Sunlight exposure and multiple sclerosis in a tropical country

We analysed past and current sun exposure in multiple sclerosis (MS) patients as compared with matched controls in Mexico, a country with tropical climate.

In a case-controlled study that include 83 MS patients and 166 matched controls, we inquired about sunlight exposure in two different periods: during adolescence and during the immediate past 5 years. Indicators were: exposure on quotidian and weekend outdoor activities with direct sunlight contact as expressed on frequency by mean number of days, daytime (morning, noon, afternoon), number of hours, visits to sunny places, and use of sunblocking agents. Additional elements were socioeconomic status, skin colour, and antecedent of varicella infection during childhood.

MS patients showed a larger proportion of white skin. MS patients had more sunlight exposure during adolescence (80% versus 60%, P = 0·002); this tendency prevailed on current indicators (46% versus 30%, P = 0·02). However, current exposure on weekends (10% versus 22%, P = 0·02) and visits to the beach (64% versus 98%, P = 0·002) were lower in MS than in controls.

Mexico gets more sunlight through the year than areas with high incidence of MS; nevertheless, its prevalence has greatly increased over the last decades, making it a relevant emerging disease. Our results indicate that in a tropical country, there is no association between sunlight exposure and the risk to develop MS, given the immunological effects of sunlight exposure either through UV radiation or vitamin D metabolism.