THE EFFICIENCY OF DIETARY-PROTEIN UTILIZATION IS INCREASED DURING PUBERTY

We investigated whether the efficiency of dietary protein utilization for growth increases during the pubertal growth spurt in both nondiabe tic and diabetic subjects. We measured leucine oxidation and retention (intake minus oxidation) in orally fed nondiabetic (n = 9) and diabet ic (n = 9) human subjects, aged 7-17 yr. Eight subjects were Tanner st age I, and 10 were Tanner stages III-V; groups were not matched for ge nder. After 3 days of consuming a diet containing approximately 1 g/kg .day protein, subjects drank a commercial liquid nutrition formula, co n containing L-[1-C-13]leucine, every 30 min for a total of 6 h to pro vide 1 g protein/kg.day. Isotopic enrichment of CO2 was used to calcul ate the fractional leucine oxidation rate and, together with alpha-ket oisocaproate isotopic enrichment, to calculate total leucine oxidation . Leucine oxidation rates decreased with puberty in both nondiabetic s ubjects (36.0 +/- 10.4 vs. 23.9 +/- 4.2 mu mol/kg fat-free mass (FFM). h, prepubertal and pubertal, respectively; P < 0.05) and diabetic (33. 6 +/- 4.94;, us. 27.3 +/- 3.4 mu mol/kg FFM.h, prepubertal and puberta l, respectively: P < 0.1) subjects. Leucine retention increased with p uberty in both nondiabetic (0.27 +/- 3.2 vs. 15.7 +/- 5.3 mu mol/kg FF M.h, prepubertal and pubertal, respectively; P < 0.001) and diabetic ( 1.9 +/- 4.9 vs. 13.2 +/- 4.4 mu mol/kg FFM.h, prepubertal and pubertal subjects, respectively; P < 0.05) subjects. The data suggest that the pubertal growth spurt is associated with a marked increase in the eff iciency of dietary protein utilization for growth.