Besides causing weight loss Roux-en-Y gastric bypass (RYGB) has profound effect on glucose metabolism leading to remission of type 2 diabetes early after surgery. However the mechanisms for this improvement remain uncertain. The aim of this study is to investigate the changes in insulin sensitivity and beta-cell function 1 week and 3 months following RYGB using oral and intravenous test.

Investigators plan to study 24 obese patients already enrolled for Roux-en-y gastric bypass surgery (RYGB). 8 with type 2 diabetes (DM2), 8 with impaired glucose tolerance (IGT) and 8 with normal glucose tolerance (NGT) before, within the first week and 3 month after RYGB using a liquid mixed meal test (MMT) and an insulin modified frequently sampled intravenous glucose tolerance test (IM-FSIGT). Furthermore, an oral glucose tolerance test (OGTT) will be performed before and after 3 months. Blood will be sampled in fasting and during the tests measuring plasma glucose, insulin and C-peptid (OGTT, MMT and IM-FSIGT) and GLP-1, glucagon, GIP and GLP-2 (MMT). Beta-cell function will be assessed from the MMT (insulinogenic index - IGI), OGTT (IGI) and the IM-FSIGT (Acute insulin response, AIR) in order to examine whether changes in beta-cell function after RYGB depend on an oral stimulus. Insulin sensitivity will be assessed in fasting (HOMA-IR), during the IM-FSIGT (minimal model: Si) and from MMT/OGTT (Matsuda index). Insulin clearance/hepatic extraction of insulin will be assessed in fasting and during the intravenous and oral test.

Inclusion Criteria:

• patients eligible for RYGB, Age 18-60 years, BMI 35-60 kg/m2 at time of referral to operation, blood pressure <145/85, C-peptid>700 pmol/l.

Exclusion Criteria:

• Obesity caused by medical treatment for psychiatric disease. Mental retardation. Alcohol or drug abuse. Severe cardiopulmonary disease. History of peritonitis, ventricular disease, upper gastrointestinal surgery, recurrent oesophagitis or severe complications to general anesthesia. Bad compliance. Treatment with thyroid hormones or antithyroid treatment. Treatment with anorectic medicine later than 3 months prior to surgery. Furthermore prior to surgery each patient has to loose 8 % of bodyweight to reduce the risk of operative complications.

Before each test day all glucose lowering medication will be paused for an appropriate amount of time depending on the type of medicine.

Morbid obese patients who are already enrolled for Roux-en-y gastric bypass surgery will be invited to participate (investigators do not assign patients to the surgery). 8 with type 2 diabetes, 8 with impaired glucose tolerance and 8 with normal glucose tolerance. Glucose tolerance will be assessed with an oral glucose tolerance test (OGTT) prior to surgery and prior to the other tests. Plasma glucose 120 minutes after OGTT will define glucose tolerance. Normal: < 7,8 mM. Impaired: 7,8 - 11,1 mM. Diabetic: >11,1 mM.

Subjects are recruited from the outpatient clinic of endocrinology and gastrosurgical clinic at Hvidovre University Hospital.