Can Pre-operative Lean Mass Predict Weight Loss After Roux-en-Y Gastric Bypass?

Inclusion Criteria: * Patients ≥ 18 years AND * Meeting inclusion criteria for RYGB (www.smob.ch): BMI ≥35 kg/m2, failure of weight-reducing programs over 2 years (for patients with a BMI ≥50 kg/m2, 1 year), understanding of the necessity to change the lifestyle and eating habits post-operatively, signed consent for lifelong post-operative follow-up in a recognized bariatric center AND * Scheduled RYGB at the HUG OR previous RYGB at the HUG with a present follow-up in Clinical Nutrition and dietetics at the HUG AND * Understanding French Exclusion Criteria: * Inability or refusal to give consent. * Planification for another type of bariatric surgery than RYGB * Contra-indications to bariatric surgery as stated under www.smob.ch: insufficient nutritional knowledge in nutrition to lose weight, pregnancy, kidney failure (Creatinine ≥ 300 umol/l without dialysis, Crohn's disease, active cancer or remission \< 2 years, active psychiatric disease, substance abuse (alcohol, cannabis, opioids), absence of compliance, lack of understanding of the requirements and necessary lifestyle changes reported by physician or dietician.