Is the north to south gradient in inflammatory bowel disease a global phenomenon?

Inflammatory bowel diseases (IBDs), such as Crohn's disease and ulcerative colitis, are multifactorial diseases that result from an overly aggressive immune response towards an environmental trigger in a genetically susceptible host. Much has been learned about susceptibility genes; however, environmental triggers are still largely unknown. In the 1990s, a large study from Europe suggested a north-south gradient regarding the incidence of IBD; however, this was never shown convincingly for the USA. In the present study, the authors used and extended data from the Nurses' Health Study I and II and were able to demonstrate that indeed the risk of developing IBD is significantly lower for the southern latitudes of the USA and did not depend on ancestry or lifestyle factors such as smoking. The authors speculate that an increased exposure to sunlight provides protection from IBD via the vitamin D pathway.