Vitamin D deficiency (defined as 25(OH)-vitamin D serum level <50 nmol/ l) is associated with irritable bowel disease (IBD). National guidelines recommend the administration of 800 -4000 IU cholecalciferol daily for an effective treatment, especially during the winter (poor sun exposition). Cumulative intermittent administration monthly or weekly is possible. The study aims to compare inflammation activity (primary outcome) after monthly or weekly treatment with soft capsules containing 24'000 IU cholecalciferol compared to no vitamin D supplementation. Quantification of 25(OH)-vitamin D serum values is a secondary outcome. The investigators will use newly developed soft capsules.

Monthly supplementation: Monthly administration of a capsule containing 24'000 IU vitamin D, corresponding to a dose of approximately 800 IU per day, as comedication to the usual treatment during 6 months.

Weekly supplementation: Weekly administration of a capsule containing 24'000 IU vitamin D, corresponding a dose of approximately 3500 IU per day, as comedication to the usual treatment during 6 months.

Control: Usual treatment without vitamin D supplementation. All 3 groups: In a follow-up phase of 6 months, all participants can choose whether they want a monthly vitamin D supplementation or not.

Inclusion Criteria:

• diagnosis of Crohn disease or ulcerative colitis

Exclusion Criteria:

• hypercalcaemia