Montefiore Medical Center (MMC) Investigational pharmacy and one of the study staff (excluding the PI) will be un-blinded to the protocol for doing the randomization table, dispensing, checking the dose of study medication and scheduling the study visits (done by the study staff person). For the pediatric age group (13-17 years) it will be single blinded trial with single ascending dose sequential study. Pediatric study subject's will be blinded to the dose of Sitagliptin and they all will start with the placebo part and then proceed with the study parts with Sitagliptin 25 mg, 50 mg and 100 mg dose in a sequential order. The participants will be admitted to the CRC on four occasions for the Parts A, B, C and D and each admission will be 3-4 weeks apart.