All patients will have two evaluations: one before the bypass surgery and one 3 months after. All evaluation will include 2 standard meals. Patients have to be fasted, and they will take in a randomized order the same glucose quantity: 30g, for the breakfast. After each meal, blood sample will be collected during 3 hours at time 0, 10, 20, 30, 40, 50, 60 minutes and every 30 minutes until time 180.