Geographic Variations and Risk of Crohnʼs Disease and Ulcerative Colitis: Results from Two Large Prospective Cohorts: ACG IBD Award

Abstract

Purpose: Geographic variation according to latitude of residence in the incidence of Crohn's disease and ulcerative colitis has been reported in Europe. However, comparable data in the U.S. are lacking. We assessed the geographic variation in risk of Crohn's disease and ulcerative colitis in a geographically diverse population of U.S. women enrolled in two large ongoing prospective studies. Methods: Our study population included female health professionals enrolled in the U.S. Nurses Health Study I and II (NHS) who were followed from 1976 to 2002 and 1989 to 2003, respectively. Information about geographic location at the time of birth, age 15, and age 30 were asked in the 1992 questionnaire in the NHS I and in the 1993 questionnaire in the NHS II. Incident cases of Crohn's disease and ulcerative colitis were confirmed by review of medical records by two gastroenterologists. Baseline data for other covariates were obtained from the initial questionnaire and updated biennially. We used Cox proportional hazards models to calculate hazard ratios (HR) and 95% confidence intervals (CIs) for risk of Crohn's disease and ulcerative colitis. Results: Over 4,209,454 person-year follow-up, we confirmed 284 cases of Crohn's disease and 332 cases of ulcerative colitis. The mean age of diagnosis was 51 for CD and 48 for UC. The incidence of Crohn's disease and ulcerative colitis increased significantly with increasing latitude (Ptrend < 0.01). In particular, geographic location at age 30 was more strongly associated with incidence of disease. Based on geographic location at age 30, compared to women in northern latitudes, women in southern latitudes had a multivariateadjusted HR of 0.51 (95% CI, 0.32-0.82) for Crohn's disease and 0.65 (95% CI, 0.45-0.94) for ulcerative colitis. Conclusion: In two large prospective cohorts, we showed a significant northsouth gradient in the association between latitude and incidence of Crohn's disease and ulcerative colitis in the United States. This differential risk may be explained by differences in UV light exposure and vitamin D status.