EFFECTS OF OBESITY ON SUBSTRATE UTILIZATION DURING EXERCISE

Abstract

1722 Skeletal muscle in obese individuals is characterized by reduced postabsorptive rates of fatty acid oxidation and a reduced oxidative capacity, thereby leading to an increased lipid accumulation within muscle. To further examine the influence of obesity on skeletal muscle fatty acid metabolism, this study determined whether fat oxidation is impaired in obese men during exercise. Seven obese (O) and 7 lean (L) sedentary otherwise healthy men (BMI = 33.7 ± 1.0 and 23.7 ± 1.0 kg/m2, respectively) of similar age participated in this study. O and L groups were matched for VO2max (48.0 ± 3.3 and 48.2 ± 3.0 ml/min/fat free mass (FFM), respectively) so that 60 min of cycle ergometry was performed at the same relative (50% VO2max) and absolute (1.53 ± 0.11 for O and 1.40 ± 0.11 l/min for L) exercise intensity. Plasma lactate levels were similar in O and L during exercise (2.5 ± 0.7 vs. 2.1 ± 0.7 mM). Total fatty acid oxidation determined with indirect calorimetry tended (p = 0.055) to be higher in the O group (18.7 ± 1.2 vs. 15.1 ± 1.2 μmol/FFM/min), and there was a trend (p = 0.08) for carbohydrate (CHO) oxidation to be higher in the L (122.2 ± 12.3 vs. 89.6 ± 12.3 μmol/FFM/min). The O group derived a significantly greater proportion of their energy from fatty acid oxidation than the L group (45 ± 4 vs. 31 ± 4%, p = 0.02). A strong association (r = 0.81, p = .0005) was observed between rates of CHO oxidation and work rate (W) for all subjects, but no association was observed between rates of fatty acid oxidation and work rate. These results suggest that during moderate intensity exercise, rates of total fatty acid oxidation determined with indirect calorimetry are not decreased in obese compared to lean men matched for aerobic fitness. In conclusion, contrary to the concept that fat oxidation is reduced in obesity, we find that sedentary obese men are not limited in their ability to oxidize fatty acids during exercise compared to lean sedentary men.