OBJECTIVE To test the hypothesis that weight rebound following slimming diets may be caused by an adaptive alteration in fuel utilisation involving a suppression of fat oxidation thus favouring fat storage in adipose tissue. METHODS Repeat measurements before and after two 14 d cycles of controlled weight loss using a very low energy diet (1.9MJ/d). METHODS Eight moderately obese women (body weight: 85.6 +/- 10.1 kg, BMI: 31 +/- 2 kg/m2, age: 42.6 +/- 10.1 years). METHODS Energy expenditure and substrate balances using 24-h whole-body indirect calorimetry and naturally labelled 13C-glucose. RESULTS Aggregate weight loss was 5.1 +/- 0.8 kg. Twenty-four hour energy expenditure declined by 12% (8359 +/- 282 to 7366 +/- 191 kJ/d, p < 0.001). Net fat utilisation was not significantly depressed (4009 +/- 366) to 3613 +/- 191 kJ/d, NS), and the proportion of energy derived from fat was unchanged at 48.0% before weight loss and 49.0% after weight loss. CONCLUSIONS The well-recognised phenomenon of reduced energy expenditure is unlikely to be a major cause of weight regain. The results do not support the theory that altered fuel selection in post-obese subjects may be the cause of difficulty in maintaining weight loss.