American Academy of Neurology - People who spent more time in the sun as children may have a lower risk of developing multiple sclerosis (MS) than people who had less sun exposure during childhood, according to a study published in the July 24, 2007 issue of Neurology®, the medical journal of the American Academy of Neurology. Neurologists and researchers have long noted that sunlight exposure in childhood seems to prevent multiple sclerosis, but this research confirms the previously suspected link.

For the study, researchers surveyed 79 pairs of identical twins with the same genetic risk for multiple sclerosis in which only one twin had MS. The twins were asked to specify whether they or their twin spent more time outdoors during hot days, cold days, and summer, and which one spent more time sun tanning, going to the beach and playing team sports as a child.

The study found the twin with multiple sclerosis spent less time in the sun as a child than the twin who did not have MS. Depending on the activity, the twin who spent more hours outdoors had a 25 to 57 percent reduced risk of developing MS. For example, the risk of developing MS was 49 percent lower for twins who spent more time sun tanning than their siblings.

“Sun exposure appears to have a protective effect against MS,” said study authors Talat Islam, MBBS, PhD, and Thomas Mack, MD, MPH, with the Keck School of Medicine of the University of Southern California in sunny Los Angeles. “Exposure to ultra violet rays may induce protection against MS by alternative mechanisms, either directly by altering the cellular immune response or indirectly by producing immunoactive vitamin D.”

The study also found the protective effect of sun exposure against multiple sclerosis was seen only among female twin pairs, but Mack says this novel finding must be viewed with caution since only a few male twins were involved in the study.

This multiple sclerosis study was supported by grants from the Multiple Sclerosis Society of Canada, the National Institute of Neurological Disorders and Stroke, the National Cancer Institute, and the National Institute of Environmental Health Sciences.

“Our findings note the importance of sun exposure among people with identical genetic risk for MS,” said Mack. “High priority should be given to research into how sun exposure reduces MS risk if we are to unravel the mystery of what causes multiple sclerosis.”