To measure postabsorptive fat oxidation (F_ox) and to assess its association with body composition (lean body mass [LBM] and body fat mass [BFM]) and pubertal development.

We studied 235 control (male/female ratio = 116/119; age [mean ± SD]: 13.1 ± 1.7 years; weight: 45.3 ± 10.5 kg; LBM: 34.3 ± 7.1 kg; BFM: 11.0 ± 4.5 kg) and 159 obese (male/female ratio = 93/66; age: 12.9 ± 2.1 years; weight: 76.2 ± 19.1 kg; LBM: 47.4 ± 10.9 kg; BFM: 28.8 ± 9.2 kg) adolescents. Postabsorptive F_ox was calculated from oxygen consumption, carbon dioxide production, and urinary nitrogen as measured by indirect calorimetry and Kjeldahl's method, respectively. Body composition was determined by anthropometry.

Postabsorptive F_ox (absolute value and percentage of resting metabolic rate) was significantly (p < 0.001) higher in the obese adolescents (76.7 ± 26.3 gm/24 hours, 42.3% ± 18.7%) than in the control subjects (40.0 ± 26.3 gm/24 hours, 28.7% ± 17.0%), even if adjusted for LBM. F_ox corrected for BFM was similar in control and in obese children, but was significantly lower in girls compared with boys (control male subjects: 62.1 ± 29.1 gm/24 hours, control female subjects: 51.6 ± 28.4 gm/24 hours, obese male subjects: 57.3 ± 29 gm/24 hour, obese female subjects: 45.0 ± 28.4 gm/24 hours). BFM and LBM showed a significant positive correlation with F_ox. By stepwise regression analysis the most important determinant of F_ox was BFM in obese and LBM in control children. There was a significant rise in F_ox during puberty; however, it was mainly explained by changes in body composition.

Obese adolescents have higher F_ox rates than their normal-weight counterparts. Both LBM and fat mass are important determinants of F_ox. (J Pediatr 1998;132:98-104)