We investigated whether the efficiency of dietary protein utilization for growth increases during the pubertal growth spurt in both nondiabetic and diabetic subjects. We measured leucine oxidation and retention (intake minus oxidation) in orally fed nondiabetic (n = 9) and diabetic (n = 9) human subjects, aged 7-17 yr. Eight subjects were Tanner stage I, and 10 were Tanner stages III-V; groups were not matched for gender. After 3 days of consuming a diet containing approximately 1 g/kg. day protein, subjects drank a commercial liquid nutrition formula, containing L-[1(-13)C]leucine, every 30 min for a total of 6 h to provide 1 g protein/kg. day. Isotopic enrichment of CO2 was used to calculate the fractional leucine oxidation rate and, together with alpha-ketoisocaproate isotopic enrichment, to calculate total leucine oxidation. Leucine oxidation rates decreased with puberty in both nondiabetic subjects (36.0 +/- 10.4 vs. 23.9 +/- 4.2 mumol/kg fat-free mass (FFM).h, prepubertal and pubertal, respectively; P < 0.05) and diabetic (33.6 +/- 4.9% vs. 27.3 +/- 3.4 mumol/kg FFM.h, prepubertal and pubertal, respectively; P < 0.1) subjects. Leucine retention increased with puberty in both nondiabetic (0.27 +/- 3.2 vs. 15.7 +/- 5.3 mumol/kg FFM.h, prepubertal and pubertal, respectively; P < 0.001) and diabetic (1.9 +/- 4.9 vs. 13.2 +/- 4.4 mumol/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 efficiency of dietary protein utilization for growth.