Obesity and type 2 diabetes (T2DM) are increasing in the US. One third of patients seeking bariatric surgery have T2DM. Although all surgeries result in significant weight loss and often 'cure' the T2DM, the rapid onset and the magnitude of the benefits of gastric bypass (GBP) on T2DM has thus far baffled clinical scientists. Limited data suggest that the improvement in T2DM after GBP occurs very rapidly, and may not be wholly accounted for by weight loss. Secretion of incretins (gut peptides secreted in response to meals which enhance insulin secretion) is impaired in T2DM and improves after GBP, possibly due to the specific anatomical changes after this surgery. While some determinants of impaired insulin secretion, such as glucotoxicity, improve equally after diet or surgical weight loss, the improvement in the incretin effect after GBP might be specific to this surgery. The AIM of this study is to determine whether the magnitude of the incretin effect on insulin secretion is greater after GBP than after an equivalent diet-induced weight loss. We will compare, in obese patients with diabetes, randomized to very low calorie diet or to GBP, the effect of an equivalent weight loss on the incretin effect (difference in insulin secretion after comparable oral and intravenous (IV) glucose loads). As more obese diabetic patients undergo GBP, understanding the mechanisms that produce improvement in their diabetes is increasingly important.