Higher levels of vitamin D in the blood may lower the risk of multiple sclerosis (MS), research suggests.
Vitamin D may curb the activity of rogue cells
Previous studies have suggested vitamin D may have a protective effect - but the evidence has been inconclusive.
A Harvard School of Public Health team measured levels of the vitamin in large numbers of US military personnel.
The study, published in the Journal of the American Medical Association, found the risk of MS fell as blood levels of the vitamin rose.
MS is among the most common neurological diseases affecting around two million people worldwide.
The researchers uncovered 257 cases of MS among more than seven million military personnel who had given blood samples to the US Department of Defense.
They found that among white personnel, there was a 41% decrease in MS risk for every 50 nanomoles per litre increase in 25-hydroxyvitamin D, a form of the vitamin found in the blood.
Those whose vitamin level was in the top 20% had a 62% lower risk of MS than those whose level was in the bottom 20%.
The researchers found no such association among black and hispanic personnel, but said this could be down to the smaller size of these sample groups .
Writing in the journal, the researchers said more evidence was needed before doctors suggested taking vitamin D supplements to ward off MS.
But lead researcher Dr Alberto Ascherio said: "If confirmed, this finding suggests that many cases of MS could be prevented by increasing vitamin D levels.
"The results of this study converge with a growing body of experimental evidence supporting the importance of vitamin D in regulating the immune system and suppressing auto-immune reactions, which are thought by most experts to play a key role in the development of MS."
Scientists formulated the theory that vitamin D plays a significant role in protecting against MS after data which suggested the condition was more common in countries furthest from the equator.
People in these countries are exposed to less sunlight, which triggers a chemical reaction in the body leading to vitamin D production.
Previous research has shown that vitamin D supplements can prevent or favourably affect the course of a disease similar to MS in mice.
And the Harvard team has published a study which found that women who take vitamin D supplements are 40% less likely to develop MS.
MS is thought to be an auto-immune disease caused by specialised T helper 1 cells attacking myelin - the insulatory material which sheathes the nerves.
There is evidence that vitamin D reduces the activity of these rogue cells.
Chris Jones, chief executive of the MS Trust, said: "There is growing evidence here of a relationship between vitamin D and the risk of developing MS which provides yet another piece of the jigsaw of MS.
"The problem is, still, that we don't know the nature of the picture we're trying to build, nor how many pieces are missing.
"Yet these are tantalising clues, along with other recent research in the area showing that a birth month of May gives lower risk of MS than November, possibly reflecting the mother's exposure to sunlight - which promotes the body's production of vitamin D - during pregnancy."
A spokesman for the MS Society agreed the results were interesting, and suggested that vitamin D levels might be a factor in predicting whether someone is more likely to develop MS.
However, he said: "Further work is needed to confirm if that is so. In the meantime, we underline the researchers' caution that vitamin D supplements should not be used for MS prevention until and unless they are proved effective."