A Study to Investigate Why Overweight People Regain Weight After Losing Weight in a Behavioral Weight Loss Program

The goal of this clinical trial is to determine the physiological basis for the differences in weight regain among adults (25-59 years old) with obesity following participation in a behavioral weight loss program. Eligible participants will undergo a baseline evaluation after which they will enter an up to 20 week behavioral weight loss program with the goal of losing at least 7 percent of their baseline weight within 35 weeks. Participants who meet the weight loss goal will be asked to remain weight stable for 2 weeks after which they will undergo a clinical examination. They will then be observed for 1 year during which they will undergo 2 additional clinical examinations, one 4 months after completing the weight loss program, and the other 12 months after completing the weight loss program.

Primary Hypothesis: Changes, integrated values and patterns in energy intake (EI), energy expenditure (EE), resting energy expenditure (REE) and non-resting energy expenditure (NREE), contribute to the variability in weight change during the 12 months following weight loss. Assessment time points: * Baseline (BL): prior to starting the weight loss intervention * T0: at the end of a period of weight stabilization following at least 7 percent weight loss * T4: four months (17 weeks) following T0 * T12: twelve months (52 weeks) following T0 Secondary Hypotheses: Changes, integrated values and patterns in components of EI and EE (listed below) contribute to variability in weight change during the 12 months following weight loss. EI: * Psychosocial, food attitude, and food environment assessments * Diet composition * Neural activation in food choice/decision-making and cue/taste/reward (functional magnetic resonance imaging \[fMRI\]) * Measures of food choice, delay discounting, and eating in the absence of hunger * Gastric emptying, appetite-related peptides, and glucose and lipid excursions in response to a meal EE: * Physical activity * 24-hour urine catecholamine * Muscle contractile efficiency * Mitochondrial function in biopsied skeletal muscle * Sleep quality and patterns * Muscle and adipose tissue gene expression * Plasma metabolome, proteome, exosomes and exposome * Stool microbiome Specific Aims: Specific Aim 1: Examine total energy expenditure (TEE) and energy intake (EI) at, and between, baseline, the end of the weight stabilization period following at least 7 percent weight loss, 17 weeks and 52 weeks after the end of the weight stabilization period following at least 7 percent weight loss with respect to variability in weight change between the end of the weight stabilization period following at least 7 percent weight loss and 52 weeks after the end of the weight stabilization period following at least 7 percent weight loss. Specific Aim 2: Examine resting energy expenditure (REE) and non-resting energy expenditure (NREE) at, and between, baseline, the end of the weight stabilization period following at least 7 percent weight loss, 17 weeks and 52 weeks after the end of the weight stabilization period following at least 7 percent weight loss with respect to variability in weight change between the end of the weight stabilization period following at least 7 percent weight loss and 52 weeks after the end of the weight stabilization period following at least 7 percent weight loss. Specific Aim 3: Assess endophenotypes of EI (e.g. diet composition, food-choice, decision-making, delay discounting, cue/taste reward, gastric emptying, appetite-related peptides, glucose and lipid excursions in response to a meal, sleep patterns, food and social environments) at and between time points at which they are measured (e.g., baseline, the end of the weight stabilization period following at least 7 percent weight loss, 17 weeks and 52 weeks after the end of the weight stabilization period following at least 7 percent weight loss) and examine their contributions to the variability in weight change between the end of the weight stabilization period following at least 7 percent weight loss and 52 weeks later. Specific Aim 4: Assess endophenotypes of EE (e.g., actigraphy measures, sleep patterns, muscle contractile efficiency, mitochondrial function, muscle and adipose tissue gene expression, plasma metabolome, microbiome, exposome, and environment) at, and between, time points at which they are measured (e.g., baseline, the end of the weight stabilization period following at least 7 percent weight loss, 17 weeks and 52 weeks after the end of the weight stabilization period following at least 7 percent weight loss) and examine their contributions to the variability in weight change between the end of the weight stabilization period following at least 7 percent weight loss and 52 weeks later.