Vitamin D supplementation in type 2 Diabetes Mellitus.

1. Written informed consent;

2. Patients with T2DM treated with lifestyle advises whether or not in combination with metformin;

3. Adults (≥ 18 years);

4. HbA1c values between 7.0 - 8.5% for the last 3 months.

1. Impaired renal function: creatinine * 150 *mol/l or an estimated creatinin clearance < 50 ml/min, to the discretion of the investigator;

2. Cardiac problems:

A. Decompensated heart failure (NYHA III and IV);

B. Diagnosis of unstable angina pectoris;

C. Myocardial infarction within the last 12 months.

3. Mental retardation, or psychiatric treatment for schizophrenia, organic mental disorder or bipolar disorder currently or in the past;

4. Insufficient knowledge of the Dutch language;

5. Serum 25OHD < 15 nmol/l or > 150 nmol/l;

6. Hypercalcemia of any reason;

7. Urolithiasis;

8. Granulomatous diseases influencing vitamin D levels;

9. Diabetes mellitus other than type 2;

10. Recent or current use of insulin or incretin mimetics;

11. Using other oral antidiabetics than metformin;

12. Pregnancy and/or lactation;

13. Substance abuse, other than nicotine;

14. Participation in any other trials, involving investigational products within 30 days prior to trial entry;

15. Any condition that the Investigator and/or Coordinating Investigator feels would interfere with trial participation or evaluation of results.