Women who take vitamin D supplements are 40% less likely to develop multiple sclerosis, research suggests.
Sun exposure may ward off MS
The Harvard University team found eating a diet rich in vitamin D alone was not enough to provide the same protection.
Nearly 190,000 women took part in the study, published in Neurology.
Researcher Dr Kassandra Munger said: "It's exciting to think something as simple as taking a multivitamin could reduce your risk of developing MS."
However, she said further research was needed to confirm the findings.
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.
Separate studies have also shown that vitamin D supplements can prevent or favourably affect the course of a disease similar to MS in mice.
And other research has found that people with MS tend to have insufficient levels of vitamin D, that periods of low vitamin D occur before times of high disease activity, and periods of high vitamin D precede times of low disease activity.
MS it thought to be an autoimmune 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.
Christine Jones, chief executive of the MS Trust said: "It's well established that, broadly, MS is more prevalent in temperate climates and the possibility of a link between sunlight and vitamin D has long been mooted.
"These results therefore make interesting reading and we look forward to reading further trial data in due course."
Mike O'Donovan, chief executive of the Multiple Sclerosis Society, said: "This is an interesting study of the potential role of vitamin D in reducing the risk of MS which certainly warrants further investigation."