The main objective of the study is to evaluate the protein requirement associated with postoperative weight loss caused by by-pass (shunt which leads to a certain degree of malabsorption) or sleeve gastrectomy (more technically conservative with less malabsorption), through the study of nitrogen balance. The benefit of protein supply easily absorbed and with high essential amino acids content (proteins "fast" soluble milk) on protein metabolism and muscle function will also be studied. Endotoxemia and intestinal flora variations were measured before and after bariatric surgery.

This is a prospective longitudinal study of the effect of bariatric surgery on protein requirements and protein metabolism, and protein supplementation interest, appropriate to the digestive tract changes, for the mass and muscle function. Three groups of patients will be studied: One group undergoing sleeve gastrectomy in Clermont-Ferrand (group 1: n = 20) will be followed by current recommendations. A second group undergoing sleeve gastrectomy in Clermont-Ferrand (group 2: n = 20) will receive during three months, protein supplement (powder whey protein ("fast")) at dose of 40g / day. 10 patients in group 1 and 10 patients in group 2 will participate in protein metabolism study. The third group undergoing gastric bypass in Lyon (group 3: n = 20) will be followed by the current recommendations and will participate in study of endotoxemia.

Inclusion Criteria:

- Males and females.

• Age between 18 to 60 years.
• Body mass index: BMI> 40 kg/m2,
• Biological Review considered satisfactory by the investigator based on the topics covered
• Serology HIV ( Human Immunodeficiency Virus )and HCV (Hepatitis C Virus ) negative.
• Subject giving his written informed consent
• Affiliated to National Health Insurance

Exclusion Criteria:

• Subject under 18 or over 60 years.
• Balance Organic considered abnormal by the investigator.
• Serology HIV or HCV positive.
• Pregnant or lactating.
• For women of childbearing age: β-HCG assay positive and not having reliable contraceptives (oral contraceptive, IUD (Intrauterine Device), implant or hormone patch, abstinence).
• Medical or surgical history (judged by investigator to be incompatible with the study).
• Subject with unstable psychiatric condition
• Blood donation in the two months preceding the study.
• Heavy alcohol consumption (> 2 to 3 glasses per day depending on sex) or presence of an addiction.
• Tobacco significant (> 5 cigarettes / day or equivalent in cigars or pipe tobacco).
• Subjects with an allergy or intolerance to dairy products.
• Intense sporting activity (> 5 hours / week).
• Being in exclusion on the National Volonteers Data file or having perceived more than 4500 Euros in the year for clinical study participation.
• No one under guardianship or not subject to social security.
• Subjects deprived of their liberty by judicial or administrative decision.
• Refusal to sign the information sheet and written consent for participation
• Subject with a cardiovascular or neoplasic evolutive disease
• Subject with a severe infection in the 3 months period before inclusion (assessed by doctor during inclusion consultation)
• Hypercorticism and dysthyroïdism
• Subject with a known neuro-muscular disease
• Subject with a chronic or acute inflammatory disease in the 3 months period before inclusion
• Subject treated by: corticoids, immunosupressor, anabolic agents, growth factor or having stopped treatment for less than 3 months before inclusion.

Furthermore for subjects having undergone a by-pass at Lyon:

• Subject consuming pre or probiotics many times per day, having an impact on intestinal flora.
• Subject with antibiotics during the 3 months before the beginning of the study.
• Subject with an intolerance to dairy products and refusing to consume dairy prod