Fat oxidation over a range of exercise intensities: fitness versus fatness

Maximal fat oxidation (MFO), as well as the exercise intensity at which it occurs (Fatₘₐₓ), have been reported as lower in sedentary overweight individuals but have not been studied in trained overweight individuals. The aim of this study was to compare Fatₘₐₓ and MFO in lean and overweight recreationally trained males matched for cardiorespiratory fitness (CRF) and to study the relationships between these variables, anthropometric characteristics, and CRF. Twelve recreationally trained overweight (high fatness (HiFat) group, 30.0% ± 5.3% body fat) and 12 lean males (low fatness (LoFat), 17.2% ± 5.7% body fat) matched for CRF (maximal oxygen consumption (V̇O₂ₘₐₓ) 39.0 ± 5.5 vs. 41.4 ± 7.6 mL·kg–¹·min–¹, p = 0.31) and age (p = 0.93) performed a graded exercise test on a cycle ergometer. V̇O₂ₘₐₓ and fat and carbohydrate oxidation rates were determined using indirect calorimetry; Fatₘₐₓ and MFO were determined with a mathematical model (SIN); and % body fat was assessed by air displacement plethysmography. MFO (0.38 ± 0.19 vs. 0.42 ± 0.16 g·min–¹, p = 0.58), Fatₘₐₓ (46.7% ± 8.6% vs. 45.4% ± 7.2% V̇O₂ₘₐₓ, p = 0.71), and fat oxidation rates over a wide range of exercise intensities were not significantly different (p > 0.05) between HiFat and LoFat groups. In the overall cohort (n = 24), MFO and Fatₘₐₓ were correlated with V̇O₂ₘₐₓ (r = 0.46, p = 0.02; r = 0.61, p = 0.002) but not with % body fat or body mass index (p > 0.05). Fat oxidation during exercise was similar in recreationally trained overweight and lean males matched for CRF. Consistently, substrate oxidation rates during exercise were not related to adiposity (% body fat) but were related to CRF. The benefits of high CRF independent of body weight and % body fat should be further highlighted in the management of obesity.