 The study is an open-label, randomized, three-way crossover and single-dose study.Subjects with newly onset T2DM are enrolled. They are assigned to take a single dose of sitagliptin 100 mg, saxagliptin 5 mg, or blank control in a randomized order separated by a washout period of 7-14 days.Then the two-step hyperglycemic clamp is initiated. 