The thermic effect of food at rest, during 30 min of cycle exercise, and postexercise with two sequences of exercise and meal (before or after exercise) was compared in eight lean (mean +/- SE, 12.8 +/- 0.7% body fat) and eight obese men (29.7 +/- 0.6% fat) to determine whether exercise before or after a meal enhances thermogenesis. The groups were matched for age, height, and lean body mass (LBM) in order to study the relationship between thermogenesis and body fat independent of LBM. Metabolic rate was measured by indirect calorimetry on five mornings, in randomized order, after an overnight fast. Treatments on respective days were 1) 3-h rest, no meal; 2) 3-h rest after a 750-kcal mixed meal (14% protein, 31% fat, 55% carbohydrate); 3) during and 3 h after 30 min of cycling, no meal; 4) during and 3 h after 30 min of cycling, meal 30 min before exercise; and 5) 3 h after 30 min of cycling, meal immediately after exercise. The thermic effect of food, which is the fed minus fasted caloric expenditure, was significantly greater for the lean than the obese men under the resting (mean +/- SE 53 +/- 5 vs. 26 +/- 5 kcal over 3 h for the lean and obese groups, P less than 0.01), exercise (26 +/- 4 vs. 4 +/- 2 kcal over 30 min, P less than 0.01), and both postexercise conditions. However, for the lean men the thermic effect of food was significantly greater for the meal-before-exercise than the resting and the meal-after-exercise conditions.(ABSTRACT TRUNCATED AT 250 WORDS)