Context: Obesity is associated with altered patterns of substrate utilization at rest and during exercise.

Objective: The relationship between obesity and fat oxidation during recovery from exercise was examined.

Hypothesis: The postexercise shift toward fat oxidation is blunted in the obese state, reflected by higher RER (respiratory exchange ratio), blunted GH (growth hormone) and increased cortisol values compared with lean controls.

Design: Each subject completed two 160 min protocols (Baseline and Exercise). During Baseline, subjects rested for 160 min and during Exercise, completed 30 min of cycling at ventilatory threshold followed by 130 min of rest.

Setting: The University of Alberta.

Subjects: Healthy untrained (V0_{2 max} < 45 ml/kg/min or < 3.35 l/min) lean (< 16% body fat, n = 6) and obese (> 25% body fat, n = 7) men, aged 30-39 yr.

Main Outcome Measures: RER, GH, cortisol, oxygen consumption, heart rate, tympanic temperature and lactate were obtained during both protocols at matched time intervals and analyzed by repeated measures ANOVA.

Results: During Baseline, there were no differences detected between lean and obese groups for any of the measured variables. In contrast, during Exercise, peak GH levels were blunted (P < 0.05) and cortisol levels were elevated (P < 0.05) in the obese compared with lean but RER was similar. The differences in GH and cortisol persisted during the postexercise period accompanied by higher RER values (P < 0.05) and reduced total oxygen consumption (P < 0.05) in the obese group.

Conclusion: These findings indicate that exercise-induced fat oxidation is diminished in obese men.