Vitamin D may help fight multiple sclerosis (MS) by preventing damage-causing immune cells from entering the brain, U.S. researchers said Monday.

The findings, published online in the U.S. journal Proceedings of the National Academy of Sciences, may help better explain why the so-called "sunshine vitamin" may prevent or ease symptoms of the neurodegenerative disease.

The quest to understand the role of the nutrient began with the observation that MS is more prevalent in regions of the world farthest from the equator where there is less sunshine, the main natural source of vitamin D.

MS is believed to be an autoimmune disorder, caused when the immune system wrongly attacks a person's own cells: the fatty protein called myelin that surrounds nerve cells and helps them send electrical signals that control movement, speech and other functions. Destruction of myelin leads to debilitating symptoms such as blurred vision, weakness and numbness.

"With this research, we learned vitamin D might be working not by altering the function of damaging immune cells but by preventing their journey into the brain," said study leader Anne Gocke, an assistant professor of neurology at the Johns Hopkins University School of Medicine, in a statement. "If we are right, and we can exploit Mother Nature's natural protective mechanism, an approach like this could be as effective as and safer than existing drugs that treat MS."

In a person with MS, immune system cells known as T cells in the body's lymph nodes are used to seek and destroy myelin in the brain.

For their study, Gocke and her colleagues simultaneously gave mice the rodent form of MS and a high dose of vitamin D and found that the mice showed no symptoms of the disease.

They also found a large number of T cells in the bloodstream of the mice, but very few in their brains and spinal cords.

"Vitamin D doesn't seem to cause global immunosuppression," Gocke said. "What's interesting is that the T cells are primed, but they are being kept away from the places in the body where they can do the most damage."

Gocke added vitamin D may slow a process of making a sticky substance that allows the T cells to grab onto blood vessel walls, which allows the T cells to remain in circulation and keeps them from migrating to the brain.

Once vitamin D is withdrawn, MS-like flare-ups in mice can occur very quickly, the researchers said.

"Vitamin D may be a very safe therapy," said Peter Calabresi, a professor of neurology at Johns Hopkins University and a co-author of the study. "But we still have to be careful with it. It's not just a vitamin. It's actually a hormone."

A clinical trial testing vitamin D supplements in multiple sclerosis patients is underway at Johns Hopkins University, the researchers said.