OBJECTIVE:We investigated to what extent changes in metabolic rate and composition of weight loss explained the lessthan-expected weight loss in obese men and women during a diet-plus-exercise intervention. DESIGN: In all, 16 obese men and women (41 ± 9 years; body mass index (BMI) 39 ± 6 kg m À 2 ) were investigated in energy balance before, after and twice during a 12-week very-low-energy diet(565-650 kcal per day) plus exercise (aerobic plus resistance training) intervention. The relative energy deficit (EDef) from baseline requirements was severe (74%-87%). Body composition was measured by deuterium dilution and dual energy X-ray absorptiometry, and resting metabolic rate (RMR) was measured by indirect calorimetry. Fat mass (FM) and fat-free mass (FFM) were converted into energy equivalents using constants 9.45 kcal per g FM and 1.13 kcal per g FFM. Predicted weight loss was calculated from the EDef using the 0 7700 kcal kg À 1 rule 0 . RESULTS: Changes in weight ( À 18.6 ± 5.0 kg), FM ( À 15.5 ± 4.3 kg) and FFM ( À 3.1 ± 1.9 kg) did not differ between genders. Measured weight loss was on average 67% of the predicted value, but ranged from 39% to 94%. Relative EDef was correlated with the decrease in RMR (R ¼ 0.70, Po0.01), and the decrease in RMR correlated with the difference between actual and expected weight loss (R ¼ 0.51, Po0.01). Changes in metabolic rate explained on average 67% of the less-than-expected weight loss, and variability in the proportion of weight lost as FM accounted for a further 5%. On average, after adjustment for changes in metabolic rate and body composition of weight lost, actual weight loss reached 90% of the predicted values. CONCLUSION: Although weight loss was 33% lower than predicted at baseline from standard energy equivalents, the majority of this differential was explained by physiological variables. Although lower-than-expected weight loss is often attributed to incomplete adherence to prescribed interventions, the influence of baseline calculation errors and metabolic downregulation should not be discounted.