August 11, 2009

The latitude gradient of MS risk (higher risk toward the poles, lower risk toward the equator) has been reported in a number of regions including Australia, Europe, and North America. The prevailing theory about why this gradient exists is that greater exposure to ultraviolet rays from the sun is protective against MS, perhaps through stimulating internal production of vitamin D which modulates the immune system. This connection between latitude, MS risk, and UV exposure still has yet to be fully validated though.

A new study in North America takes this validation process a step further by comparing MS risk statistics for different regions with the UV index (UVI) for these regions. UVI is influenced by a number of factors including elevation, ozone, and cloudiness, so it's not solely a matter of latitude. Since there is no comprehensive geographic prevalence study for North America (or even just for the US, for that matter), data on MS risk was compiled from a number of regional studies as well as other sources such as an analysis of MS prevalence in US veterans.

The investigators compared the UVI data with the MS prevalence data, and found significant correlations between the two. They also demonstrated that the risk of MS in the lowest UVI states (Washington, Oregon, Vermont, North Dakota, and Minnesota) was 80% to 440% greater than that of the highest UVI states (Florida, Arizona, New Mexico, Louisiana, and Alabama). A similar negative correlation between UVI and MS prevalence was also seen in an analysis of regions across the US and Canada.

While this study doesn't confirm the link between UV radiation and MS risk, it does help to support it. It also highlights the need for a nationwide MS prevalence study or perhaps a registry of some type, which would greatly facilitate this type of study in the future and prevent scientists from having to cobble together multiple types of data from different sources.