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

The goal of this observational study is to evaluate whether preoperative fat-free mass predicts post-operative weight loss in patients undergoing Roux-En-Y gastric bypass. The primary objective of this study is to evaluate whether preoperative fat-free mass predicts weight loss in the two years following RYGB. We will collect the following data, assessed as part of routine follow-up: * height, weight, hip and waist circumferences * body composition by bioimpedance analysis and dual energy-X-ray absorptiometry * Muscle strength and physical activity * Nutritional parameters: resting energy expenditure, eating behavior and digestive tolerance", plasma micronutrient status. . * Cardiovascular risk factors: fasting glucose, blood levels of Hb1Ac, triglycerides, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, blood pressure * Co-morbidities: joint pain and arthritis, sleep apnea, gastro-oesophageal reflux, metabolic dysfunction-associated fatty liver disease (MAFLD), cancer, stroke, heart attack. Quality of life: quality of life by the "Bariatric Analysis and Reporting Outcome (BAROS) questionnaire", symptoms of dumping syndrome by the "Sigstad diagnostic score system" Birthdate, birthweight and term of their children