Pharmacokinetic Study: Effect of 25(OH)D and Vitamin D3 on Serum 25(OH)D

Inclusion criteria: * Postmenopausal women * Age 50 to 75 years * Body mass index 18-29 kg/m2 * 25-hydroxyvitamin D levels 20 to 60 nmol/l * Caucasian * Generally healthy Exclusion criteria: * Serum calcium \> 2.6 nmol/L * Use if dietary supplements (\> 400 IU vitamin D per day, \> 600 mg of calcium per day) * Hypertension * Diseases that carry the risk of hypercalcemia * Use of any drug that interferes with bone metabolism in the last 12 months (bisphosphonates estrogen receptor modulators, PTH, calcitonin) * Oral HRT in the last 6 months * Extreme diets * Fracture or fall in the last 3 months * Current smoking or alcohol abuse * Planning on a sunny vacation in the course of the trial * Kidney stone history * Creatinine clearance \< 30 ml/min * Co-medications: anticoagulants, PTH, corticosteroids, thiazide diuretics, digoxin, anticonvulsants, malabsorption,z C