Comparison of thermic effects of constant and relative caloric loads in lean and obese men.

Controversy regarding defective thermic effect of food (TEF) in obesity might be related to differences among studies in the caloric loads. To clarify further the role of blunted thermogenesis in obesity, responses to the same absolute caloric load (720 kcal) and a relative load, which was 35% of each subject's resting metabolic rate (RMR), were compared in 11 lean (L) and 11 obese (O) men. The relative load was slightly larger for O than L (752 +/- 27 vs 683 +/- 21 kcal; means +/- SEM, NS). TEF, calculated as 3-h postprandial minus fasting RMR, was greater for L than O for both the 720-kcal (69 +/- 4 vs 31 +/- 3 kcal/3 h, p less than 0.01) and relative loads (64 +/- 4 vs 37 +/- 3 kcal/3 h, p less than 0.01). For L, TEF was greater for the 720-kcal load than for the relative load whereas for O, TEF was greater for the relative than for the 720-kcal meal. However, expressed as a percent of the calories ingested, TEF for the absolute and relative meals was identical for each group, in both cases lower for O (4.2 +/- 0.4% vs. 4.7 +/- 0.3%) than for L (9.7 +/- 0.4% vs 9.3 +/- 0.8%); p less than 0.01. These results demonstrate the impact on thermogenesis of the basis on which the meal is dosed and provide further evidence for defective thermogenesis in obesity.