Leisure-time physical activity, sun exposure, and multiple sclerosis risk: a population-based case-control study.

BACKGROUND: Physical activity benefits symptoms and function in people with multiple sclerosis (MS), but its relevance to MS susceptibility remains uncertain and may be difficult to separate from correlated lifestyle and environmental factors, including sun exposure. METHODS: This Swedish population-based case-control study included 3,008 incident MS cases and 5773 matched controls. Analyses were restricted to participants whose MS onset, or corresponding reference year among controls, occurred within 5 years before study inclusion. Leisure-time physical activity 5 years before inclusion was classified into four levels. Odds ratios (ORs) with 95% confidence intervals (CIs) were estimated using logistic regression overall and by sun exposure. Time spent outdoors was additionally considered in a subgroup. RESULTS: In models adjusted for age, sex, and residential area, the highest versus lowest level of physical activity was associated with lower MS risk (OR 0.78, 95% CI 0.67-0.92), but the association was attenuated after multivariable adjustment (OR 0.94, 95% CI 0.80-1.11). High physical activity was associated with lower risk among individuals with low sun exposure (adjusted OR 0.76, 95% CI 0.58-0.99), whereas no association was observed among those with high sun exposure (adjusted OR 0.97, 95% CI 0.78-1.20). In the subgroup, additional adjustment for outdoor activity had little influence on the estimates. CONCLUSION: The overall association between leisure-time physical activity and MS risk was attenuated after adjustment for lifestyle and environmental factors. Inverse associations among individuals with low sun exposure and in analyses accounting for outdoor activity suggest a possible context-dependent association.