Whole-body protein turnover in response to hyperinsulinemia in humans postabsorptively with [15N]glycine as tracer.

Studies using stable isotopes to determine the effect of insulin on whole-body protein turnover have given conflicting results. The precursor approach to studying healthy subjects in a postabsorptive state shows reductions in breakdown and oxidation; with end product methods in parenterally fed patients no such changes are seen. To explain these discrepancies, we measured protein turnover with and without euglycemic hyperinsulinemic clamping postabsorptively in nine healthy subjects by using single-dose [15N]glycine with calculations based on ammonia and urea end product excretion. With and without clamping, respectively, insulin reduced nitrogen (22.1 and 48.2 mg.kg-1.9 h-1, P < 0.01) and urea (15.8 and 37.5 mg.kg-1.9 h-1, P < 0.05) but increased ammonia (7.7 and 5.0 mg.kg-1.9 h-1, P < 0.05) excretion. Although the urea end product method suggested that insulin tended to reduce both protein breakdown and synthesis, the protein metabolism changes detected with the ammonia end product method tended to be in the opposite direction. The [15N]glycine ammonia end-product method may be inappropriate for studies during insulin infusion because of insulin's effect on ammonia excretion.