Sun exposure over the life course and associations with multiple sclerosis

Abstract

ObjectiveTo examine sun exposure and multiple sclerosis (MS) over the life course (ages 5–15 and 16–20 years, every 10 years thereafter).MethodsCases with MS (n = 151) and age-matched controls (n = 235) from the Nurses' Health Study cohorts completed summer, winter, and lifetime sun exposure history questionnaires. Cumulative ambient ultraviolet (UV)-B (based on latitude, altitude, cloud cover) exposure before MS onset was expressed as tertiles. Seasonal sun exposure was defined as low vs high hours per week (summer [≤9 vs >10 h/wk]; winter [≤3 vs >4 h/wk]). Relative risks (RRs) and 95% confidence intervals (CIs) were estimated via conditional logistic regression with adjustment for body mass index, ancestry, smoking, and vitamin D supplementation.ResultsMost participants were white (98%); the mean age at MS onset was 39.5 years. Living in high (vs low) UV-B areas before MS onset was associated with a 45% lower MS risk (adjusted RR 0.55, 95% CI 0.42–0.73). Similar reduced risks (51%–52%) for medium or high exposure were observed at ages 5 to 15 years and at 5 to 15 years before MS onset (adjusted p < 0.05). At age 5 to 15 years, living in a high (vs low) UV-B area and having high (vs low) summer sun exposure were associated with a lower MS risk (RR 0.45, 95% CI 0.21–0.96).ConclusionLiving in high ambient UV-B areas during childhood and the years leading up to MS onset was associated with a lower MS risk. High summer sun exposure in high ambient UV-B areas was also associated with a reduced risk.