Inherited and Environmental Risks Acting on Body Weight

Inclusion Criteria: * Age 20-45 years * Differs in habitual diet from co-twin (study 1). * Willing to undergo 7-day overfeeding and hypocaloric feeding periods (study 2) * Member of the Washington State Twin Registry Exclusion Criteria: * History of bariatric surgery or active participation in weight-loss program * Major medical or neurological disorder (e.g., diabetes, multiple sclerosis) * Known gallbladder disease, anemia, impaired thyroid, kidney or liver function or hypertriglyceridemia (\>350 mg/dl) (study 2) * Current use of weight-loss medications or other medications known to alter appetite (e.g., atypical anti-psychotics) * High levels of vigorous physical activity (\>300 min./week) * Pregnancy, menopause, or breastfeeding * Individuals with severe food allergies and dietary restrictions (e.g., vegan/vegetarian, gluten-free, others) (study 2) * Co-twin unwilling or unable to participate * MRI contraindication (e.g., implanted metal, claustrophobia) * Lifetime eating disorder * Current smoking or heavy alcohol use (≥ 2 drinks per day for females and ≥ 3 drinks per day for males) * BMI \> 45 kg/m2 (study 1) * BMI \> 35 kg/m2 (study 2) * Weight \> 350 pounds or determined to be safe by MRI tech (MRI limit) * Inability to participate in a controlled diet program (study 2) or undergo other study procedures * Non-English speaking-or-reading individuals