Greater exposure to sunlight during the winter months — part of a person’s lifetime exposure to ultraviolet radiation — can help to lower the risk of multiple sclerosis (MS), a large U.S. cohort study suggests.
Increasing evidence suggests that poor exposure to ultraviolet radiation from sunlight, especially during childhood, is a potential risk factor for MS.
The mechanisms underlying this association are thought to be linked to the impact of the sun’s ultraviolet radiation on the immune system, or in the synthesis of vitamin D — whose deficiency is suggested to increase the risk of MS, although this connection was recently challenged.
“There is epidemiological evidence suggesting that low exposure to ambient UVR [ultraviolet radiation] during early life may be associated with MS and earlier symptom onset,” the researchers wrote.
To further investigate the link between ultraviolet radiation and MS, researchers analyzed people taking part in the U.S. Radiologic Technologists (USRT) study, a decades-long investigation into the health effects of potential low doses of radiation experienced by radiologic technologists.
A total of 39,801 participants also completed a series of questionnaires about places of residence and sun exposure — specifically, time spent outdoors on weekends and weekdays, history of sunburns and skin sensitivity to sunlight — and general health and lifestyle questions. The study also analyzed those with MS, including year of diagnosis.
Among participants, there were 569 self-reported MS cases, with medical records available for 203 of them. A review by MS-specialized neurologists confirmed an MS diagnosis in 148 cases.
The study population was predominately female (more than 90%), with a mean age of 44 at MS diagnosis.
Researchers estimated the participants’ exposure to ultraviolet radiation by crossing residency information with satellite data from NASA’s Total Ozone Mapping Spectrometer (TOMS) project, launched in 1996 to measure ozone levels.
Results showed a trend for an increased risk of MS with lesser exposure to the sun’s ultraviolet radiation during winter months, but no effects were detected during summer months. The effects were consistent across groups younger than age 40.
While exposure to sunlight in summer is most likely sufficient for vitamin D production regardless of place of residency, areas where the sunlight is significantly poor during the winter poses a risk for vitamin D deficiency.
“Vitamin D targets nervous system tissues, regulating important neurotrophic factors in the brain, and also exerts effects on the differentiation and functioning of immune cells,” the researchers wrote.
Researchers also highlighted the potential effect of the ultraviolet radiation on the immune system independent of its role in vitamin D production.
“Our findings are consistent with the hypothesis that UVR [ultraviolet radiation] exposure reduces MS risk and may ultimately suggest prevention strategies,” the study concluded.