Effect of DPP-4 inhibitor or Metformin on insulin therapy in patients with type 2 diabetes

INTERVENTION: sitagliptin metformin CONDITION: Type 2 diabetes PRIMARY OUTCOME: Change of glucose, glucagon and C‐peptide response to MTT after sitagliptin or metformin treatment. SECONDARY OUTCOME: Change of HbA1c, fasting plasma glucose, glucagon and C‐peptide after sitagliptin or metforimin treatment. INCLUSION CRITERIA: 1) Patients with type 2 daiabetes 2) Patients without change of treatment for the past three months 3) HbA1c(NGSP) 6.9%<=and10.5%> 4) Over 20 years old,Under 75 years old 5) A consent form for participating in this study is submitted 6) Possible to take the test meal (a set of Chicken in white sauce, crackers, soft pudding)