The aim of this study is to determine if a phenotype defined by 24-EE responses to overfeeding and underfeeding is related to weight loss or gain in 4 different under- and overfeeding settings: an inpatient weight loss group for obese individuals who will be administered one of two diets based on a 50% reduction of their daily energy needs for six weeks (standard reduction diet or calorie reduced diet that is still high in protein; 6 week weight reduction, 10 week stay, n=20/standard reduction diet group and n=10/relative high protein diet), and two overfeeding groups for lean, weight gain resistant individuals: a low-protein overfeeding group at 150% of daily energy needs for 6 weeks overfeeding (6% protein, 64% carbohydrate, 30% fat; 10 week stay, n=10), and a normal protein overfeeding group at 150% of daily energy needs for 6 weeks overfeeding (20% protein, 50% carbohydrate, 30% fat; 10 week stay, n=10). We will also examine additional metabolic and behavioral measurements to determine how they relate to weight change and changes in 24-EE. These include sympathetic nervous system activity, behavior, adipose and muscle tissue energy content, and abdominal adipocyte size. The study will evaluate the relationship between the percent increase in 24-EE in response to overfeeding/underfeeding and the amount of weight change over time in each subject. These findings may provide important information for predicting and adapting specific individualized interventions for obesity.