By Amy Norton
WEDNESDAY, Oct. 7, 2015 (HealthDay News) -- People with multiple sclerosis tend to develop it later if they had regular sun exposure as teenagers, a new study suggests -- adding to evidence linking the disease to a lack of sunlight and vitamin D.
The study found that sun exposure during adolescence seemed to influence the age at which people developed MS: The more summer sun they soaked up, the later their symptoms appeared.
Of nearly 1,200 Danish adults with MS, those who'd spent time in the sun every summer day developed symptoms two years later, on average, versus people who'd gotten less sun.
The findings do not mean that basking in the sun will prevent or treat MS, experts stressed.
But the results do support past research suggesting that vitamin D plays some role in the disease, according to Nicholas LaRocca, vice president of health care delivery and policy research for the National Multiple Sclerosis Society, in New York City.
Sunlight triggers the body's synthesis of vitamin D, and some studies have linked both sun exposure and higher levels of vitamin D in the blood to a lower risk of multiple sclerosis.
No one knows if that's a cause-effect relationship. But clinical trials are underway to see whether vitamin D supplements can help slow MS progression, said LaRocca, who was not involved in the current study.
Until those trial results are in, it's too soon to make any specific vitamin D recommendations, according to LaRocca.
But, he added, since adequate vitamin D is important for overall health, people with MS could talk to their doctors about taking a supplement.
"They may be advised to have their vitamin D level tested first," LaRocca said.
Multiple sclerosis involves an abnormal immune system attack on the protective sheath surrounding nerve fibers in the brain and spine. That leads to symptoms like muscle weakness, numbness, vision problems and difficulty with balance and coordination.
Typically, MS symptoms flare up periodically, followed by periods of remission. Over time, the disease can cause worsening problems with walking and mobility.
The precise cause of MS is unknown, but research suggests it arises from a combination of genetic vulnerability and certain environmental triggers. Inadequate vitamin D -- a nutrient needed for normal immune function -- is considered one of the suspects.
The new findings support the theory that "avoiding sunlight" could be one of the triggers for MS, said study lead researcher Dr. Julie Laursen, of the Danish Multiple Sclerosis Center, in Copenhagen, Denmark.
Since sun exposure is closely connected to people's vitamin D levels, it's possible that the vitamin explains the later MS onset, Laursen said. However, she stressed, the findings do not "directly" support that.
The researchers found no relationship between MS onset and patients' reported use of multivitamins or vitamin D as teenagers.
According to Laursen, it's possible that sunlight, like vitamin D, has its own beneficial effects on the immune system.
In the study, adults with multiple sclerosis were asked about their "summer sun" habits and supplement use during their teens. They were also asked to recall their weight at age 20.
It turned out that MS arose later -- at the average age of 33 -- among people who'd gotten some sun every day as teenagers. Among people who'd gotten less sun, MS developed at age 31, on average.
People's weight at age 20 also seemed to matter: Those who'd been overweight developed MS almost two years earlier, on average, than those who'd been at normal weight at 20.
Body fat, Laursen explained, also happens to be related to vitamin D: People who are overweight tend to have lower blood levels of the vitamin.
Again, though, it's not clear that vitamin D explains the connection between weight and multiple sclerosis, Laursen said.
The study does, however, support the theory that adolescence is a critical time in MS development, according to Laursen. She said more research is needed to see whether there are roles for sun exposure, body weight, vitamin D, or all three.
LaRocca agreed. "It's a complex picture," he said.
In the meantime, LaRocca said, people can talk to their doctors about whether a vitamin D supplement is a good idea. They can also get the vitamin through certain foods, he added -- including fatty fish, and cereals and dairy products fortified with vitamin D.
The study findings were published online Oct. 7 in the journal Neurology.
The National Multiple Sclerosis Society has more on the possible causes of MS.