The mechanism behind the meal-induced increase in energy expenditure in humans and its reduction in obesity was examined in 15 normal-weight and seven obese subjects. The subjects were studied by indirect calorimetry in the basal state and during 2 hours after a mixed meal corresponding to 40% of the 24-hour basal energy requirement. Artificial thermal insulation was applied over the abdominal area before the study in seven of the normal-weight subjects. Thermistor catheters were inserted into a hepatic vein, the pulmonary artery, and a systemic artery for blood sampling and recording of blood temperatures. Basal hepatic venous drainage of heat in relation to the splanchnic oxygen uptake in the normal-weight subjects was low (12 ± 1 J/mL O₂) and became even lower after the meal (5 ± 3 J/mL). In the obese individuals and the insulated subjects, blood-drained splanchnic heat amounted to 19 to 21 J/mL oxygen both before and after the meal. The postprandial increase in whole body energy expenditure was diminished both in the obese (12% ± 1% above basal) and in the insulated subjects (15% ± 1%) compared with the noninsulated controls (22% ± 2%). In normal-weight subjects, there is a leakage of heat across the abdominal wall. Reduction or prevention of this leakage by artificial thermal insulation or by obesity is accompanied by a reduction of the postprandial increase in energy expenditure.