Inflammatory bowel disease rates lower in sunnier regions
MedWire News: Women living in southern regions of the USA have a significantly lower risk for Crohn's disease (CD) and ulcerative colitis (UC) than those living in northern regions, study findings indicates.
"A leading explanation for this 'north-south' gradient in the risk of UC and CD may be differences in exposure to sunlight or UVB [ultraviolet B] radiation, which is generally greater in southern latitudes," write Andrew Chan (Massachusetts General Hospital, Boston, USA) and colleagues in Gut.
The researchers explain that the pathogenesis of CD and UC is largely unknown, but it is thought that environmental and lifestyle factors must play an important role because the genetic risk has been shown to be low.
Furthermore, European studies have linked latitude and geographic variations to the risk for inflammatory bowel disease.
To investigate whether a similar association is present in the more genetically diverse US population, Chan and team studied data for women who enrolled in the Nurses' Health Study I and II in 1976 and 1989, respectively.
Information on state of residence at the time of birth, at age 15 years, and age 30 years was collected in 1992 in Nurses' Health Study I and in 1993 in NHS II. Reported diagnoses of incident CD or UC to the end of 2003 were confirmed by medical record review.
Complete residence data were available for 175,912 women. Among these women, there were 257 cases of CD and 313 cases of UC documented during 3,428,376 person-years of follow-up.
The researchers found that the incidence of CD and UC decreased significantly with decreasing latitude. Residence at age 30 years more strongly associated with risk than residence at birth or at 15 years of age.
Indeed, women residing in southern latitudes (<37° latitude) at 30 years of age had a significant 53% lower risk for subsequently being diagnosed with CD and a significant 38% lower risk for a subsequent UC diagnosis, compared with women residing in northern latitudes (>41-42° latitude).
Residing in southern latitudes at birth was not associated with a reduction in the risk for CD (hazard ratio [HR]=1.02), but was associated with a nonsignificant reduction in the risk for UC (HR=0.69).
Similarly, southern residence at 15 years of age was associated with nonsignificant reductions in the risk for CD (HR=0.67) and UC (HR=0.86) compared with northern residence.
The risk for CD and UC among women living in middle latitude regions (37-41° latitude) did not differ significantly from that among women living in northern regions.
The researchers note that the results were consistent even after accounting for differences in self reported ancestry and smoking, "suggesting that other environmental or lifestyle factors correlated with geographical variation may mediate these associations."
Exposure to UV radiation may be one such factor as it is "the greatest environmental determinant of plasma vitamin D, and there is substantial experimental data supporting a role for vitamin D in the innate immunity and regulation of inflammatory response," Chan et al remark.
"Alternatively, differences in exposure to ambient air pollutants according to latitude may account for geographical variation in the risk of UC and CD," they add.
The researchers conclude that further studies are required to determine the underlying lifestyle and environmental factors that mediate the association between inflammatory bowel disease and geographic location.
By Laura Cowen