The findings, which back up previous European research, could eventually lead to new types of treatment as well as preventive measures, suggest the authors.
Inflammatory bowel disease (IBD) includes Crohn’s disease and ulcerative colitis. Both conditions can be extremely painful and require extensive surgery, often severely affecting quality of life.
Despite recent advances in identifying genetic factors that might be involved in the development of IBD, the causes still remain largely unknown, and because the overall genetic risk is low to moderate, environmental and lifestyle factors are likely to have a key role, say the authors.
They base their findings on data obtained from two studies tracking the long term health of nurses, one of which started in 1976 (Nurses’ Health Study I), and the other of which began in 1989 (Nurses’ Health Study II).
Information on area of residence at birth and at ages 15 and 30 was collected for both studies, and any diagnosis of IBD recorded up to 2003.
The 238,000 participants, who were enrolled between the ages of 25 and 55, were mailed every two years to update their health information. None had any history of IBD when they enrolled in the studies.
States in the US were divided up into northern, middle, and southern tiers of latitude for each of the four time zones (eastern, central, mountain, and pacific).
The residence of just under 176,000 women was recorded in 1992, subsequent to which 257 cases of Crohn’s disease and 313 of ulcerative colitis were diagnosed by 2003.
A diagnosis of IBD was significantly associated with increasing northerly latitude, with residence by the time a woman was 30 more strongly linked to the development of the condition.
Compared with women residing in northern latitudes, those living in southerly climes were 52% less likely to develop Crohn’s disease by the age of 30 and 38% less likely to develop ulcerative colitis.
Smoking, which is thought to affect IBD risk, had no influence on the findings.
"A leading explanation for this north-south gradient in the risk of ulcerative colitis and Crohn’s disease may be differences in exposure to sunlight or UVB radiation, which is generally greater in southern latitudes," comment the authors.
"UV radiation 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," they continue.
They add that their results lend credence to the importance of biological pathways in influencing geographical differences in the development of IBD.
"Understanding such pathways could eventually lead to the development of novel lines of therapy as well as interventions that may modulate the risk of incident disease," they conclude.
