March 23, 2018
Spending time outside in the summer appears to have a stronger association with reduced future risk for multiple sclerosis (MS) rather than time spent with direct sunlight on the skin, a new study suggests.
This finding implies that " the mechanism may not be mediated just by vitamin D," lead author, Helen Tremlett, PhD, University of British Columbia, Vancouver, Canada, commented to Medscape Medical News. "While vitamin D may play a role, our results suggest that other mechanisms may be involved as well. Sunlight can have other immunological effects apart from just boosting vitamin D."
The study also showed that the association between sun exposure and reduced MS risk appears to be present throughout the life course — not just during childhood or adolescence.
"We know sun exposure may alter MS risk, but there are lots of pieces of the puzzle missing. This study adds a little more information," Tremlett said.
"Our data suggest that it is not just childhood exposure to sun that appears to lower the risk but this relationship continues into young adulthood right up to a few years before MS develops."
The study was published online in Neurology on March 7.
The researchers analyzed data from the US Nurses' Health Study and compared women with new-onset MS (cases) with age-matched controls.
They sent all cases and controls a questionnaire about their sun and outdoor exposure over their life course, including time spent outdoors in summer and winter, time spent in direct sunlight (in "minimal clothing"), sunburn episodes, use of sunscreen, and place of main residence. Information on natural skin and hair color and tendency to tan or burn as a child or adolescent was also collected.
They used a novel measure of sun exposure, ultraviolet (UV) flux (the amount of UV penetration to earth), which is calculated by combining information on latitude, altitude, and cloud cover available from weather station records.
Results were adjusted for body mass index at age 18 years (to account for differences in general health and activity levels associated with being outside more), smoking status (ever vs never), and supplemental intake of vitamin D (from baseline dietary questionnaires).
Responses were received from 90% of those contacted, giving information on 151 women with MS and 235 matched controls from all across the United States. Most participants were white (98%); the mean age at MS onset was 39.5 years.
Results showed that living in an area with higher ambient UVB levels (top tertile) was associated with a 36% to 45% reduced risk for MS across all age groups combined compared with those with the lowest tertile UVB exposure.
For specific age group exposure, increased ambient UVB levels at ages 5 to 15 and 31 to 40 years were associated with statistically significant 51% and 65% reduced risks for MS, respectively. Although significance was not reached for the remaining age groups after full model adjustment, a similar direction of effect was observed.
The relationship between time spent outdoors during childhood and MS risk became apparent only when considered in the context of an individual's ambient UVB environment; a 55% lower risk for MS was associated with more time spent outdoors in summer when residing in areas of high ambient UVB.
The researchers also report reduced outdoor exposure in both summer and winter for individuals once they developed MS, which they say "may have a negative effect on MS progression and disease activity as well as having broader health implications."
This study was funded by the Martha Piper Research Fund, University of British Columbia, Canada, and supported by the National Institutes of Health.