Effects of Caloric Restriction Alone Versus Postoperative Caloric Restriction Following Bariatric Surgery on Glucose Metabolism in Patients With Diabetes Mellitus

Background: * Bariatric surgery is the most effective way to achieve significant, long-term weight loss. It has also been shown to be an effective therapy for obese individuals with type 2 diabetes: more than 70 percent of patients no longer need medications for diabetes after surgery. This resolution of diabetes is predominately caused by marked weight loss resulting in improved insulin sensitivity. However, the beneficial effects of bariatric surgery on type 2 diabetes cannot be accounted for entirely by weight loss, because many bariatric surgery patients have resolution of diabetes within 1 week following bariatric surgery, even before they lose a clinically significant amount of weight. * One possible reason for the rapid resolution of diabetes after bariatric surgery .is that during the first week after surgery, patients can eat very little (about 300 Calories per day). It is well known that reducing calories to this level improves diabetes. Another possibility is that changes in the flow of food through the intestines may improve diabetes. Evidence for this comes from the observation that patients after gastric bypass have better glucose levels than those who have gastric banding. Researchers are interested in determining how much of the improvement in diabetes in the first week after Roux-en-Y gastric bypass (RYGBP) surgery is due to restricting calories, and how much is due to other factors, such as bypassing the upper part of the small intestine. Objectives: * To determine the change in total body insulin sensitivity after RYGBP compared to caloric restriction without surgery. * To study possible reasons for improvements in diabetes after RYGBP. Eligibility: \- Individuals 18 to 60 years of age who have a body mass index (BMI) greater than 35 and have type 2 diabetes. Design: \- This is not a randomized study, and patients will not receive bariatric surgery as part of this study. Two groups of patients will be studied: those scheduled for RYGBP surgery and those not undergoing surgery. * RYGBP Surgery Participants: * Up to 3 weeks before surgery, participants will spend 2 nights and days at the Vanderbilt University Clinical Research Center or the NIH Clinical Center for testing to learn about how their bodies handle sugar and use energy. During the 5 days prior to these tests, participants will be asked to not take diabetes medications, and will check blood sugar at least twice a day. * From 8 days before surgery, participants will begin an 800 Calorie per day liquid diet to prepare for surgery. * After surgery and discharge, participants will be readmitted to the Clinical Research Center at Vanderbilt or NIH for further tests and diet monitoring. Diabetes medications may be adjusted or stopped altogether based on the results of the tests. * Non-surgery Participants: * Participants will spend 2 nights and days in the NIH Clinical Center for testing to learn about how their bodies handle sugar and use energy. During the 5 days prior to these tests, participants will be asked to not take diabetes medications, and will check blood sugar at least twice a day. * After the tests, participants will begin an 800 Calorie per day liquid diet for 8 days. * After 8 days, participants will be readmitted to the Clinical Center at NIH for 1 week of further tests and a 300 Calorie per day diet. Diabetes medications may be adjusted or stopped altogether based on the results of the tests.