Protein turnover during puberty in normal children

To investigate whether insulin resistance of puberty involves protein metabolism, we compared whole body leucine kinetics in 20 prepubertal Tanner I (TI), and 21 pubertal Tanner II-IV (TII-IV) healthy children. Leucine flux (LR(a)), oxidation (LOX), and nonoxidative disposal (NOXLD) were measured during primed constant infusion of [1-C-13]leucine at baseline and during a stepwise hyperinsulinemic (10 and 40 mU . m(-2). min(-1)) euglycemic clamp in combination with indirect calorimetry. At baseline LR(a) and LOX were lower in TII-IV vs. TI [LR(a): 3.59 +/- 0.17 vs. 4.05 +/- 0.18 mu mol . min-1 . kg(-1) fat-free mass (FFM), P = 0.036; LOX: 0.45 +/- 0.03 vs. 0.59 +/- 0.04 mu mol . min(-1) FFM(-1), P = 0.005], but NOXLD was similar. Insulin-like growth factor I (IGF-I) levels correlated inversely with LR(a), NOXLD, and LOX. Energy expenditure correlated positively with LR(a), LOX, and NOXLD. During the clamp absolute and percent suppression in LR(a) were significantly lower in TII-IV than TI. In conclusion, 1) proteolysis and protein oxidation are lower during puberty compared with prepuberty, whereas protein synthesis is unchanged; 2) insulin action in inhibiting proteolysis is decreased during puberty; and 3) increased pubertal IGF-I levels may play a role in decreased protein degradation.