High residential sun exposure is associated with a low risk of incident Crohn's disease in the prospective E3N cohort.

Background: Vitamin D insufficiency has been suggested to be associated with high risk of Crohn's disease (CD). In France, where food fortification is limited, the major source of vitamin D is through sun exposure. The aim of this work was to analyze the relationship between residential sun exposure and the risk of incident CD or ulcerative colitis (UC).
Methods: The E3N cohort consists of women living in France, aged 40 to 65 years and free of major diseases at inclusion in 1990. Among the 91,870 women included in the study, we identified 123 incident cases (45 CD, 71 UC, and 7 indeterminate colitis). To assess residential sun exposure, we used a database containing mean daily ultraviolet radiation (UVR) dose for each French county. The relationship between residential sun exposure and risk of incident inflammatory bowel diseases was explored using Cox models.
Results: Higher levels of residential sun exposure were associated with a significant decreased risk of CD (hazard ratio [HR] for the third versus the first tertile of UVR dose, 0.49; 95% confidence interval (CI), 0.23-1.01; P for trend = 0.04), but not of UC (HR, 1.21; CI, 0.61-2.11). In women with available data on dietary vitamin D intake, we observed a lower risk of CD with higher residential UVR (HR, 0.29; 95% CI, 0.11-0.80; P for trend = 0.01). Dietary vitamin D intake was neither associated with the risk of CD (HR, 0.41; 95% CI, 0.14-1.24; P for trend = 0.14) nor UC (HR, 1.61; CI, 0.61-4.23).
Conclusions: In this prospective cohort of women, high residential sunlight exposure was associated with decreased incidence of CD, but not UC.