The purpose of this study is to determine the effects of the drug, sitagliptin, on the rate at which the stomach empties, and the release of gut hormones and blood glucose concentrations, after a mashed potato meal in healthy subjects. Sitagliptin has been shown to reduce the blood glucose (sugar) response to a meal and this may potentially be due to slowing of stomach emptying. This is particularly relevant to people who have diabetes, in whom normalization of elevated blood glucose levels is important to maintain health.