Levels of Vitamin D Are Low After Crohn's Disease Is Established But Not Before

BACKGROUND & AIMS: Low serum levels of vitamin D have been associated with Crohn's disease (CD). However, it is unclear whether low vitamin D levels cause CD or CD reduces serum vitamin D. METHODS: United States military personnel with CD (n = 240) and randomly selected individuals without CD (controls, n = 240) were matched by age, sex, race, military branch, and geography. We measured 25-hydroxyvitamin D in sera 8-3 years (pre-2) and 3 years to 3 months before diagnosis (pre-1) and 3 months before through 21 months after diagnosis (pre-0). We genotyped VDR and GC vitamin D related polymorphisms. We used conditional logistic regression, including adjustments for smoking, season, enlistment status, and deployment, to estimate relative odds of CD according to vitamin D levels and interactions between genetic factors and levels of vitamin D. RESULTS: Levels of vitamin D before diagnosis were not associated with CD in pre-2 (P trend = .65) or pre-1 samples (P trend = .84). However, we found an inverse correlation between CD and highest tertile of vitamin D level in post-diagnosis samples (P trend = .01; odds ratio, 0.51; 95% CI, 0.30-0.86). Interactions were not detected between vitamin D levels and VDR or GC polymorphisms. We observed an association between VDR Taq1 polymorphism and CD (independent of vitamin D) (P = .02). CONCLUSIONS: In serum samples from military personnel with CD and matched controls, we found no evidence for an association between CD and vitamin D levels up to 8 years before diagnosis. However, we observed an inverse-association between post-diagnosis vitamin D levels and CD. These findings suggest that low vitamin D does not contribute to development of CD-instead, CD leads to low vitamin D.