Impact of body fat mass and percent fat on metabolic rate and thermogenesis in men

To clarify further the independent relationships of body composition parameters to energy expenditure, resting metabolic rate (RMR) and postprandial thermogenesis were studied in four groups who were matched for absolute fat mass (study 1) and relative fatness (study 2). In study 1, five lean [group A, 15.4 ± 0.6% (±SE) body fat] and five obese men (group B, 25.0 ± 0.9% fat) were matched on body fat mass (13.0 ± 0.9 vs. 14.4 ± 0.8 kg, respectively). Fat-free mass (FFM) and total weight were greater for group A than B. RMR was measured for 3 h in the fasted state and after a 720-kcal mixed meal. RMR was greater for group A than B (1.38 ± 0.08 vs. 1.14 ± 0.04 kcal/min, P < 0.05). The thermic effect of food, calculated as 3 h postprandial minus fasting RMR, was greater for group A than B (65 ± 6 vs. 23 ± 9 kcal/3 h; P < 0.05). In study 2, two groups (n = 6 men/group) were matched for percent body fat (33 ± 1% fat for both) but differed in lean, fat, and total weights: 50.8± 3.1 kg FFM for the lighter (group C) vs. 68.0 ± 2.8 kg FFM for the heavier (group D) group, P < 0.05. RMR was lower for group C than D (1.17 ± 0.06 vs. 1.33 ± 0.04 kcal/min, P < 0.05), but the thermic effect of food was not significantly different (31 ± 3 vs. 20 ± 6 kcal/3 h). After adjustment for differences in FFM among the four groups, no significant differences in RMR were observed. These results demonstrate that the putative impaired thermogenesis in obesity is specifically a function of relative fatness rather than fat mass and confirm the idea that RMR is determined by FFM and is not independently related to obesity.