Each study will be 3-4 weeks apart. Prior to each part, hemoglobin must be >12 g/dl and a female subject should test negative for pregnancy. Part B: Will be identical as part A except that Sitagliptin dose will be 50 mg. Part C: Will be identical as part A except that Sitagliptin dose will be 100 mg. Part D: Will be identical as part A except that the study subject will receive a placebo. In all the Parts B, C, D urine pregnancy test will be done before the visit for all female subjects. Also after each Part A, B, C, D the subject will be on Dexcom™ Continuous Glucose Monitoring System for 7 days and after that they will send that back to us using a prepaid mailing envelope.