Determinants of Exercise-induced Fat Oxidation in Obese Women and Men

Endurance training at an intensity eliciting maximal fat oxidation may have a beneficial effect on body weight and glucose metabolism in obese patients. However, the exercise intensity at which maximal fat oxidation occurs and the factors limiting fat oxidation are not well studied in this population. Obese, otherwise healthy men (n = 38) and women (n = 91) performed an incremental exercise test up to exhaustion on a cycle ergometer. Substrate oxidation was estimated using indirect calorimetry. Magnetic resonance tomography and spectroscopy were conducted to assess body fat distribution and intramyocellular fat content. We determined the exercise intensity at which maximal body fat oxidation occurs and assessed whether body composition, body fat distribution, intramyocellular fat content, or oxidative capacity predict exercise-induced fat oxidation. Maximal exercise-induced fat oxidation was 0.30 +/- 0.02 g/min in men and 0.23 +/- 0.01 g/min in women (p < 0.05). Exercise intensity at the maximum fat oxidation was 42 +/- 2.2% VO2max in men and 43 +/- 1.7% VO2max in women. With multivariate analysis, exercise-induced fat oxidation was related to fat-free mass, percent fat mass, and oxidative capacity, but not to absolute fat mass, visceral fat, or intramyocellular fat content. We conclude that in obese subjects the capacity to oxidize fat during exercise appears to be limited by skeletal muscle mass and oxidative capacity rather than the availability of visceral or intramyocellular fat.