Past exposure to sun, skin phenotype, and risk of multiple sclerosis: case-control study

Objective: To examine whether past high sun exposure is associated with a reduced risk of multiple sclerosis. Design: Population based case-control study. Setting: Tasmania, latitudes 41-3°S. Participants: 136 cases with multiple sclerosis and 272 controls randomly drawn from the community and matched on sex and year of birth. Main outcome measure: Multiple sclerosis defined by both clinical and magnetic resonance imaging criteria. Results: Higher sun exposure when aged 6-15 years (average 2-3 hours or more a day in summer during weekends and holidays) was associated with a decreased risk of multiple sclerosis (adjusted odds ratio 0.31, 95% confidence interval 0.16 to 0.59). Higher exposure in winter seemed more important than higher exposure in summer. Greater actinic damage was also independently associated with a decreased risk of multiple sclerosis (0.32, 0.11 to 0.88 for grades 4-6 disease). A dose-response relation was observed between multiple sclerosis and decreasing sun exposure when aged 6-15 years and with actinic damage. Conclusion: Higher sun exposure during childhood and early adolescence is associated with a reduced risk of multiple sclerosis. Insufficient ultraviolet radiation may therefore influence the development of multiple sclerosis.