Vitamin D Study in Finland: Implications for the Rest of the World
More Evidence Links Vitamin D Deficiency and Multiple Sclerosis
New research adds to evidence that vitamin D deficiency is associated with MS. Does that mean vitamin D supplementation could prevent the disease?
By Alice Callahan
Medically Reviewed by Samuel Mackenzie, MD, PhD
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Going back to at least the 1960s, researchers have noticed something interesting about the distribution of multiple sclerosis (MS): The disease is uncommon close to the equator, where sunlight is most abundant. At higher latitudes, where the sun shines for less time and at a lower intensity, the prevalence of MS increases.
Since sunlight is a necessary ingredient for the synthesis of vitamin D in the skin, the hypothesis that vitamin D deficiency may increase a person’s risk of developing MS was born. New research adds support for this idea, but many questions remain.
Does Vitamin D Play a Role in Prevention of MS?
A study published in October 2019 in the journalNeurologyadds to evidence that vitamin D deficiency may increase a person’s risk of developing multiple sclerosis, even many years later. The study focused on a large group of Finnish women who gave blood samples early in pregnancy between 1983 and 2009. The researchers identified 1,092 women who were eventually diagnosed with MS, on average nine years after their blood samples were drawn. They compared the vitamin D levels in their stored blood samples to those of women who didn’t develop MS.
In the study, vitamin D deficiency was more common in the MS group, affecting 58 percent of women who eventually developed MS compared with 52 percent of women who did not. The researchers calculated that women with deficient vitamin D blood levels (defined as less than 30 nanomoles per liter [nmol/l]) had a 43 percent higher risk of developing MS compared with those with adequate levels (at least 50 nmol/l). Vitamin D deficiency was common in this group of women, particularly before 2004, when Finland first started recommending that pregnant women take a vitamin D supplement.
The study's lead author, Kassandra Munger, a doctor of science in nutritional epidemiology and a research scientist at Harvard T.H. Chan School of Public Health in Boston, says that the results of this study and others show a clear pattern. “If your levels are low, your risk is high, and as your levels increase, your risk seems to decline,” she says.
Previous studies have found similar results, but they were smaller and done in populations with higher average vitamin D levels. For example, another study conducted by Dr. Munger and her colleagues, published in December 2006 inJAMA, found that in U.S. military personnel, high vitamin D levels were associated with a lower risk of MS.
What if Low Vitamin D Is Just Correlated With MS, but Is Not a Cause?
The recent study of Finnish women, as well as previous studies with a similar observational design, can only identify correlations between variables. In other words, they can conclude that people who end up diagnosed with MS are more likely to have had low vitamin D levels earlier in life, but they can’t definitively show that low vitamin D played a role in causing MS.
“In epidemiology, we’re always concerned about that,” says Munger. “Are there alternative explanations, or is something else going on?” For example, perhaps it’s not vitamin D specifically, but sun exposure, that affects MS risk.
However, Munger points to recent genetic studies, including one published in April 2019 in the journalNeurologyand several more published in the last two years, that show that genes associated with lower vitamin D levels are also associated with an increased risk of multiple sclerosis.
“Those studies have all suggested that there quite likely is a causal relationship between vitamin D and MS, confirming what the observational studies are showing and removing some of that doubt,” Munger says.
Other MS Risk Factors Are Still Important to Consider
It’s important to note that most of the research on vitamin D and MS risk has focused on non-Hispanic white populations, and it might not apply to people of color, who generally produce less vitamin D in their skin. In fact, the 2006 study of U.S. military personnel found no relationship between vitamin D and MS development in Hispanic or black people, and a recent report from Mexico also found no association in that population.
Low vitamin D is also not the only risk factor identified for MS. “MS is definitely multifactorial,” says Sharon Stoll, DO, an assistant professor of neurology at the Yale School of Medicine in New Haven, Connecticut, citing other known risk factors like smoking and infection with Epstein-Barr virus.
But, Dr. Stoll says, “Vitamin D deficiency can set someone up who already has genetic components to develop multiple sclerosis, and supplementation could prevent that.”
Could Vitamin D Supplementation Prevent MS?
The answer to this question isn’t certain, but an editorial accompanying the study of Finnish women says it’s worth considering.
“It is time to take an active approach to preventing MS, at a minimum targeting those individuals with an elevated risk of MS, including smokers, the obese, and those with a family history of MS,” write Ruth Ann Marrie, MD, PhD, a professor of neurology at the University of Manitoba Max Rady College of Medicine in Winnipeg, and Christopher A. Beck, PhD, as associate professor of biostatistics and orthopedics at the University of Rochester Medical Center in Rochester, New York.
But as the editorial notes, there are still many questions about who should take vitamin D and in what doses. Current U.S. nutritional guidelines, developed by the National Academy of Sciences Institute of Medicine, recommend that most people consume 600 IU of vitamin D per day, a figure based on the amount needed for bone health. Good dietary sources of vitamin D include fatty fish, fortified dairy products, and other fortified foods like orange juice, but many people don’t eat enough of these foods to meet the vitamin D requirement without a supplement.
The Institute of Medicine guidelines also say that vitamin D blood levels of 50 nmol/l are adequate for most people, but Munger says higher levels may be required for MS prevention.
“To get to a level of 75 to 100 nmol/l — where we see a decrease in risk — most people would need 1,000 IU per day,” she says.
That amount is well within the safe range of vitamin D supplementation. The Institute of Medicine recommends not taking more than 4,000 IU of vitamin D per day, as the vitamin can be toxic in high doses, causing adverse effects like cardiovascular and kidney damage.
“People who are at high risk for MS, like first-degree relatives, for example, of someone with MS, may want to ensure that their vitamin D levels are adequate, that they're taking a supplement or that they're having responsible, judicious sun exposure, because we know that there are also risks with that as well,” says Munger.
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