Vitamin D Inadequacy on Postprandial Glucose in Type 2 Diabetes Mellitus

• insulin sensitivity and β-cell function indexes from the meal test [ Time Frame: 20 days ]
  Within 20 days of the screening visit, the eligible participants will receive a 3-h mixed meal test for postprandial glucose excursion.
• Meal-related glycemic excursion indexes (∆G, ∆T, BR)from the meal test [ Time Frame: 20 days ]
  Within 20 days of the screening visit, the eligible participants will receive a 3-h mixed meal test for postprandial glucose excursion.

• Vitamin D deficiency
  patients with serum vitamin D levels of the lower tertile
• Vitamin D inadequacy
  patients with serum vitamin D levels of the middle tertile
• Reference group
  patients with serum vitamin D levels of the upper tertile

Inclusion Criteria:

• Ambulatory type 2 diabetes patients.
• Men or women.
• Aged 20 ~ 75 years old.
• Stable oral anti-diabetes therapy for at least 8 weeks.
• Willing to participate in the study by signing an informed consent.
• Willing to undergo a screening visit including medical history taking and fasting blood sampling.
• Willing to undergo a standardized mixed meal test at a separate visit.

Exclusion Criteria:

• Patients of type 2 diabetes with the following: History of HHNK; Fasting glucose higher than 250 mg/dL; Treated with insulin, alpha-glucosidase inhibitor, incretin mimetics, DPP-IV inhibitors or rapid-acting insulin secretagogus (the meglitinides) in the past 8 weeks;
• HbA1c > 8.5 %.
• History or evidence of parathyroid or calcium-related diseases.
• History or evidence of endocrine diseases including hyperthyroidism, hypothyroidism, adrenal disease and pituitary disease.
• History of major renal, liver, heart, blood and neurological disease, judged by the investigation physicians.
• History or evidence of alcoholism or drug abuse.
• History of surgical operation of upper gastrointestinal tract and liver. Women who are pregnant.
• History of malignancy within the last 5 years
• History of mental incapacity or language barriers that may preclude adequate understanding or cooperation in the study.
• Under systemic glucocorticoid therapy in the past 8 weeks
• Under supplementation of vitamin D, of any kind, in the past 4 weeks
• Under hormonal replacement therapy in the past 8 weeks
• Under treatments for osteoporosis including calcitonin and biphosphate in the past 8 weeks.
• History or evidence of difficult venous access
• Subjects with the following laboratory values: hemoglobin < 9 g/dL, WBC < 3000/mL, platelet < 100,000/mL, serum Cr > 2.0 mg/dL, ALT > 3x ULN, total bilirubin > 2x ULN.
• Current or concomitant illness that would interfere with the subject's ability to perform the study or that would confound the study results, judged by the investigation physicians.