A half-century ago, doctors from Europe and North America who spent time in central Africa were struck by the absence of multiple sclerosis there. Indeed, the farther from the equator people lived, the more prevalent multiple sclerosis (MS) seemed to become. Scandinavians faced a higher risk than most other people. Thus arose the “latitude hypothesis” of MS, suggesting that a lack of direct sunshine somehow contributed to the nerve-damaging immune malfunction underlying the disease.
Although the geographical connection was strong, says Michael J. Goldacre, an epidemiologist at the University of Oxford in England, “it seemed almost too obvious to be true.”
But a study from southern California now lends new credence to the sunshine theory of MS protection by removing a persistent confounder in such studies—the variability in people’s genes. The researchers sifted through a large database to find records of 179 sets of identical twins in which one had MS and the other didn’t. Estimating these individuals’ childhood sun exposures, the scientists found that the twins with MS on average had gotten less sun.
The study bolsters a 2003 report from Australia that associated greater sun exposure and a history of sunburns in childhood with reduced risk of MS. Also, Goldacre and his colleagues discovered in 2004 that people with MS were only half as likely as the general population to develop skin cancer—a condition linked with exposure to ultraviolet radiation.
“There’s clear evidence from multiple publications to suggest this is something that’s real,” says Avery August of Pennsylvania State University in University Park, an immunologist not part of these studies. “There’s a genetic component [to MS] but also an environmental component,” he says.
In the new study, epidemiologist Thomas M. Mack and his team at the University of Southern California in Los Angeles analyzed questionnaires that the twins had completed before 1993 to obtain data on childhood sun exposure. The surveys included questions about outdoor sports and time at the beach.
The twin who spent more time outdoors had a 25 to 57 percent lower risk of developing MS, depending on the activity recorded, the researchers report in the July 24 Neurology. The people without MS had spent significantly more time than their siblings sunbathing, beachcombing, and getting out on hot days.
“This is a very sound piece of work,” Goldacre says. “Dermatologists may feel that [advice to] spend some time in the sun is not a wholly welcome message. But it’s all a matter of this being good for you in small doses.”
Curiously, the latitude effect in MS seems to fade after adolescence. While this study and earlier ones hint that ultraviolet rays set a child’s immune system on a normal course for life, they don’t prove it, August says.
Specifically, the studies don’t show how sunshine would thwart the rogue immune attacks on nerves, which cause a loss of muscle coordination in MS patients.