A randomization procedure will be used to avoid a bias in the assignment of the study subjects to the two treatment groups (oral vitamin D, oral placebo). Randomization lists, preparation of study drugs and placebo, labelling and drug accountability will be done by the hospital pharmacy (Kantonsapotheke Zürich). The randomisation list will be preserved by the hospital pharmacy. The oral intake will be directly observed by a physician assistant or study nurse. As placebo and verum are manufactured having identical appearance, taste and smell, study participants can not differ between placebo and verum.