In this study, the investigators will also use the drug Sitagliptin, which is an FDA-approved drug for the treatment of type 2 diabetes mellitus. In this study, use of Sitagliptin is considered investigational since it is not being used for treatment of diabetes but is instead being used to understand how GLP-1 works and to better understand how medications like Sitagliptin work in patients with type 2 diabetes mellitus.