Vitamin D intake is inversely associated with rheumatoid arthritis: Results from the Iowa Women's Health Study

Abstract

Objective: Vitamin D is a potent regulator of calcium homeostasis and may have immunomodulatory effects. The influence of vitamin D on human autoimmune disease has not been well defined. The purpose of this study was to evaluate the association of dietary and supplemental vitamin D intake with rheumatoid arthritis (RA) incidence. Methods: We analyzed data from a prospective cohort study of 29,368 women of ages 55–69 years without a history of RA at study baseline in 1986. Diet was ascertained using a self‐administered, 127‐item validated food frequency questionnaire that included supplemental vitamin D use. Risk ratios (RRs) and 95% confidence intervals (95% CIs) were estimated using Cox proportional hazards regression, adjusting for potential confounders. Results: Through 11 years of followup, 152 cases of RA were validated against medical records. Greater intake (highest versus lowest tertile) of vitamin D was inversely associated with risk of RA (RR 0.67, 95% CI 0.44–1.00,P for trend = 0.05). Inverse associations were apparent for both dietary (RR 0.72, 95% CI 0.46–1.14,P for trend = 0.16) and supplemental (RR 0.66, 95% CI 0.43–1.00,P for trend = 0.03) vitamin D. No individual food item high in vitamin D content and/or calcium was strongly associated with RA risk, but a composite measure of milk products was suggestive of an inverse association with risk of RA (RR 0.66, 95% CI 0.42–1.01,P for trend = 0.06). Conclusion: Greater intake of vitamin D may be associated with a lower risk of RA in older women, although this finding is hypothesis generating.