Abstract

Increasing fat oxidation is a common goal of exercise training for managing obesity. While constant load exercise at the intensity associated with the level of maximal fat oxidation (MFO) has been shown to be effective, there is a growing interest in the use of moderate-intensity interval exercise. This study compares fat oxidation rate from a FAT_max test with a moderate-intensity interval training session in obese men. Twelve sedentary obese males (age 27.2±4.1 years; BMI 30.5±3.2 kg/m²; %fat mass 32.5±4.5) completed two exercise sessions: a graded exercise test (GXT) to determine MFO and maximal aerobic power (VO2_max), and an interval cycling session during which respiratory gases were measured. The 30-min interval session involved 5-min repetitions of workloads 20% below and 20% above the FAT_max intensity (FAT-INT). Mean VO2_max was 31.8±5.6 ml/kg/min and all participants achieved ≥3 of the designated VO2_max test criteria. The average MFO was low (0.14±0.08 g/min) and occurred at 34±6%VO2_max. The MFO identified during the GXT was not significantly different compared with the average fat oxidation rate in the FAT-INT session (0.16±0.09 g/min). During the FAT-INT session, fat oxidation rate increased with time; the highest rate (0.18±0.11 g/min) in minute 25 was significantly higher than the rate at minute 5 and 15 (p=0.004 and 0.04 respectively). In the FAT-INT, energy expenditure (EE) at 15 min (5.06±0.9 kcal/min) and at 25 min (5.13±0.9 kcal/min) were significantly greater than the EE at the MFO intensity (4.5±1 kcal/min) (p=0.003 and 0.001 respectively). Fat oxidation contributed 27±13% of the EE at the MFO workload during the GXT, 27±14% during the first half of interval training session, and was 32±14% during the second half of the FAT-INT session. There was no significant difference between blood lactate concentration measured at the MFO intensity and the end of FAT-INT session. This study shows in a cohort with low aerobic power, fat oxidation during the FAT-INT session was comparable in the first 15 min and greater in the second 15 min, than the MFO determined during a GXT. Future research may consider if the varying workload in moderate-intensity interval training helps adherence to exercise without compromising fat oxidation.