By: Amanda Chan–
People who have had lifelong exposure to high levels of sunlight are less likely than people with less exposure to develop multiple sclerosis, a new study suggests.
The risk of having a preliminary symptom of multiple sclerosis decreased by 30 percent for every 1,000 kilojoules of exposure to ultraviolet light, the study said.
These levels of sun exposure were accumulated over a lifetime (you might be exposed to 6 kilojoules of UV light on a summer day), so people shouldn’t sit in the sun for extended periods of time without sunscreen and expect to lower their multiple sclerosis risk, said study researcher Dr. Robyn Lucas, a fellow at the National Centre for Epidemiology and Population Health at Australian National University.
“There is strong evidence that the risks of high doses of UV radiation in a single exposure greatly outweigh any possible benefits,” Lucas told MyHealthNewsDaily.
The benefits coming from higher vitamin D levels were already known, but because it was sun exposure that this study linked to decreased multiple sclerosis risk, it’s possible that only sun-generated vitamin D — and not that provided by food sources or vitamin D supplements — provides those benefits, Lucas said.
The findings of the study appear tomorrow (Feb. 8) in the journal Neurology.
The importance of sun exposure
Researchers looked at the sun exposure histories of 216 Australians, ages 18 to 59, who had an early sign of multiple sclerosis but were not diagnosed with the disease, as well as 395 people who did not have any MS symptoms. The participants reported how much sunlight they were typically exposed to, and the researchers also measured their skin damage from sun exposure and their melanin levels. The participants’ vitamin D levels were measured by blood tests.
Over their lifetimes, people in the study had been exposed to 500 to 6,000 kilojoules of UV light. Researchers found that those with the most skin damage from sun exposure were 60 percent less likely to have had a first sign of multiple sclerosis than people who had the least damage.
And people with the highest vitamin D levels were less likely to have a first sign of multiple sclerosis than people with the lowest vitamin D levels, the study said.
The researchers also found that multiple sclerosis was 32 percent more common in the Australian regions farthest from the equator than the regions closest to the equator — a difference they attributed to differences in sun exposure, vitamin D levels and skin type.
The secret to vitamin D’s effects
This study’s findings revealed the relationship between the first sign of multiple sclerosis, called the first demyelinating event, and sunlight, said study researcher Anne-Louise Ponsonby, an epidemiologist at Murdoch Children’s Research Institute in Australia.
The first demyelinating event can appear as a loss of sensation in a limb, blindness in one eye, or weakness in one limb that lasts more than 24 hours, Ponsonby said. Most people who have such an event will go on to develop multiple sclerosis in 10 years. The disease is diagnosed after a second event.
Vitamin D is known to affect immune cells, and the immune system plays a significant role in spurring multiple sclerosis, said Dr. Tom D. Thacher, an associate professor of family medicine at the Mayo Clinic in Rochester, N.Y., who was not involved with the study.
While the study showed that increased sun exposure is linked with a reduced risk of multiple sclerosis, it does not prove that vitamin D prevents the disease, said Thacher, who wrote an article on vitamin D insufficiency published last month in the journal Mayo Clinic Proceedings.
“Other factors besides vitamin D that are related to sun exposure could be responsible for protection from multiple sclerosis,” such as melanin production from getting a suntan, Thacher told MyHealthNewsDaily.
A study published last month in the journal Multiple Sclerosis found that people who had sufficient levels of vitamin D had higher levels of antibodies to the Epstein-Barr virus, which is linked to increased risk of multiple sclerosis.
“Low vitamin D may predispose people to certain viral infections,” said Dr. Ellen M. Mowry, author of that study and an assistant neurology professor at University of California, San Francisco, who was not involved with the new study. “Since some viral infections have been associated with MS risk, low vitamin D could also influence MS by this mechanism.”
By Nathan Seppa April 24th, 2010; Vol.177 #9 (p. 9)
Ultraviolet radiation from sunshine seems to thwart multiple sclerosis, but perhaps not the way most researchers had assumed, a new study in mice suggests.
If validated in further research, the finding could add a twist to a hypothesis that has gained credence in recent decades. The report appears online March 22 in the Proceedings of the National Academy of Sciences.
Scientists have hypothesized that MS is rare in the tropics because people synthesize ample vitamin D from exposure to the UV radiation in equatorial sunlight. What’s more, MS is more common in the high latitudes of northern parts of Europe and North America than in regions farther south. That pattern has led to the assumption that higher levels of vitamin D might prevent people from developing MS, what became known as the latitude hypothesis.
But a direct cause-and-effect relationship between vitamin D deficiency and MS has never been established. In past experiments, giving vitamin D supplements to mice with an MS-like disease required giving the animals harmful amounts of the nutrient, notes Hector DeLuca, a biochemist at the University of Wisconsin–Madison.
“It just didn’t add up,” he says. “We decided to go back and see if maybe UV light by itself was doing something.”
In MS, the fatty myelin sheaths that insulate nerves in the central nervous system are damaged by attacks by the immune system. In a series of experiments in mice, DeLuca and his team induced a condition comparable to human MS by injecting the animals with proteins that instigate similar myelin damage.
The researchers exposed some mice to UV radiation before and after giving the animals the damaging injection. Another group of mice got the injection but not the UV exposure.
The mice exposed to UV rays suppressed the effects of MS-like disease better than the control mice, the researchers found, even though the amount of radiation wasn’t enough to greatly increase the animals’ blood concentrations of vitamin D.
In another test, the researchers gave injected mice varying doses of vitamin D supplements, but no UV radiation. The supplements failed to control the disease onset, severity or progression.
“We concluded that UV light is doing something beyond [making] vitamin D,” DeLuca says.
There’s no question that the latitude hypothesis has merit, says George Ebers, a neurologist at the University of Oxford in England. “MS risk is geographically related.” But that risk is more complicated than exposure to UV radiation during an MS attack, as this mouse model used. For example, previous research has shown that children in northern latitudes who are born in May, after their mothers had spent a winter with little sunshine, are more likely to develop MS than are kids born in November, he says.
Ebers notes that mice in this study were exposed or not exposed to UV over a matter of weeks and were in the throes of an MS-like disease during the study. “That’s completely separate … from the question of whether your risk is boosted or diminished by where your mother lived,” he says.
Apart from the timing issue, MS risk might well be influenced by a biological mechanism apart from vitamin D blood levels, but many questions remain, Ebers says. Those include how UV radiation might inhibit MS and, more specifically, what is the effect of UV rays in suppressing the immune system. “It’s quite possible that UV exposure will have a number of other mechanisms and be involved in hormonal circuits,” he says.
DeLuca and his colleagues speculate that UV radiation is playing a mysterious role in MS that is independent of vitamin D production. “We’re doing experiments trying to find out what it is,” he says.