In the first large-scale study to examine the issue, researchers from the Harvard School of Public Health reported a strong association between vitamin D levels within the body and MS risk among whites, but not among blacks and Hispanics.
The study is published in the Dec. 20 issue of The Journal of the American Medical Association.
Senior researcher Alberto Ascherio, MD, DrPH, tells WebMD that roughly half of white Americans and two-thirds of black Americans could be considered to have insufficient levels of vitamin D. Because exposure to sunlight is a major source of the vitamin for most people, vitamin D levels are usually lowest in the wintertime.
"Our findings suggest that vitamin D may have a direct impact on multiple sclerosis risk," Ascherio says. "If we confirm that the vitamin is protective, we could potentially prevent thousands of cases of MS a year in the United States alone."
In earlier studies, Ascherio and Harvard colleagues reported that women who took multivitamins with at least 400 international units (IU) of vitamin D appeared to have a lower risk of MS than women who did not.
Their newest study involved a study population of more than 7 million members of the U.S. Army and U.S. Navy with blood samples stored in a Department of Defense repository.
MS Risk and Racial Groups
Because dark-skinned people do not make vitamin D through the skin as easily as light-skinned people, separate analyses were conducted among whites, blacks, and Hispanics. UV rays from the sun allow vitamin D to be made in the skin.
The investigation revealed that among whites, people with the highest circulating vitamin D levels had the lowest MS risk. Compared with whites with the lowest levels, those with the highest were found to have a 62% lower risk for developing the disease.
The strongest association was seen among the youngest study participants.
Vitamin D and the Immune System
Ascherio says the findings add to the mounting evidence supporting a role for vitamin D in regulating the immune system and suppressing autoimmune reactions.
Earlier research from the Harvard team suggested a protective role for vitamin D against rheumatoid arthritis, another autoimmune disease.
Other researchers have reported that vitamin D deficiency may increase the risk of a wide range of medical maladies, including heart disease, diabetes, unexplained muscle and joint pain, and certain cancers.
William F. Finn, MD, who has been studying vitamin D for many years, agrees that large, randomized trials are needed to confirm the vitamin’s role in protecting against MS or any of these diseases.
But he says it is not too soon to recommend that people take vitamin D in supplement form.
Most multivitamins contain 400 IU of vitamin D, which has been thought to be a sufficient daily dosage. But Finn argues that most people need between 800 and 1,000 IU a day, especially in the winter when they are getting less of the vitamin from sun exposure.
He says blacks and other dark-skinned people may need even more than that.
Finn is a professor of medicine at the University of North Carolina at Chapel Hill.
"It is important to discuss this with your doctor, but I believe that most people could benefit from getting more vitamin D," he says. "Vitamin D deficiency or insufficiency is a very common problem in the United States."