The 2-yr proposed project is aimed at understanding whether or not energy metabolism is altered in reduced-obese subjects as compared to never-obese control subjects. A major hypothesis of the proposal is that the reduced-obese do not constitute a homogeneous group and that energy metabolism may be altered in some but not all reduced- obese subjects. Using the NWCR, the project will begin by examining determinants of resting metabolic rate in reduced-obese subjects. A foil ow-up study will examine total energy expenditure (using doubly labeled water) in subjects who have maintained short (1-2 year ) versus long (greater than 5 year) weight reduction. Finally, the project will assess fat oxidation in response to dietary manipulation in reduced-obese subjects. No previous studies of energy expenditure in reduced obese subjects have used subjects with who have maintained their weight loss for longer than 5 years. in fact, most previously reduced-obese subjects were studied within 6 months of their weight loss. Substantial weight loss can likely produce perturbation in energy metabolism that may last for long periods of time.