The goal of this project is to examine the physiological mechanisms that contribute to improvements of glucose homeostasis in type 2 diabetes mellitus (DM) following gastric bypass Roux-en-Y bariatric surgery (GBP). The central hypotheses are: 1) that surgical changes in the anatomy of nutrient absorption achieved by GBP change incretin hormone secretion which in turn improves pancreatic islet function; and 2) that improvement in insulin secretion induced by the changes in incretin function, together with improvement in insulin resistance induced by weight loss, normalize glucose homeostasis in type 2 DM.