How Metabolism Affects Weight Loss and Gain

Prior treatment

• Current nicotine use or nicotine use within the last month prior to screening
• Illicit drug use (e.g., amphetamines, cocaine, heroin, marijuana)
• Non-MRI-compatible metallic implants (e.g., artificial joints, metal plates)
• Use of weight-loss medications

Other illness

• 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)
• Any other condition not mentioned above that, in the opinion of the investigator, could interfere with study participation or compromise patient safety

Lab values

• Prediabetes (HbA1c \>5.6% or fasting blood glucose \>100 mg/dL) or diabetes mellitus

Other

• Regular high-intensity physical activity (≥1 hour/day)
• Pregnancy or breastfeeding
• Clinically significant claustrophobia

• 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