Vitamin D deficiency (defined as 25(OH)-vitamin D serum level <50 nmol/l) is cured with oral supplementation. National guidelines recommend the administration of 800 IU cholecalciferol daily for an effective treatment, especially during the winter (poor sun exposition). Cumulative intermittent administration monthly (24'000 IU) or weekly (5'600 IU) is possible. Commercially available intermittent pharmaceutical forms in Switzerland are liquid (oily or alcoholic solution). However, conventional dosing of 800 IU (daily or cumulative per week/month) is often inadequate to obtain optimal serum value >75 nmol/l. The study aims at comparing the rise of vitamin D3 levels after treatment between newly developed soft capsules and monthly solution (primary outcome), and to evaluate the effect of a loading dose (without infringing the cumulative maximal dose of 4'000 IU per day) on the achievement of optimal values >75 nmol/l (secondary outcome). The investigators will use newly developed soft capsules and alcoholic solution, which are commercially available in Switzerland.