Biliopancreatic diversion (BPD) surgery results in greater resolution of type 2 diabetes than all other bariatric surgical procedures, and it is hypothesized that this procedure has specific beneficial effects on glucose homeostasis beyond weight loss alone. The BPD procedure is performed in more than 150 patients/year by surgeons at the Division of Obesity and Metabolic Disorders, Catholic University of the Sacred Heart, School of Medicine, in Rome, Italy. The purpose of this study is to provide a better understanding of the effect of the BPD bariatric surgical procedure on insulin action and pancreatic beta cell function. It is hypothesized that weight loss achieved with BPD surgery will have greater effects on insulin sensitivity and beta cell function than weight loss induced by Roux-en-Y gastric bypass (RYGB).