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). The study aims to compare 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.

Group Control, Alcoholic Drinking Solution: 5 ml of an alcoholic drinking solution with 24'000 IU vitamin D p.o. once a month for 3 months (3 doses).

Group IMP, Capsules: one capsule with 24'000 IU vitamin D p.o. once a month for 3 months (3 doses).

Group IMP + loading dose, Loading Dose: one capsule with 24'000 IU vitamin D p.o. once a week during an individually calculated number of weeks.

All 3 groups: Measurement of vitamin D in serum before therapy and one week after the last dose.

Inclusion Criteria:

• vitamin D deficiency by serum level <50 nmol/l

Exclusion Criteria:

• hypercalcaemia