How Metabolism Affects Weight Loss and Gain

Inclusion Criteria: * Group 1 (Lean): BMI 18.5-22.0 kg/m², reporting difficulty gaining weight * Group 2 (Obese): BMI \>30 kg/m², reporting difficulty losing weight * Healthy status as determined by medical history, physical examination, and laboratory tests * Stable weight (less than 5% fluctuation) over the past 6 months Exclusion Criteria: * Prediabetes (HbA1c \>5.6% or fasting blood glucose \>100 mg/dL) or diabetes mellitus * Conditions affecting appetite or energy expenditure (e.g., Cushing's syndrome, uncontrolled hyper-/hypothyroidism) * Gastrointestinal disorders that impact nutrient absorption (e.g., inflammatory bowel disease, malabsorption syndromes, ulcers) * Psychiatric conditions influencing eating behavior (e.g., active depression, anorexia nervosa, bulimia nervosa, borderline personality disorder) * Acute, unstable cardiovascular disease requiring hospitalization within the last 6 months (e.g., stent placement) * Cancer that required treatment in the past 5 years * Chronic kidney disease (Stage IV or higher, per NKF criteria) * Active infectious disease (e.g., HIV, hepatitis) * Current nicotine use or nicotine use within the last month prior to screening * Illicit drug use (e.g., amphetamines, cocaine, heroin, marijuana) * Regular high-intensity physical activity (≥1 hour/day) * Non-MRI-compatible metallic implants (e.g., artificial joints, metal plates) * Pregnancy or breastfeeding * Use of weight-loss medications * Clinically significant claustrophobia * …