Fat oxidation and adiposity in prepubertal children: Exogenous versus endogenous fat utilization

Fat balance plays an important role in fat mass regulation. The mechanisms by which fat intake and fat oxidation are controlled are poorly understood. In particular, no data are available on the origin, i.e. exogenous (meal i ntake) or endogenous (adipose tissue lipolysis), of fat oxidized during the postprandial period in children and the proportion between these two compo nents. In this study we tested the hypothesis that there is a relationship between adiposity and the oxidative fate of fat taken with a mixed meal in a group of 15 children with a wide range of fat mass (9-64%). The combinati on of stable isotope analysis ([C-13] enriched fatty acids added to a mixed meal) and indirect calorimetry allowed us to differentiate between the exo genous and endogenous resting fat oxidation rate over the 9-h postprandial period. During the 9 hours of the postprandial period, the children oxidize d an amount of fat comparable to that ingested with the meal [26.8 (+/-2.31 )g vs. 26.4 (+/-2.3) g, respectively, P = ns]. On average, exogenous fat ox idation [2.99 (+/-3.0)g/9 h] represented 10.8% (+/-0.9) of total fat oxidat ion. Endogenous fat oxidation, calculated as the difference between total f at oxidation and exogenous fat oxidation, averaged 23.4 (+/-1.9) g/9 h and represented 88.2% (+/-0.9) of total fat oxidation. Endogenous fat oxidation as well as exogenous fat oxidation were highly correlated to total fat oxi dation (r = 0.83, P < 0.001; r = 0.84, P < 0.001, respectively). Exogenous fat oxidation expressed as a proportion of total fat oxidation was directly related to fat mass (r = 0.56, P < 0.03), while endogenous fat oxidation e xpressed as a proportion of total fat oxidation was inversely related (r = -0.57, P < 0.03) to the degree of adiposity. The enhanced exogenous fat oxi dation observed when adiposity increases in the dynamic phase of obesity ma y be viewed as a protective mechanism to prevent further increase in fat ma ss and hence to maintain fat oxidation at a sufficient rate when the body i s exposed to a high amount of dietary fat, as typically encountered in obes e children.