The new study adds to a growing body of research suggesting a link between vitamin D and MS, an autoimmune disease that affects the brain and spinal cord that is believed to afflict more than a quarter-million Americans. The research has been published in the medical journal Neurology.
Vitamin D is made by the body in response to sun exposure and is found naturally in some foods such as fatty fish. The so-called "sunshine vitamin" is also added to milk and other foods in the U.S., though doctors say it is difficult to get the recommended amount of vitamin D from food alone.
Researchers have speculated that low vitamin D levels increase risk for developing a number of chronic health problems, including diabetes, heart disease, various cancers—and MS.
The new study looked at blood samples collected since 1975 from 164,000 people living in northern Sweden that were taken as part of other health studies. The bulk of the samples came from pregnant women.
The study's lead researcher, Jonatan Salzer, a doctor at Umeå University Hospital, says his team wanted to know if there were links between the development of multiple sclerosis and vitamin D levels in women, as well as between vitamin D in pregnant women and their children who later developed MS.
The researchers found 192 cases of MS among women and 37 cases of MS in children.
All the blood was collected before any of the subjects developed the disease, researchers said. For purposes of comparison, blood samples from those who developed MS were matched with at least two samples taken from healthy people who were the same age and had blood collected on the same date. The vitamin D levels were measured and compared.
Dr. Salzer says adults who had high blood levels of vitamin D were 61% less likely to develop MS than people with lower vitamin D levels. However, the study found no link between vitamin D levels in mothers whose children later developed MS.
No research to date has shown a definitive link between vitamin D and the development or progression of MS because it is possible people with MS simply have low vitamin D levels. However, there are separate, ongoing studies looking at giving vitamin D supplements to people with MS to see if they can reduce the incidence of disease flare-ups.
"There's mounting evidence" that low vitamin D levels influence the disease, says Andrew Solomon, an assistant neurology professor at the University of Vermont, who specializes in treating MS patients.
Solomon, who was not involved in the Swedish study, recommends his patients take 2,000-4,000 international units a day of vitamin D.
The Swedish researchers considered high vitamin D levels to be equal to or greater than 75 nanomoles per liter of blood.
In the U.S., people with vitamin D levels at or above 50 is considered adequate, according to the Institute of Medicine, which advises U.S. policy makers.
However, there is concern that vitamin D levels may be declining as children and adults increasingly spend less time outdoors and slather on more sunscreen when they do venture out. In winter, experts say, people in the northern half of the U.S. don't receive enough UVB rays from the sun to make vitamin D, regardless of whether they use sunscreen.
In 2010, the IOM raised the amount of vitamin D Americans should consume to 600 international units per day from 200 for most children and adults, based on evidence that vitamin D and calcium protects bones.
However, the IOM said it was not clear there were other health benefits linked to vitamin D and warned people from consuming too much.
The Swedish study included funding from Biogen Idec, Merck Serono and Umea University.