OBJECTIVE: Exercise has been proposed as a tool for the prevention of obesity. Apart from an effect on energy expenditure, in particular low-intensity (LI) exercise might also influence substrate metabolism in favour of fat oxidation. It is however unclear what is the most beneficial exercise regime for obese people. We therefore studied the effect of either high-intensity (HI) or LI exercise on 24 h energy expenditure (24 h EE) and substrate metabolism. METHODS: Eight healthy obese male volunteers (age: 38+/-1 y, BMI: 31+/-1 kg/m(2), W(max): 235+/-16 W) stayed in the respiration chamber for two nights and the day in-between. In the chamber they cycled either at a HI (three times 30 min in a interval protocol (2.5 min 80/50% W(max))) or LI (three times 60 min continuously at 38% W(max)) protocol with an equicaloric energy expenditure. In the chamber subjects were fed in energy balance (37/48/15% of energy as fat/carbohydrate/protein). RESULTS: The 24 h EE was not significantly different between protocols. In both protocols, sleeping metabolic rate (SMR) was elevated after the exercise (average+6.7%). The 24 h respiratory quotient (24 h RQ) was not different between protocols. During exercise, RQ was higher in the HI compared to the LI protocol (0.93 vs 0.91 resp., P<0.05), whereas in the postexercise period RQ tended to be lower in the HI compared to the LI protocol (P=0.06). CONCLUSION: 24 h EE is not differently affected by HI or LI exercise in obese men. Similarly, the differences in HI and LI exercise, RQ are compensated postexercise leading to similar substrate oxidation patterns over 24 h independently of the level of exercise intensity.