Bone Marrow Fat and Bariatric Surgery-Mediated Bone Loss

Inclusion Criteria: Postmenopausal women (defined as last menses \>12 months) * Obesity with a BMI ≥ 35 kg/m², * Patients with an indication for bariatric surgery with two surgical techniques, RYGB and SG, but who have never had a bariatric surgery procedure previously, * Patients who have signed the informed consent. Exclusion Criteria: * History of an osteoporotic fracture 2- Current corticosteroid therapy or corticosteroid therapy within the past 3 months, C-onditions or treatments affecting bone metabolism (breast cancer treated with aromatase inhibitors, malabsorption, stomach cancer, primary hyperparathyroidism, uncontrolled hyperthyroidism…), * History of radiation therapy to the lumbar spine or the hip being studied, * Patients on hormone replacement therapy (HRT) or patients already on anti-osteoporotic treatment (bisphosphonates, strontium ranelate, teriparatide, or denosumab) other than calcium and/or vitamin D supplementation, * Chronic kidney disease with a creatinine clearance (CKD-EPI formula) ≤ 30 mL/min, * Weight \> 160 kg, as the bone densitometer table is limited to this weight for the examination, * Patients who have previously undergone bariatric surgery, * Patients with orthopedic implants in both hips or the lumbar spine, * Individuals unable to give personal consent, * Individuals under legal guardianship or conservatorship, * Patients not enrolled in a social security program. * Any contraindication to MRI: (claustrophobia, patients with a pacemaker/defibrillator, cochlear implant, neurostimulator, or orthopedic implants in the hip, pelvis, or lumbar spine, etc.).