Does exercise duration affect Fat_max in overweight boys?

Abstract

To compare the assessment of Fat_max using a single graded exercise test with 3 min stages against 30 min prolonged exercise bouts in overweight boys. Ten overweight boys (8–12 years) attended the laboratory on seven separate occasions. On the first visit, body anthropometrics and peak aerobic capacity ($\dot{V}$O_2peak) were assessed. Following this, each participant attended the laboratory after an overnight fast for six morning cycling sessions. During the first session, participants completed a continuous, submaximal graded exercise protocol with seven 3 min stages (GRAD) at 35, 40, 45, 50, 55, 60 and 65% $\dot{V}$O_2peak. The final five visits consisted of a 30 min bout of prolonged exercise (PROL) performed in a counterbalanced order at 40, 45, 50, 55 and 60% $\dot{V}$O_2peak. There was no effect of exercise duration on Fat_max or the absolute rate of fat oxidation during PROL (p > 0.05). At the group level, GRAD and PROL provided similar estimates of Fat_max (GRAD: 53 ± 10% $\dot{V}$O_2peak; PROL: 53 ± 10% $\dot{V}$O_2peak; p = 0.995); however, individual variation between the two protocols is shown by a systematic bias and residual error of 0 ± 11% $\dot{V}$O_2peak. Fat oxidation rates remained stable across 30 min of steady-state exercise in overweight boys. Furthermore, Fat_max was similar at 3, 10, 20 and 30 min of exercise, suggesting that for exercise lasting ≤30 min, exercise duration does not affect Fat_max. However, Fat_max determined with GRAD may need to be interpreted with caution at the individual level given the variation in Fat_max between protocols.