Obesity promotes chronic kidney disease and is accelerating its shift to the stage of renal replacement. Bariatric surgery is a treatment for severe or morbid obesity whose renal benefit is currently unknown. The glomerular filtration rate (GFR) is the best parameter to define the kidney function. It can be estimated using formulas by assaying endogenous markers (creatinine, cystatin C) or measured with an exogenous tracer glomerular filtration (51 Cr-EDTA).

Bariatric surgery alters the production of endogenous markers and the extracellular volume (VEC) with 2 important consequences on the assessment of GFR: the formula to estimate GFR is not possible to compare renal function before and after surgery because of the variation in production of endogenous glomerular filtration markers in the same subject; decreasing VEC predicted decreased GFR after surgery, since these parameters are in part proportional to each other.

Our working hypothesis is that bariatric surgery protects the kidneys of patients with chronic kidney disease. To demonstrate this, investigators propose to compare the ratio DFG / VEC before and after gastric bypass.

Goals The main objective is to measure the effect of gastric bypass on the report DFG / VEC. Secondary objectives are to assess the effect of gastric bypass on the albumin / urine creatinine and evaluate the performance of the main GFR estimating formulas in people with severe or morbid obesity.

Inclusion Criteria:

• Topic GFR estimated by having the simplified formula of MDRD <60 ml / min / 1.73 m²
• Topic with BMI ≥ 40 kg / m², or ≥ 35 kg / m²
• Subject having at least one comorbidity could be improved after surgery after failure of medical treatment: hypertension , OSAHS and other severe respiratory disorders, severe metabolic disorders, particularly type 2 diabetes , debilitating musculoskeletal diseases, steatohepatitis nonalcoholic .
• Topic who signed the informed consent
• Topic affiliated to the social security
• Male or female Topics
• Elderly 18 to 70 inclusive

Exclusion Criteria:

• Non- Caucasian person because the necessary correction factors and not always available for the formula to estimate GFR ( Cockcroft , Salazar and Corcoran ) .
• No major under guardianship
• Nobody hospitalized without his consent and not protected by law
• Private person of liberty
• Pregnant Woman, knowing that a pregnancy test will be performed for women of childbearing age . Results will be communicated to the patient by a doctor of his choice.
• Rapidly progressive glomerulonephritis
• Nephrotic proteinuria ranking