Study Finds Early Sun Exposure Associated
with Reduced Risk of MS
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Medical Update Memo
August 8, 2007
Summary
Researchers from Duke University 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 of Neurology (2007;69:381-388).
This study was funded by the MS Society of Canada
and the National Institutes of Health.
Details
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 US 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 US 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.
ASK MS Information System Code: 2.3.2.1.a
Disponible en français.
Disclaimer
The Multiple Sclerosis Society of Canada is an independent, voluntary health
agency and does not approve, endorse or recommend any specific product or
therapy, but provides information to assist individuals in making their own
decisions.
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