinsulinemic-hyperglycemia on protein and carbohydrate homeostasis was assessed using L-[1-13C]-leucine and [3- 3H]glucose combined with open-circuit indirect calorimetry. After a 30-min basal period, healthy human volunteers were subjected to two sequential experimental periods (150 min each) during which insulin was continuously infused at a rate to elicit maximal effects (10.0 mU · kg-1 · min-1, resulting in 220-fold basal levels) in conjunction with an infusion of L- amino acids to maintain euleucinemia. Plasma glucose was maintained near basal (94 ± 2 mg/dl) during period I and at twofold basal (191 ± 4 mg/dl) during period II. The endogenous rate of leucine appearance (index of proteolysis in μmol · kg-1 · h-1) dropped by 80% from basal during period I (P < 0.01) but only by 44% during period II. Although hyperglycemia stimulated the rate of proteolysis threefold (18.0 ± 10.2 to 49.8 ± 7.2, P < 0.001), neither leucine oxidation nor nonoxidative leucine rate of disappearance (an estimate of protein synthesis) was altered. During both glycemic conditions, the rates of hepatic glucose production were completely suppressed, and this was accompanied by 3- and 11-fold increases in the estimated rates of protein and carbohydrate oxidation, respectively, and an 80% decrease in the rate of lipid oxidation. The estimated rate of glucose storage was increased fourfold during period I and by an additional 2.3-fold (to ~10-fold basal) in period II. The present study demonstrates that, during hyperinsulinemia, acute elevations of plasma glucose to two times basal levels result in a marked stimulation of whole body proteolysis during hyperinsulinemia.