Deficient vitamin D levels may increase MS risk by over 40 percent.
However, it is known that women are at much higher risk of developing the disease than men. And new research in a large sample of women has found a risk factor: low vitamin D levels.
The new study was published in the journal Neurology, and the first author of the paper is Dr. Kassandra Munger, of the Harvard T.H. Chan School of Public Health in Boston, MA.
Dr. Munger explains that to date, "There have only been a few small studies suggesting that levels of vitamin D in the blood can predict risk." But the new research examines a much larger cohort.
Studying vitamin D and MS risk
Dr. Munger and colleagues examined the data available from the blood tests of more than 800,000 Finnish women. The blood was collected as part of the prenatal testing in the Finnish Maternity Cohort.
Using data from national medical registries, the researchers also examined which women received an MS diagnosis during a period of 9 years.
Of all the study participants, 1,092 women developed MS 9 years after they had their blood tested. Dr. Munger and team compared these women with 2,123 age-matched study participants who did not go on to develop MS.
The team defined vitamin D deficiency as under 30 nanomoles per liter. Insufficient levels were considered to fall between 30 and 49 nanomoles per liter, and normal levels were defined as 50 nanomoles per liter and above.
Dr. Munger and team used conditional logistic regression to adjust for possible confounders, such as the year during which the blood sample was taken, the number of times that the woman had been pregnant, and her number of pregnancies carried to term.
Low vitamin D raises MS risk by 43 percent
Of the participants who had MS, 58 percent had vitamin D deficiency. By comparison, 52 percent of the women who did not develop MS had deficient levels of the vitamin. Finnish women have historically been reported to have low vitamin D levels.
Interestingly, the researchers found that women with vitamin D deficiency were 43 percent more likely to develop MS than their counterparts who had normal levels of the vitamin.
Women with the deficiency were also 27 percent more likely to develop MS compared with women who had insufficient levels.
Additionally, the study found that with each vitamin D increase of 50 nanomoles per liter, the risk of MS decreased by 39 percent.
"Our study, involving a large number of women, suggests that correcting vitamin D deficiency in young and middle-age women may reduce their future risk of MS."
Dr. Kassandra Munger
Strengths and limitations of the study
Strengths of the study include the large population sample as well as the national medical registries used to collect the data, which, as the authors explain, minimized selection bias.
The authors also highlight potential limitations to their research. Firstly, the study sample, although large, was limited to white women. For this reason, the results may not be applicable to men or people of other ethnic backgrounds.
Secondly, the team notes the possibility of reverse causation - that is, that the study participants already had MS, but without any symptoms, when they enrolled in the study.
However, they point out that determining serum levels of vitamin D at an average of 9.3 years before receiving an MS diagnosis - as they did in the study - greatly reduced this possibility.
"More research is needed on the optimal dose of vitamin D for reducing risk of MS," explains Dr. Munger. "But striving to achieve vitamin D sufficiency over the course of a person's life will likely have multiple health benefits."