Inflammatory bowel disease (IBD) is a chronic immunologically mediated inflammatory condition of the gut. Calprotectin is measured in the faeces as surrogate marker of intestinal inflammation. Levels are elevated in patients with active IBD (cut-off at 50-60 μg/g). Insufficiency in vitamin D or cholecalciferol, defined as 25(OH)-vitamin D serum level <50 nmol/l, has been associated with IBD that is, with increased disease activity such as relapse, disease progression, surgeries and hospitalization.

To our knowledge, no study has investigated yet the effect of vitamin D supplementation on the disease activity of adult patients with IBD. Our research question is the following: Does the supplementation of cholecalciferol within the recommended doses influence significantly the disease activity in adult patients with IBD? The 5C-study aims at quantifying the inflammation activity of adult patients with IBD after the weekly or monthly administration of soft capsules containing 24'000 IU cholecalciferol that is, within the recommended doses, compared to controls with no vitamin D supplementation.