OBJECTIVE: To investigate whether efficiency of weight gain during a short period of overfeeding is related to adaptive differences in basal metabolic rate (BMR) and physical activity. SUBJECTS: Fourteen healthy females (age 25+/-4 years, BMI 22.1+/-2.3 kg/m2). DESIGN AND MEASUREMENTS: Subjects were overfed with a diet supplying 50% more energy than baseline energy requirements for 14 days. Overfeeding diets provided 7% of energy from protein, 40% from fat and 53% from carbohydrates. Body composition was determined using hydrodensitometry and isotope dilution, total energy expenditure (TEE) with doubly labeled water and basal metabolic rate (BMR) with indirect calorimetry. Physical activity (PA) was recorded with a tri-axial accelerometer. RESULTS: Body weight increased by 1.45+/-0.86 kg (mean+/-S.D.) (P<0.0001), fat mass increased by 1.05+/-0.75 kg. Energy storage was 57.0+/-17.9 MJ, which is the difference between energy intake (207.2 MJ) and energy expenditure (150.2 MJ) during overfeeding. There was no difference between metabolically efficient and metabolically inefficient subjects in changes in BMR and PA. CONCLUSION: These results indicate that the metabolic efficiency of weight gain was not related to adaptive changes in energy expenditure.