Effects of Vitamin D on Beta Cell Function and Insulin Sensitivity in Pre-diabetes and Diabetes Mellitus Type 2

Inclusion Criteria: * Meeting criteria for IFG, IGT, IFG+IGT or diabetes mellitus at OGTT. * BMI ≤ 32 kg/m2. * HbA1c ≤ 7.0 % (MonoS) or ≤ 63 mmol/mol (IFCC). * Fasting plasma glucose \< 9 mmol/l * S-25-OH-vitamin D3 \< 75 nmol/l. Exclusion Criteria: * Treatment with any vitamin D preparation. * Regular sun-bathing in solarium. * Hypercalcemia at screening, defined as free s-calcium \> 1.35 mmol/l. * Hyperphosphatemia at screening, defined as s-phosphate \> 1.5 mmol/l. * Sarcoidosis or other granulomatous disease. * Treatment with phenytoin, barbiturates, rifampicin, isoniazid, cardiac glycosides, orlistat or colestyramin. * Impaired hepatic function. * Impaired renal function * Cardiac disease defined as: * Unstable angina pectoris * Myocardial infarction within the last 6 months * Congestive heart failure NYHA class III and IV * Cerebral stroke within the last 6 months. * Anti-diabetic medication of any kind. * Females of childbearing potential who are pregnant, breast-feeding or intend to become pregnant or are not using adequate contraceptive methods.