The primary aims of this study are to investigate the metabolic and neural adaptations in 60 obese adult volunteers participating in lifestyle interventions resulting in weight loss through a structured meal replacement program or caloric restriction plus vigorous exercise. The secondary aim is to determine whether the magnitude of metabolic adaptation or the changes in the brain s reward circuitry or responsiveness to food cues are related to the ubiquitous weight loss plateau after 6-10 months or the rate of weight regain in the subsequent months.