Low exposure to sunlight is a risk factor for Crohn's disease

Low sunshine exposure might contribute to the pathogenesis of inflammatory bowel disease (IBD).

To assess the geographic distribution of IBD incidence in relation to sunshine exposure in France to test the hypothesis that higher sun exposure is associated with lower IBD risk.

Using the national health insurance database, incidence rates of Crohn's disease (CD) and ulcerative colitis (UC) were estimated for each of the 94 French administrative areas ('départements'), between 2000 and 2002. The surface UV radiation intensity was obtained by combining modelling and satellite data from Meteosat, the European meteorological satellite. Relationships between incidence rates and sun exposure were tested for significance by using a Poisson regression. We mapped smoothed relative risks (sRR) for CD and UC, using a Bayesian approach and adjusting for sun exposure, to search for geographical variations.

Areas with a smoothed RR of CD incidence significantly above 1 corresponded to areas with low sunshine exposure, whereas those with high or medium sunlight exposure had smoothed RRs either lower than 1 or not significantly different from 1. There was no association between sun exposure and UC incidence.

This geographic study suggests that low sunlight exposure is associated with an increased incidence of Crohn's disease. Further studies are needed to determine if this association is causal.