Fat Oxidation Rates during Exercise in Prepubertal Children: The Role of Obesity, Fitness, and Lean Mass

This study aimed to determine fat oxidation values during a graded cyclo-ergometer exercise in prepubertal children considering cardiorespiratory fitness (CRF) and body size based on obesity status and sex, as well as to elucidate whether body mass index or fat accumulation mediate the relationship between CRF and fat oxidation.

We evaluated 118 prepubertal children (59 girls) aged 11.5 ± 2.0 yr (Tanner ≤II). Body composition was assessed using dual-energy x-ray absorptiometry and anthropometry. CRF (peak oxygen uptake (V̇O 2peak )) and fat oxidation rates were determined during a graded cycling test.

In absolute values, obese prepubertal children displayed higher maximal fat oxidation (MFO) compared with overweight and normal-weight counterparts ( P = 0.006). However, after adjusting by CRF, the difference in MFO disappeared ( P = 0.104). When MFO was expressed with respect to body size and adjusted by CRF, it was significantly greater in normal-weight children ( P = <0.001). Similar results were found in boys and girls. Furthermore, fat accumulation and body mass index acted as mediators in the relationship between CRF and fat oxidation, explained a 38% of the variance.

Fat oxidation levels are comparable among prepubertal children, regardless of obesity status, upon CRF adjustment. Nevertheless, when contextualized within body size, obese and overweight children demonstrated reduced fat oxidation capacity compared with their normal-weight peers during exercise and with no sex-related differences. Furthermore, fat mass seems to be a mediator factor between CRF and fat oxidation.