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.

The investigators 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

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.

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.

Patients with scheduled gastric bypass at the Geneva University Hospitals (HUG)