By Amy Burkholder
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(CNN) -- The "sunshine vitamin" may do more than promote healthy bones; it may ward off multiple sclerosis, one of the most common neurological diseases affecting young adults.
A study conducted by the Harvard School of Public Health offers some of the strongest evidence yet that people with higher levels of vitamin D in their blood were at lower risk for later developing the disease.
Researchers compared levels of vitamin D in 257 blood serum samples, which were among more than 7 million from U.S. military personnel stored in the Department of Defense Serum Repository.
Multiple sclerosis cases were identified through Army and Navy physical disability databases and medical record reviews.
Researchers found that among white people, the risk for multiple sclerosis was lowest among those with the highest vitamin D levels, and highest for those with the lowest vitamin D levels.
Among black people, who researchers say have a naturally lower level of vitamin D in their blood because of the pigment in their skin, no significant associations between vitamin D and multiple sclerosis risk were found.
The authors found there were insufficient numbers of muliple sclerosis cases and of high levels of serum vitamin D among these groups to detect any protective effect.
While previous research has indicated that vitamin D may provide a protective effect, evidence has been inconclusive.
"We established with a certain degree of confidence that people with high vitamin D levels have a lower risk of developing MS. What we don't know for sure if increasing their vitamin D levels will actually prevent MS," said study author Dr. Alberto Ascherio of the Harvard School of Public Health.
Multiple sclerosis, or MS, is a disorder of the central nervous system in which the body's immune system attacks the brain and spinal cord, specifically the myelin, the substance that insulates nerve cells and helps messages travel among them.
It affects 400,000 people in the United States. About 200 cases are diagnosed each week.
Heredity a factor
Doctors still don't know what causes MS, but many are convinced heredity plays a role.
Most of us have a one in 750 chance of developing MS, but that can jump to one in 40 if a relative has the disease. Women with MS tend to outnumber men 3-1.
Where you live and exposure to sunlight are also factors.
Research has shown in regions where there is more sunlight and vitamin D levels are higher, there are fewer cases of MS than in those with less sunlight.
People with MS tend to have lower than normal blood levels of vitamin D compared with the general population.
Vitamin D is unique. The fat-soluble vitamin is found naturally in foods such as milk, cheese, fish and fortified juices and cereals. It can also be produced in the body by exposure to sunlight.
How does this translate? Should we be baking in the sun, or loading up on vitamin D supplements as a preventive measure?
Researchers concede that large-scale clinical trials are needed before a conclusive link is established and that a broad recommendation to increase vitamin D intake requires stronger evidence than observational studies alone.
They say you should hold off on the use of vitamin D supplements for MS prevention until efficacy is proved.
The MS Society of America, which supported this study, agrees.
"The bottom line here, this adds to the evidence of association between vitamin D and multiple sclerosis," said Dr. Nicholas LaRocca, associate vice president of health care Development & Policy Research.
"But I don't believe we're at the point right now to make the recommendation that people should go out and take vitamin D. We'd need more studies before we'd be comfortable with that."
The researchers don't know how the people studied got their vitamin D, whether it was from food, supplements or sunlight.
And while there have been a slew of recent studies touting the positive effects of vitamin D, suggesting it may affect everything from diabetes to cancer, how much you need depends largely on the individual.
The FDA approves of 200 IU for 19-50 year olds; 400 IU for people over 50. Most vitamin pills contain 400 IU of vitamin D.
By comparison, a light-skinned person in a bathing suit can produce more than 10,000 IU with a half-hour in the sun, according to Dr. Meir Stampfer of the Harvard School of Public Health.
More is not better: The Institute of Medicine warns that excessive intake of supplemental vitamin D can have serious, toxic effects on the body, including excessive calcium levels in the blood, high blood pressure, nausea, poor appetite, weakness, constipation, impaired kidney function and kidney damage.