FRIDAY, July 27, 2007 (HealthDay News) -- A new study of identical twins suggests that children who spend more time in the sun have a lower risk for developing multiple sclerosis (MS) as adults.
"Evidence is building up that something in relation to sunlight and/or vitamin D exposure during childhood may play a protective role," said study co-author Dr. Thomas M. Mack, of the department of preventive medicine, Keck School of Medicine, University of Southern California, Los Angeles. "It's now been suggested by several different studies that this is the case, and if it's true, it would be important."
The study is published in the July 24 issue of Neurology.
The findings echo those of a recent Harvard School of Public Health study, released in December and published in the Journal of the American Medical Association. That study found that among 140 white men and women, those with the highest levels of sunlight-derived vitamin D were 62 percent less likely to have developed MS than those with the lowest levels. The finding was not replicated in a smaller patient pool of either blacks or Hispanics, however.
According to the National Multiple Sclerosis Society (NMSS), MS is an autoimmune disease of the central nervous system that currently affects more than 400,000 Americans. More than 2.5 million men and women worldwide suffer from the disorder.
While it is unclear what causes MS, the often-crippling disease is thought to develop when the body's own immune system begins attacking a fat and protein-laden substance called myelin that insulates nerve fibers.
Numbness, tingling, loss of coordination and balance, blindness, fatigue, and even paralysis can ensue, as normal communications between brain and body progressively collapse.
The majority of MS patients are first diagnosed between the ages of 20 and 50, and female patients outnumber males two-to-one.
In the study, Mack's team assessed the sun exposure of 79 pairs of identical twins in the United States and Canada, in which at least one twin in each pair had been diagnosed with MS.
Most of the twins were girls, and among those with MS, most had been diagnosed between the ages of 20 and 40.
Each subject was asked about his or her childhood history of outdoor activity, as well as that of the twin.
Time spent tanning, going to the beach, and playing team sports during childhood was also noted. No absolute sun exposure measurements were recorded. Rather, the authors assessed relative degrees of sun exposure between twins, based on personal recall.
All participants were also asked to reveal any history of childhood infections as well as smoking habits.
The result: The twin with MS usually had been exposed to less sun overall as a child than the twin without the disease, the researchers found.
They observed, however, that this protective effect was only apparent among female twins. The lack of evidence among male twins could simply be a function of the relatively small number of male-male twins included in the study, the researchers said.
The degree to which the risk for developing MS was reduced as a result of increased sun exposure ranged from 25 percent to 57 percent, depending on what activity the disease-free twin had engaged in.
For example, the researchers determined that non-MS twins who had spent more childhood time sun-tanning than their sibling had a nearly 50 percent reduced risk of developing MS as an adult.
It's not clear how sun exposure might protect against the illness. Ultraviolet rays might trigger a beneficial cellular immune response directly, or perhaps sunlight helps stave off the disease indirectly, by boosting vitamin D production.
To better understand the mystery behind sun exposure and its link to MS risk, the researchers said future sun-MS studies should be given "high priority."
"If it's true that sunlight is protective and/or vitamin D is protective, then there's one group of people who ought to think seriously about it, and that is young parents who have MS," noted Mack. "Because the likelihood that a child of a parent with MS will go on to get MS is 3 or 4 percent. Which is many, many times the likelihood that the average person could get MS."
"So, I think if I was a young parent, and I or my wife had MS, and I had a child, I would want to take every step I could take to prevent my child from getting the disease," added Mack. "But," he cautioned, "the problem is, of course, that we know that too much sunlight is the cause of melanoma. So, that's a dilemma. We want to give the child some exposure but not too much."
Dr. John Richert, executive vice president for research and clinical programs at the NMSS, which co-sponsored the study, said that "we all have to take note of" the new findings.
"This is one of a series of reports over the last couple of years that have at least indirectly implicated the role of sun exposure and vitamin D production with susceptibility to MS," he noted. "And this builds a stronger and stronger case that sun is one of the factors that can contribute to the development of MS."
Richert observed that MS in general is more common the farther away from the equator one lives, particularly during childhood, further supporting the sunlight-MS link.
However, he also agreed that too much sunlight brings its own risks.
"Certainly, in terms of relative risk -- skin cancer that can develop from sun exposure versus the potentially diminished risk for MS -- we don't have equations to really balance these," Richert stressed. "And there's a lot more work that still needs to be done before any kind of recommendations can be made about sun exposure or vitamin D intake."
For additional information on risk factors for MS, visit the National Multiple Sclerosis Society.