Abstract
Exposure to the sun may protect against multiple sclerosis Br Med J 2003;327:334). In France, geographic distributions of multiple sclerosis and Crohn disease(CD)are similar (IBD2006;12:218 and J Neurol Neurosurg Psychiatry 2007;78:707). Aim: to determine if geographic distribution of CD and UC is influenced by sun exposure. Methods: This study included patients to whom IBD reimbursement was attributed in France, between 2000 and 2002. Data provided relate to age, sex, postcode area of residence and IBD type. Ground UV from sunshine data was expressed in kJ/m2 and was provided by month and by French “departement” for the years 1984 to 2002. We calculated a mean monthly UV dose for the studied period, which was classified into quintiles, and linked with incidence data tabulated by departement. Age and sex-adjusted incidence rate ratios (IRR), with their 95% confidence intervals, were estimated per quintile of UV dose by using the Poisson regression. A spatial analysis, with Bayesian approach was then performed. Results: 14,213 new patients with CD (8,172 women) and 12,452 with UC (5,984 women) were registered by the 5 health insurance funds. There was a negative correlation between age and gender-adjusted CD incidence rates, and local UV dose (IRR= 1.42, 95%CI [1.27 1.58] for the 1st quintile, IRR = 1.12, 95%CI [1.00 1.26], for the 2nd quintile, IRR = 0.90, 95%CI [0.78 1.04] for the 3rd quintile, IRR = 0.85, 95%CI [0.74 0.98], for the 4th quintile, (5th quintile = reference). Local UV dose improved the Bayesian model of geographic distribution of CD and its effect was stronger than local spatial autocorrelation. As shown in the figure, areas with a smoothed relative risk (sRR)of CD significantly higher than 1 corresponded to departments with low sunlight exposure, whereas those with high sunlight exposure had sRR either lower than 1 or not significantly different from 1. Conclusion. Low sunlight exposure could be a risk factor for CD.
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