Meal-induced thermogenesis in obese patients before and after weight reduction

The purpose of this study was to find out whether human obesity is associated with a diminished meal-induced thermogenesis and, if so, to what extent this response is influenced by weight reduction. Ten obese subjects (body mass index 42 +/- 2) and 10 age- and sex-matched non-obese volunteers were studied with continuous indirect calorimetry in the basal state and after the ingestion of a standardized test meal. Six obese subjects (body mass index 44 +/- 2) were examined on two occasions, once before and once after gastric banding and an average weight reduction of 18 +/- 3 kg. Basal oxygen uptake and energy expenditure were 30% (P less than 0.001) greater in the obese subjects compared to non-obese controls. After the meal, pulmonary oxygen uptake and energy expenditure increased rapidly and reached a relatively constant level after 60 min; for pulmonary oxygen uptake the average rise above basal was less in the obese (17.7 +/- 1.6%) than the non-obese (27.8 +/- 1.9%, P less than 0.001); the increase in energy expenditure was 18.5 +/- 1.7% in obese and 30.8 +/- 2.1% in non-obese subjects (P less than 0.001). After weight reduction, oxygen uptake and energy expenditure in the basal state were 20% lower (P less than 0.05) than before weight reduction. The average post-prandial increase in oxygen uptake was greater after weight reduction (24.8 +/- 2.0%) than before (16.7 +/- 1.6%, P less than 0.001). Corresponding values for energy expenditure were 27.2 +/- 2.2 and 18.2 +/- 2.2% (P less than 0.001). It is concluded that: (1) the thermogenic response to a mixed meal is lower in obese compared to non-obese individuals; and, (2) this impaired response is partly normalized after weight reduction. These findings suggest that a diminished meal-induced thermogenesis is a secondary phenomenon rather than a primary pathogenic factor in human obesity.