OBJECTIVE The capacity for lipid and carbohydrate (CHO) oxidation during exercise is important for energy partitioning and storage. This study examined the effects of obesity on lipid and CHO oxidation during exercise. METHODS Seven obese and seven lean [body mass index (BMI), 33 +/- 0.8 and 23.7 +/- 1.2 kg/m(2), respectively] sedentary, middle-aged men matched for aerobic capacity performed 60 minutes of cycle exercise at similar relative (50% VO(2max)) and absolute exercise intensities. RESULTS Obese men derived a greater proportion of their energy from fatty-acid oxidation than lean men (43 +/- 5% 31 +/- 2%; p = 0.02). Plasma fatty-acid oxidation determined from recovery of infused [0.15 micromol/kg fat-free mass (FFM) per minute] [1-(13)C]-palmitate in breath CO(2) was similar for obese and lean men (8.4 +/- 1.1 and 29 +/- 15 micromol/kg FFM per minute). Nonplasma fatty-acid oxidation, presumably, from intramuscular sources, was 50% higher in obese men than in lean men (10.0 +/- 0.6 versus 6.6 +/- 0.8 micromol/kg FFM per minute; p < 0.05). Systemic glucose disposal was similar in lean and obese groups (33 +/- 8 and 29 +/- 15 micromol/kg FFM per minute). However, the estimated rate of glycogen-oxidation was 50% lower in obese than in lean men (61 +/- 12 versus 90 +/- 6 micromol/kg FFM per minute; p < 0.05). CONCLUSIONS During moderate exercise, obese sedentary men have increased rates of fatty-acid oxidation from nonplasma sources and reduced rates of CHO oxidation, particularly muscle glycogen, compared with lean sedentary men.