Insulin reduces leucine oxidation and improves net leucine retention in parenterally fed humans

Protein metabolism during parenteral feeding was measured with and without euglycemic hyperinsulinemic clamping in five healthy human subjects using the primed continuous infusion of [13C]leucine (LEU) tracer methodology. All subjects underwent two periods of protein measurement. Subjects were randomized to have the clamp first or second. Two subjects had the clamp first, and they participated in another study in which the measurement period after the clamp was increased by another 2 h. Insulin reduced LEU oxidation (from 26 to 22 μM · kg−1 · h−1; P < 0.05) and improved net LEU balance (from 3 to 7 μM · kg−1 · h−1; P < 0.05). The order of the clamp influenced the effects of insulin. With clamping performed second, insulin reduced LEU flux (from 152 to 134 μM · kg−1 · h−1; P < 0.01), endogenous LEU rate of appearance (Ra; from 125 to 107 μM · kg−1 · h−1; P < 0.01), and non-oxidative LEU disappearance (NOLD; from 128 to 113 μM · kg−1 · h−1; P < 0.01). With clamping performed first, NOLD decreased after insulin was stopped (from 129 to 121 μM · kg−1 · h−1; P < 0.05), but no change was seen in the flux and LEU Ra, despite the return of plasma insulin and amino acid concentrations to basal levels. The reduction in NOLD was accentuated with time and did not reach plateau even after 6 h and indicated a prolonged carry-over effect for NOLD and Ra. This effect was not seen for leucine oxidation.