Background: A relationship between the latituderelated distribution of multiple sclerosis (MS) and exposure to sunlight has long been considered. Higher sun exposure during early life has been associated with decreased risk of MS.

Objective: Since Norway is an exception to the latitude gradient of MS prevalence, we tested here whether sunlight exposure or vitamin D-related dietary factors in childhood and adolescence are associated with the risk of MS.

Methods: Retrospective recall questionnaire data from 152 MS patients and 402 population controls born at and living at latitudes 66–71°N were analysed by means of conditional logistic regression analysis accounting for the matching variables age, sex, and place of birth.

Results: Increased outdoor activities during summer in early life were associated with a decreased risk of MS, most pronounced at ages 16–20 years (odds ratio (OR) 0.55, 95% CI 0.39–0.78, p = 0.001, adjusted for intake of fish and cod-liver oil). A protective effect of supplementation with cod-liver oil was suggested in the subgroup that reported low summer outdoor activities (OR 0.57, 95% CI 0.31–1.05, p = 0.072). Consumption of fish three or more times a week was also associated with reduced risk of MS (OR 0.55, 95% CI 0.33–0.93, p = 0.024).

Conclusion: Summer outdoor activities in childhood and adolescence are associated with a reduced risk of MS even north of the Arctic Circle. Supplemental cod-liver oil may be protective when sun exposure is less, suggesting that both climate and diet may interact to influence MS risk at a population level.