The goal of this research is to investigate genetic and environmental factors that contribute to obesity through brain inflammation. The main questions are 1) if identical twins, who differ in food consumption habits, have differences in adiposity markers and brain inflammation and 2) if signs of brain inflammation in response to a specific diet is modified by genetics.

In study 1, participant twin pairs that differ in habitual diet intake, will complete an in-person study visit that includes an MRI of the brain and abdomen, body composition measurement, a blood draw and questionnaires. In study 2, a different set of participant twin pairs will complete 3 in-person study visits over 2 weeks, during which all meals will be provided by the study. Visits will include brain MRI, body composition measurements, blood draws and questionnaires.

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