Vitamin D and Inflammatory Bowel Disease
Inflammatory bowel disease — Crohn's disease and ulcerative colitis — sits at the intersection of a disease with a strong latitude gradient, a population with high rates of vitamin D deficiency, and a biology (mucosal immunity, T-cell regulation, antimicrobial peptides) that vitamin D actively modulates.
Deficiency is the norm in IBD
Cross-sectional studies find 60–70% of adults with active Crohn's have 25(OH)D < 20 ng/mL, and 40–50% of ulcerative colitis patients. Reasons stack: reduced intestinal absorption (especially with terminal ileal Crohn's or resection), reduced sun exposure during flares, corticosteroid effects on vitamin D metabolism, and dietary avoidance of dairy.
The latitude and MR evidence
IBD incidence increases sharply with latitude — the Nurses' Health Study found women in the highest 25(OH)D quartile had 46% lower Crohn's incidence than those in the lowest. Mendelian randomisation studies using genetic variants that lower 25(OH)D suggest a causal link with Crohn's specifically, less clearly with ulcerative colitis.
Supplementation trials
- Jørgensen 2010 — 94 Crohn's patients in remission; 1,200 IU cholecalciferol/day for 12 months reduced relapse rate from 29% to 13% (p = 0.06 — a trend, not conventional significance).
- Yang 2013 — 108 Crohn's; 5,000 IU/day raised 25(OH)D and reduced disease activity index vs standard care.
- Cochrane 2023 review — very low-certainty evidence for reducing clinical relapse in Crohn's; insufficient data for UC. Well-powered trials are still lacking.
Practical management
- Everyone with IBD should have 25(OH)D checked at diagnosis and annually.
- Target 25(OH)D 30–50 ng/mL (75–125 nmol/L).
- Malabsorption often requires larger doses (2,000–5,000 IU/day) than in the general population; some patients need 10,000 IU/day.
- Terminal ileal Crohn's, short-bowel syndrome, or post-resection patients may need 25(OH)D (calcifediol) or IM cholecalciferol if oral is inadequate.
- Check calcium at higher doses to avoid iatrogenic hypercalcaemia.