Latitude Has More Significant Impact on Prevalence of Multiple Sclerosis Than Ultraviolet Level or Sunshine Duration in Japanese Population

Higher latitude is known to be associated with higher prevalence of multiple sclerosis (MS). We investigated the degree of impact of latitude, ultraviolet (UV) radiation, and sunshine on the prevalence of MS in Japan, which has 47 prefectures with a variety of climates. MS prevalence in each prefecture was collected from database of the Ministry of Health, Labour, and Welfare of Japan. Latitude of each prefecture was represented by that of the capital city. Data of UV radiation level and annual actual sunshine duration were obtained from databases of Japan Meteorological Agency. We performed linear correlation analyses of MS prevalence against latitude, UV radiation, and annual actual sunshine duration. MS prevalence significantly correlated to latitude (Pearson's correlation, r = 0.69, p < 0.001) and UV radiation level (r = -0.65, p < 0.001) but not to annual actual sunshine duration (r = -0.37, p = 0.011). Stepwise multiple linear regression analyses revealed significant correlation between MS prevalence and only latitude (p < 0.001). While our result shows that both latitude and the UV intensity have significant relationship to MS prevalence, the stronger relevance of the former suggests an existence of risk factors other than UV radiation.