Background: Vitamin D may mediate immunomodulatory effects in patients with inflammatory bowel disease (IBD). The relationships between disease activity and circulating levels of total, free, and bioavailable 25(OH) vitamin D (25(OH)D) are poorly defined. The aim of this study was to measure circulating components of the vitamin D axis in patients with IBD and healthy controls and to correlate these with markers of disease activity, adjusting for potential confounders.
Methods: Clinical data were obtained and serum was analyzed for 25(OH)D and vitamin D-binding protein in patients with IBD and controls. Markers of systemic and intestinal (fecal calprotectin) inflammation were measured.
Results: Serum 25(OH)D concentration was similar across 23 controls, 40 patients with Crohn's disease, and 31 with ulcerative colitis. An inverse correlation between 25(OH)D and calprotectin was noted in Crohn's disease (Pearson's r = -0.35, P = 0.040), ulcerative colitis (r = -0.39, P = 0.039), and all IBD together (r = -0.37, P = 0.003), but not with systemic markers. A similar trend was noted for free and bioavailable 25(OH)D. This inverse correlation remained after partial correlation analysis correcting for sunlight exposure, total oral vitamin D intake, and obesity and was also noted among the subgroup without small intestinal involvement.
Conclusions: Despite total, free, and bioavailable 25(OH)D concentrations being similar to those in a healthy control population, they inversely correlated strongly with intestinal inflammation. This was independent of potential malabsorption, sunlight exposure, and total vitamin D intake and obesity. Vitamin D may play an immunomodulatory role in IBD.