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The challenge in maintaining long-term weight loss is well known, however new research suggests diet quality may be the driving factor. A pilot study from our group demonstrated that a higher carbohydrate-containing diet was associated with lower total energy expenditure during weight loss maintenance (Ebbeling et al). These findings will be confirmed in the ongoing Framingham State Food Study (NCT02068885): Following weight loss on a standard diet, 150 overweight or obese adults (aged 18 to 65 years) will be randomized to one of three weight-loss maintenance diets varying in carbohydrate to fat ratios for 20 weeks. However, the specific mechanisms underlying the calorie-independent effects of diet remain unclear. Another study from our group demonstrated lower energy availability (calculated based on caloric content of circulating metabolic fuel concentrations) in the fasting and late post-prandial periods in 8 overweight or obese young adults who were maintained on a low-fat (high-carbohydrate) diet (Walsh et al). We hypothesize that this lower metabolic fuel availability on a high carbohydrate diet results in part from increased anabolic changes within the adipocyte, favoring fat storage in preference to oxidation. We will invite subjects already enrolled in the Framingham State Food Study to participate, aiming for a total of 30 subjects (with the goal of approximately equal numbers per diet group following randomization to assigned test diet in the parent study). Participants will be admitted to a research unit for a 24-hour period during weight maintenance on the test diet, during which they will undergo frequent blood sampling for the measurement of circulating metabolic fuels, hunger and satiety ratings, while consuming their assigned diet meals. Each participant will also undergo two abdominal subcutaneous fat aspiration biopsies under local anesthesia, the first immediately following weight loss (before initiating the test diet) and the second during weight maintenance, in order to perform gene expression analyses on the adipose tissue. Our main outcomes will be a comparison in energy availability (the sum of energy in the major metabolic fuels in the blood) between diet groups in the late postprandial period and changes in adipose tissue gene expression within-individuals and by diet group assignment. Other outcomes will include differences in hunger and satiety ratings, total 24-hour energy availability, and specific metabolic fuel concentrations.