A recent study published in PLOS One found that northern states had increased rates of inflammatory bowel disease hospitalizations compared to southern states. However, the researchers also discovered that season was not associated with inflammatory bowel disease hospitalization rates.
Inflammatory bowel disease (IBD) is characterized by chronic inflammation of all or part of the digestive tract. IBD includes ulcerative colitis (UC) and Crohn’s disease (CD); both of which normally cause severe diarrhea, fatigue, weight loss and pain.
Studies have consistently shown a link between low vitamin D status and IBD. Although, there has been conflicting epidemiological studies showing that both higher and lower sun exposure is linked to an increased risk of IBD. This led researchers to conduct a large epidemiological study to assess the impact of latitude, season and UV exposure on IBD hospitalizations in the United States.
The researchers used National Inpatient Sample (NIS), the largest database of hospital inpatient stays in the U.S., to retrieve discharge rates from 2001-2007. Hospital discharge rates measure the number of patients who leave a hospital after receiving care; therefore, a lower discharge rate indicates that fewer patients went to the hospital.
To assess UV light exposure, they used the daily UV index forecast from the National Weather Service Climate Prediction Center from 2001-2004. Lastly, the states were classified as northern or southern based on the latitude; states above the 40th parallel and states below the 35th parallel were considered northern and southern, respectively.
The researchers identified 76,600 discharges of patients with UC and 143,495 with CD within the northern and southern states.
Here is what the researchers found:
- The average discharge rates of both UC and CD patients were significantly lower in the southern states compared to the northern states during winter and summer seasons (p <05).
- Residents of southern states were more likely to have longer hospital stays and more likely to undergo surgery than those in northern states.
- The discharge rates were not significantly lower in the summer compared to other seasons in the southern and northern states. However, 2007 was an exception. There were significantly fewer discharges during summer in both regions (p <05).
- A higher UV index was significantly associated with lower discharge rates when examining UC and CD patients together and separately (p < 0.001, p < 0.001, p = 0.018).
The researchers concluded,
“Although IBD admissions were higher in northern latitudes and an inverse association between UV index and discharge rates was observed, we failed to find consistent differences in hospitalization rates by season when controlling for latitude.”
They went on to state,
“There was one exception, with 2007 showing a protective effect during summer for both regions as well as both UC and CD.”
The researchers proposed an explanation for these puzzling results. They stated that it was possible that the protective effect of summer during 2007 was a result of more robust data, since 2007 included the most states for analysis. The researchers also propose that other environmental factors besides sunlight and UV exposure may be responsible for the geographic differences in IBD disease severity.
Tovey, A. & Cannell, JJ. Northern latitudes but not season linked to increased rates of inflammatory bowel disease hospitalizations. The Vitamin D Council Blog & Newsletter, 2016.
Stein A. et al. 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. PlOS One, 2016.