Women who live in sunnier climates in childhood and young adulthood and who have high summer sun exposure in high UV-B areas may have a reduced risk of developing multiple sclerosis (MS) later in life, according to results from a study published online in Neurology.
“While previous studies have shown that more sun exposure may contribute to a lower risk of MS, our study went further, looking at exposure over a person’s life span,” said study author Helen Tremlett, PhD, associate professor and Canada Research Chair in Neuroepidemiology and Multiple Sclerosis, University of British Columbia, Vancouver, Canada. “We found that where a person lives and the ages at which they are exposed to the sun’s UV-B rays may play important roles in reducing the risk of MS.”
Increased production of vitamin D, caused by sun exposure, is thought to be key to this reduction in risk. Previous studies have found an association between lower levels of vitamin D and an increased risk of MS.
Dr. Tremlett and colleagues included 151 women with MS and 235 age-matched women without MS from the Nurses’ Health Study (98% white; 94% of fair to medium skin), who resided in a variety of locations throughout the United States. Women with MS were an average of 39.5 years old at MS onset. All completed questionnaires about summer, winter, and lifetime sun exposures.
The women were divided into three groups: low, moderate, and high UV-B ray exposure, based on where they lived. Latitude, longitude, and average cloud cover for each location were also considered. Researchers defined high summer sun exposure as over 10 hours/week, and high winter sun exposure as over 4 hours/week.
Women who lived in sunnier climates with the highest UV-B ray exposure had a 45% reduced risk of developing MS across all pre-MS onset age groups compared with those living in areas with the lowest UV-B ray exposure.
Women living in areas with highest UV-B ray exposure between the ages of 5 and 15 years had a 51% reduced risk of MS compared with those living in areas with the lowest UV-B ray exposure.
Among the 151 women with MS, 22% reported high exposure between ages 5 and 15 years, and 41% reported low exposure. Women who spent more time outdoors during the summer months between ages 5 and 15 years in locations where exposure to UV-B rays was the highest had a 55% reduced risk of developing MS compared with those with the lowest exposure.
“Our findings suggest that a higher exposure to the sun’s UV-B rays, higher summer outdoor exposure and lower risk of MS can occur not just in childhood, but into early adulthood as well,” said Dr. Tremlett, adding that their measurements of sun exposure may be useful for future studies.
“In addition, our research showed that those who did develop MS also had reduced sun or outdoor exposure later in life, in both summer and winter which may have health consequences.”
The significant reductions in the risk of developing MS are particularly relevant given results from the National Prevalence Initiative, presented at the European Committee for Treatment and Research In Multiple Sclerosis (ECTRIMS) 2017 annual meeting, which show that the prevalence of MS in the United States is higher than ever.
According to these preliminary results, approximately 1 million Americans have MS. This estimate is based on the work of leading experts, and is nearly twice as large as estimates based on previous research—specifically an updated study from 1975. Of note, these prevalence data need to be confirmed before anticipated publication in 2018.
In 2014, the National Multiple Sclerosis Society launched the National Prevalence Initiative to determine an accurate and economically feasible estimate of MS prevalence in the United States. Data for this initiative come from administrative datasets from the Veteran’s Health Administration, Medicare, Medicaid, and private insurers.
According to the National Multiple Sclerosis Society: “Our increased understanding of the impact of MS on people’s lives, including the individual and collective financial impact of managing MS, along with the health care and services people with MS need, is essential. A scientifically sound prevalence estimate will help researchers understand if MS is increasing, or if there are MS clusters that may hold clues to factors that may trigger MS. Additionally, it will ensure the continued acceleration of crucial research to ultimately find treatments and solutions for each person living with MS.”
The study conducted by Dr. Tremlett and colleagues was supported by the University of British Columbia, Canada, the Canada Research Chair Program, and the National Institutes of Health.