Background: Vitamin D is a hormone with immunomodulatory properties. Vitamin D deficiency has been reported for patients with inflammatory bowel disease (IBD). In this cross-sectional study, 25-hydroxyvitamin D3 (25-OH-D3) levels in patients with IBD were compared to those in patients with irritable bowel syndrome (IBS). Methods: A total of 181 patients, 156 with IBD and 25 with IBS, were included. The influence of disease activity, inflammatory markers, physical activity, and season were assessed. Results: A total of 58.6% (n = 58) of the patients with Crohn’s disease (CD) and 44.6% (n = 25) of the patients with ulcerative colitis (UC) had a 25-OH-D3 level < 50 nmol/L. CD patients showed significantly decreased 25-OH-D3 levels compared to the IBS patients (p = 0.018), but no significant difference was found for UC patients. In a linear regression model adjusted for age, gender, and BMI, a significant inverse association of C-reactive protein (CRP) (p = 0.031) and faecal calprotectin (FC) (p = 0.025) with 25-OH-D3 levels was observed for CD patients. Seasonal variation in 25-OH-D3 levels was found in CD patients, with significantly lower values in spring than in summer (p = 0.04). Conclusion: Vitamin D deficiency was common in all IBD patients, but more pronounced in CD patients, in whom it also showed a significant inverse association with inflammatory markers such as CRP and FC.

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