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

This study is prospective but the investigators will also use the retrospective data of the patients who underwent bariatric surgery before the acceptance of the study, with their informed consent. The investigators will collect the following data, assessed as part of routine follow-up: Preoperatively: * medical history * height, weight, hip and waist circumferences * body composition by bioimpedance analysis (BIA) and dual energy-X-ray absorptiometry (DXA) * handgrip strength * sarcopenia questionnaire (SARC-F) * physical activity questionnaire (IPAQ) * resting energy expenditure by indirect calorimetry * eating behaviour: duration of meals, hyperphagia, snacking, compulsions, and cognitive restrictions as variables dichotomized as yes or no, and hunger and satiety each through a visual analogue scale, where 0 corresponds to "no hunger or good satiety" and 10 to "big hunger or no satiety" * digestion symptoms: frequency of vomiting and of bowel movements * Binge eating questionnaire * blood pressure * blood sample * co-morbidities * results of liver ultrasound, gastric endoscopy * quality of life questionnaire (BAROS), Dumping questionnaire (Sigstad) * blood and tissue biobanking These assessments will be repeated after surgery at the frequency indicated below: * at month 1: all tests except DXA, sarcopenia questionnaire (SARC-F), physical activty assessment (IPAC), binge eating questionnaire, liver ultrasound and gastric endoscopy, quality of life and dumping questionnaire, blood and tissue biobank * At month 3: all tests except DXA, binge eating questionnaire, dumping, questionnaire, and only if indicated for clinical routine, liver ultrasound and gastric endoscopy. * At month 6: all tests except DXA, sarcopenia questionnaire (SARC-F), physical activty assessment (IPAC), binge eating questionnaire, quality of life questionnaire, and only if indicated for clinical routine, liver ultrasound and gastric endoscopy, only if indicated in clinical routine, blood and tissue biobank. * At year 1, 2, 3, 4 and 5: all tests except liver ultrasound and gastric endoscopy only if indicated in clinical routine * At year 1.5: all tests except DXA, sarcopenia questionnaire (SARC-F), blood and tissue biobank Study sample calculation: The investigators hypothesized, in a conservative way and based on available litterature, that the patients will decrease their lean mass by 5±10 kg within 2 years after RYGB. A sample size of 32 achieves 90% power to detect a mean of paired differences of 5 kg with an estimated standard deviation of paired differences of 10.0 and with a significance level (alpha) of 0.050 using a two-sided paired t-test. The investigators plan to evaluate whether preoperative FFM (independent variable) predicts weight loss (dependent variable) while adjusting for age, sex, preoperative BMI category (35-40, 40-50, \> 50), and FM, physical activity. This corresponds to 6 adjustment variables altogether, or 3 supplemental variables. According to the rule of Harrel that stipulates at least 10 observations per independent variables, the investigators need at least 70 participants (10\*(1+6)) to perform these adjustments. Considering a drop-out rate of 30% at 2 years, the investigators will need a sample size of: 70+30% = 91 subjects.