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Indirect evidence that ultraviolet-B radiation mitigates multiple sclerosis in the United States
Journal of Photochemistry and Photobiology B: Biology, October 2015, Pages 46 - 47
- Ultraviolet-B is inversely proportional to those dying with multiple sclerosis (MS).
- Regression to approximately zero persons with MS occurs at the Tropic of Cancer (∼23 degrees N).
- Ultraviolet-B mitigates the incidence of MS.
This article describes on the relationship of the relative prevalence of persons dying with multiple sclerosis with the latitude of the population centroid of those affected in each of the United States. Regression to zero prevalence occurs at the Tropic of Cancer, the latitude where the Sun is at zenith on the summer solstice and where ultraviolet radiation (UVR) is least attenuated. This observation supports UVR as a mitigating force in multiple sclerosis.
Keywords: Multiple sclerosis, Ultraviolet radiation, Population centroid, Tropic of Cancer.
Several publications are now available showing an inverse relationship of the prevalence of multiple sclerosis (MS) and the intensity of UVR, particularly at the UV-B wavelength of 280–320 nm  . Most work has involved the prevalence of MS in countries of varying latitude, but some studies have involved regional areas in a single country  and . There is now growing consensus that UV-B is protective either through an immunosuppressive effect or through the production of vitamin D, or possibly both mechanisms  .
This report collects the relative prevalence of MS by collecting 56,257 death records from the National Center for Health Statistics of persons who were born and died in one of the United States who had the diagnosis of MS on their death certificates. Although persons do not die specifically of MS, they do die from associated co-morbid conditions like pressure sores with sepsis, urinary tract infections, or pneumonia as terminal events  . Those who moved away from their state of birth were not included in the analysis. We assumed that the number of deaths was proportional to the prevalence of MS and that the symptoms of MS were prominent enough to be included on the death certificates. We were then able to calculate the percent of persons afflicted with MS for the population of each state based upon the Year 2000 U.S. Census.
By searching the Internet, we recorded the latitude of the 50 most populous cities or towns in each of the 50 United States and by using the relative percentage of the population of each city, calculated a weighted average of latitude and therefore created the latitude for the population centroid for each state. Latitude of these centroids was then plotted against the number of those dying with MS in each state, divided by the population of each state obtained from the 2008 U.S. Census and further normalized all the states relative to New Hampshire (e.g., 247 MS deaths in NH/1,315,809 total population = 0.00019 = ∼0.02%) lying at the approximate overall average latitude centroid (43.2 degrees) as seen in Fig. 1 . The Y-axis shows the relative prevalence of MS, the value of unity being New Hampshire (and also Kentucky); the X-axis shows the latitude centroid of each state.
Alaska had too few persons with MS born in that state and was below two standard deviations (and below zero on the Y-axis) and not included in the analysis. Similarly, North Dakota and South Dakota were high outliers (above three standard deviations) and were also excluded for a better R2 on the regression; R = 0.46, and the regression to zero on the Y-axis occurred at 23.1 degrees latitude, a value very close to the value of the Tropic of Cancer (23.4 degrees as of 2014), the latitude where the Sun is directly overhead on the summer solstice and where UV-B penetrates most effectively through the atmosphere. Supportive of our findings are recent reports that several vitamin D receptors polymorphisms appear to change with latitude  . In addition, various autoimmune diseases, including MS, are associated with season of birth and therefore to gestational UV-B exposure  .
This indirect evidence supports UV-B, which is most attenuated at increasing latitude, as the main wavelength of electromagnetic radiation that modulates the prevalence of MS.
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