The purpose of the study is to test whether additional protein intake with or without strength training will help to maintain body composition (fat-free mass) after surgical treatment of obesity by gastric by pass. The main hypothesis is that additional protein intake will limit fat-free mass loss during surgery-induced weight loss, and that strength training will have an additional beneficial effect

Design: Open randomised controlled trial comparing, in obese patients after GBP, an additional oral intake of protein with or without a muscular-strengthening physical training programme (18 weeks) to standard medical and nutritional follow-up after bariatric surgery as control (3 groups). Primary outcome: Variation at 6 months after GBP in total and appendicular fat-free mass as assessed by dual-energy X-ray absorptiometry (DXA). Duration of the study: 36 months. Duration of participation for one patient: 7 months. Number of centers: Monocentric study.

Inclusion Criteria:

• Female gender
• Age between 18 and 60 years
• Planned obesity treatment by gastric bypass
• Written informed consent

Exclusion Criteria:

• Subject not affiliated with a social security scheme
• Refusal
• Regular follow-up not feasible
• Participation in a structured physical activity program
• Recent coronary event
• Uncontrolled hypertension
• proliferative diabetic retinopathy or disabling neuropathy, treatment by dialysis, orthopedic problems
• Pregnancy
• Specific food intolerance to protein products
• Abnormal cardiac stress test