BLOOD SAMPLING AND MEDICATIONS: At approximately 0700 baseline blood samples will be drawn prior to study start for: glucose, insulin, glucagon, GLP-1, DPP-4 activity and Sitagliptin. Also, there will be blood draws at the following time points: -240 min, -180 min, -60 min, 0 min, 4 hrs. prior to the mixed meal and at 10, 20, 30, 40, 60, 90, 120, 150, 180, 210, 240, 360 min after. The insulin pump basal rate (or glargine) will be set to continue as per home regimen. Sitagliptin 25 mg will be administered at this point. At around 1100 (0 min), the pre-breakfast insulin bolus of rapid acting insulin will be given at 0 min. The insulin dose will be based on the patient's usual insulin to carbohydrate ratio. This dose will be reduced by 20-50% and the subject will be offered a standard liquid meal of Boost High Protein Drink, 12 Oz. (360 calories, 50 g of carbohydrates and 12 g of fat) and asked to consume it within a 10-15 minute period. This will be enriched with 1 gram of [1- 13C] glucose. Expired 13CO2 will be determined in breath at -240, 0, 10, 20, 30, 40 60, 90, 120, 150, 180, 210, 240, 360 min. Lunch will be provided after 240 min and an insulin bolus of rapid acting insulin will be given through the insulin pump (calculated according to subject's insulin to carbohydrate ratio and sensitivity factor) or through an insulin injection. They will stay for 2 more hours for the 360 min blood draw. Blood glucose concentrations will be measured at the bedside using an Analox glucose analyzer. Sitagliptin being a 24 hr medication pk data will be obtained at 0 and 360 min. For safety reasons the subject's will be put on a Dexcom CGMS and they will wear for a week. During this period they will keep a log sheet in which they will note the blood glucose, food and exercise activity. They will return that using a prepaid envelope which will be provided. The subject will be discharged after this. They will not be paid separately for transportation, parking or child care. A social security number will be required. It will be kept confidential. For minors, compensation will be given to the parent, but it is intended for the child.