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July 24, 2007
Researchers report that sun exposure during childhood was associated with a reduced risk of MS in a study of 79 pairs of twins in which one twin had MS. This study adds to a growing body of evidence suggesting that sun exposure may be protective against MS. Talat Islam, MBBS, PhD, Thomas Mack, MD, MPH (Keck School of Medicine, Los Angeles) and colleagues report their findings in the July 24 issue ofNeurology(2007;69:381-388). This study was funded by the MS Society of Canada and the National Institutes of Health.
MS is thought to occur in people who are genetically susceptible to the disease who encounter something in their environment that triggers its onset. Even in identical twins with the same genes, only about 30% of the time both twins develop MS. Worldwide, MS tends to occur with greater frequency in latitudes farther away from the equator than in latitudes closer to the equator. One possibility that has been explored in this and other studies is that exposure to ultraviolet radiation (UVR) from the sun may protect against MS. Recent research indicates that UVR (or vitamin D synthesized via UVR exposure) can dampen the immune attack. This might provide a biological mechanism for reduced MS where UVR exposure is higher.
Dr. Islam and colleagues investigated this possibility using the International Twin Study, a registry of North American Twins who have been diagnosed with chronic diseases, including MS. Studying genetically identical twin pairs provides the unique opportunity of separating genetic and environmental factors that might contribute to MS.
The team identified 292 pairs of monozygotic (identical) twins of whom one had MS. They sent these twins a 60-page questionnaire on factors that might affect MS development, including questions about nine sun exposure-related activities, such as more time outdoors, more time spent at the beach, and more time sunbathing. Of this group, 193 pairs responded, and of these, 79 pairs reported differences in sun exposure during childhood on at least one of the nine different sun-exposure-related activities.
In this cohort of 79 pairs, twins who reported more sun exposure-related activities during childhood had a significantly reduced risk of developing MS. This decrease ranged from 43% to 75%, depending on the activity.
“Although this study relies on the recollection of adults about their childhood activities several decades previously, the data were collected long before the amount of sun exposure was considered to represent a risk factor in MS. These findings mesh with other reports and adds to a growing body of evidence suggesting a link between sunlight exposure and resistance to developing MS,” states Dr. John R. Richert, Executive Vice President of Research and Clinical Programs at the National MS Society.
The National MS Society is funding further research into this possible environmental contribution to MS development. For example, Anthony J. McMichael, MBBS, PhD (The Australian National University, Canberra) is comparing lifetime sun exposure in 570 people who are at high risk for MS and 876 people without MS in various communities in Australia (where the north-south latitude impact on MS prevalence is marked). In another Society-funded study, Colleen Hayes, PhD (University of Wisconsin-Madison) is studying the possible protective mechanisms of calcitriol, a chemical that is a product of vitamin D synthesis.
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