Fat Oxidation Rates during Exercise in Pre-Pubertal Children: The Role of Obesity, Fitness and Lean Mass.

Abstract

Purpose: To determine fat oxidation values during a graded cyclo-ergometer exercise in pre-pubertal children considering cardiorespiratory fitness (CRF) and body size based on obesity status and sex. Moreover, to elucidate whether body mass index (BMI) or fat accumulation mediate the relationship between CRF and fat oxidation.

Methods: We evaluated 118 pre-pubertal 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 -VO 2peak ) and fat oxidation rates were determined during a graded cycling test.

Results: In absolute values, obese pre-pubertal children displayed higher maximal fat oxidation (MFO) compared to 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 BMI acted as mediators in the relationship between CRF and fat oxidation, explained a 38% of the variance.

Conclusions: Fat oxidation levels are comparable among pre-pubertal children, regardless of obesity status, upon CRF adjustment. Nevertheless, when contextualized within body size, obese and overweight children demonstrated reduced fat oxidation capacity compared to 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.