Northern Latitude but Not Season Is Associated with Increased Rates of Hospitalizations Related to Inflammatory Bowel Disease: Results of a Multi-Year Analysis of a National Cohort

Background and Aims
There is growing evidence that the incidence and severity of inflammatory bowel disease (IBD) may be geographically and seasonally related. Why these associations are observed remains unclear. We assessed the impact of geographic location, season, and exposure to ultraviolet light on disease severity by measuring national hospital IBD-related discharge rates.
Methods
Utilizing the Nationwide Inpatient Sample (NIS), we identified all patients with IBD-related discharges from 2001–2007. Patients were included if they were discharged from states above the 40th parallel (north) or at or below the 35th parallel (south); and their discharge fell within the winter (January, February, and March) or summer (July, August, and September). Groups of patients were assessed comparing north to south within each season, and summer to winter within each region. UV index was recorded from the National Weather Service data and compared to monthly discharge rates.
Results
There was a consistent pattern of increased IBD-related hospitalization rates in northern states compared to southern states for both ulcerative colitis and Crohn’s disease. Differences in IBD-related hospitalization rates by season, however, were not uniform across the years studied. UV index was significantly inversely associated although not proportional to discharge rates for both Crohn’s disease and ulcerative colitis.
Conclusions
In the US, there is a significant increased rate of IBD-related hospitalizations in the northern compared to southern states, which not fully explained by differences in UV exposure.