1. The thermic effect of a glucose load (50 g) was studied in ten control and eleven obese female subjects, using both direct and indirect calorimetry simultaneously. Experiments were done under conditions of thermal equilibrium (28° and 30% relative humidity)..
2. Thermal balance (heat production measured by indirect calorimetry minus heat losses measured directly) was negative in the control group during the fasting period (heat deficit – 14.2 ± 5.0 kJ/m2 per h), whereas that of the obese group was in equilibrium (+ 1.4 ± 4.8 kJ/m2 per h)..
3. After the glucose load, metabolic rate increased 13.0 ± 1.5 and 5.2 ± 1.3% in the control and obese groups respectively..
4. In contrast to the metabolic rate, total heat losses were not significantly altered in either group after the glucose load. Total heat losses of the obese group were significantly lower than those of the control group throughout the experimental period..
5. During the experiments the amount of heat stored was increased in both groups. Thermal balance in the control group became positive while that of the obese group remained positive..
6. During the fasting period, the control subjects oxidized more carbohydrates (90.4 mg/min) than lipids (68.8 mg/min), whereas obese subjects oxidized more lipids (103.7 mg/min) than carbohydrates (50.2 mg/min). After the glucose load, the oxidation rate of carbohydrates was increased in both groups to 158.1 mg/min in control subjects and 95.6 mg/min in obese subjects..
7. The mean skin temperature of the control subjects was significantly higher than that of the obese subjects and remained higher throughout the postprandial period..
8. These results indicate that: (a) during the fasting period, the energy sources utilized and the thermal balance of the two groups were different; (b) the thermic effect of glucose was less in the obese subjects and, therefore, might be a factor contributing to their low energy expenditure.