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.