Thermic effect of a meal over 3 and 6 hours in lean and obese men

Controversy regarding defective postprandial thermogenesis in obesity may partly be due to methodological factors such as duration of measurement. To clarify further the role of blunted thermogenesis in obesity, the thermic effect of food was compared in seven lean (mean ± SEM, 15.7% ± 1.5% body fat, by densitometry) and seven obese men (37.3% ± 3% fat) over 3 and 6 hours. The groups were matched for age (35 ± 2 and 33 ± 2 years for the lean and obese groups; range, 25 to 39 years), fat-free mass (FFM), and aerobic fitness. Resting metabolic rate (RMR) was measured by indirect calorimetry for 6 hours on two mornings, in randomized order: (1) after a 720-kcal liquid mixed meal, which was 24% protein, 21% fat, and 55% carbohydrate; and (2) in the postabsorptive state. The thermic effect of food, calculated as postprandial minus postabsorptive RMR, was significantly greater for the lean than obese men for the first 3 hours of measurement (67 ± 6 v 49 ± 3 kcal3 hours; P < .01). During the second 3 hours, the thermic effect of food was marginally, but not significantly, greater for the lean than obese men (34 ± 8 v 20 ± 4 kcal3 hours; P = .10, NS). Over the entire 6 hours, the thermic effect of food was significantly greater for the lean than obese men (100 ± 12 v 69 ± 5 kcal6 hours; P < .05). Three hours after the meal the increment above fasting RMR was greater for the lean than obese men (28% ± 7% v 13% ± 3% above fasting; P < .05). However, 6 hours after the meal the elevation above fasting RMR was not significantly different between the lean (6% ± 3%) and obese men (0% ± 2%). These results indicate that the thermic effect of food is neither delayed nor more prolonged in obese compared with lean men and provide further evidence for defective thermogenesis in obesity.