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<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<0·001); the increase in energy expenditure was 18·5 ± 1·7% in obese and 30·8 ± 2·1% in non‐obese subjects (P<0·001). After weight reduction, oxygen uptake and energy expenditure in the basal state were 20% lower (P<0·05) than before weight reduction. The average postprandial increase in oxygen uptake was greater after weight reduction (24·8 ± 2·0%) than before (16·7 ± 1·6%, P<0·001). Corresponding values for energy expenditure were 27·2 ± 2·2 and 18·2 ± 2·2% (P<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.