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What link does vitamin D have with Covid-19? Scientists split fact from fiction

Can vitamin D supplements really protect you from the coronavirus? Experts say it's time to think critically.

Yulia Reznikov

As of now, there are no drugs, vaccines, or other therapeutics approved by the FDA to prevent or treat the novel coronavirus. This hasn't stopped speculation over possible treatments: People are hungry for actionable ways to protect themselves from Covid-19 and interest in untested methods has spiked.

One popular, controversial supplement that's especially gripped the public’s attention is vitamin D.

It's a perplexing scenario: Some scientists say that, while vitamin D does play a role in immune function, it won't majorly influence how your body reacts to Covid-19. Still, preliminary, non-peer-reviewed studies posit a possible link between vitamin D deficiency with worsened Covid-19 outcomes.

Three veteran micronutrient experts tell Inverse that while it's plausible that vitamin D plays a role in a body's reaction to the novel coronavirus ultimately no studies can prove that it actually helps. Still, that doesn't stop some researchers from describing vitamin D as an "independent risk factor" for Covid-19.

What’s new in the coronavirus-vitamin D debate? — When fighting a Covid-19 infection, some people's immune systems dangerously overreact. This ignites a sometimes fatal physiological phenomenon called a cytokine storm.

In a recent preprint (that has not been peer-reviewed) a team of researchers suggests cytokine storms may be linked to a deficiency in vitamin D.

After analyzing publicly available Covid-19 patient data collected from 10 countries, the team found an inverse correlation between vitamin D levels and Covid-19 severity and mortality.

Vadim Backman, a medical engineer at Northwestern University, led the research. He tells Inverse that vitamin D deficiencies may explain some of the pressing mysteries around Covid-19: why older people and African Americans are more likely to experience severe cases of Covid-19 compared to other populations, and why some Covid-19 patients experience hyperinflammatory cytokine storms while others do not.

“Our analysis shows that vitamin D may help cut mortality risk by half,” Backman tells Inverse. “It will not prevent a patient from contracting the virus, but it may reduce complications and prevent death in those who are infected.”

Backman notes the study "wasn't perfect." His team didn't measure vitamin D blood levels directly or control for potential confounding factors that influence them, including age and comorbidities like diabetes, heart disease, and cancer. Working with the data available, the team accounted for differences in testing techniques as well as socioeconomic status and access to healthcare.

What do other scientists think about the research? — JoAnn Manson, an epidemiologist at Brigham and Women’s Hospital and chief of Harvard Medical School’s division of preventive medicine, tells Inverse that Backman and his team's hypothesis that vitamin D levels influence immunity, inflammation, and ultimately, cytokine storms, is plausible.

That's because vitamin D is known to boost innate immunity (the body’s first line of defense against invaders like a virus or bacteria), reduce inflammation, and modulate immune function.

However, Manson notes this specific study "does not provide compelling evidence" of vitamin D's influence on severity and mortality linked to Covid-19 because it didn't control for major confounding factors.

"The public should not be misled by this kind of study."

David Seres, an internal medicine doctor and director of medical nutrition at Columbia University Irving Medical Center, tells Inverse that the preprint's conclusions are "misleading to the extreme."

“The public should not be misled by this kind of study,” Seres says. “It tells us nothing about the benefit of increasing your vitamin D levels. It only describes a correlation, and does so indirectly and theoretically.”

John C. Gallagher, a professor at Creighton University School of Medicine, tells Inverse he would reject the preprint in a peer review. Gallagher, like Manson, has studied vitamin D, calcium, and other micronutrients’ impact on health for decades.

“The hypothesis is that vitamin D levels are lower in these countries where the mortality rate is higher — so what?” Gallagher says. “It is just an association study where they link the two factors together.”

In another recent preprint study, scientists analyzed the vitamin D levels of Covid-19 patients and compared them to the levels of people not infected, whose data was collected by the UK Biobank. Gallagher notes that this study was unable to use the biomarker of vitamin D to make any conclusions about its link to incidence of or mortality from Covid-19.

What else do we know about vitamin D and Covid-19? — A literature review published May 1 by the Centre for Evidence-Based Medicine at the University of Oxford looked at all the data collected so far on a Covid-19 and vitamin D.

The team failed to find any peer-reviewed studies showing vitamin D supplements are beneficial in preventing or treating Covid-19. There was also no evidence related to vitamin D deficiency predisposing people to the disease.

There are a handful of other preprints available online that suggest vitamin D deficiency may contribute to severe Covid-19 — but none of those studies have completed peer review.

"People should know that there is no evidence that taking supplementary vitamin D will have any salutary or preventive effect on Covid-19," Seres says. "Until properly and directly studied, I will be concerned that it is conceivable, even if unlikely, that supplementation of D could even be harmful."

Generally, taking the recommended dose (600 to 800 IUs daily) of vitamin D supplements is widely considered safe. But overloading on vitamin D (like taking more than 4,000 IUs per day) can cause negative side effects like pain or kidney problems.

"More is not necessarily better," Manson says.

How do we get vitamin D?

People get vitamin D from a few sources, but it most often comes from the sun. When a person is exposed to sunlight, the ultraviolet B-rays in that light hit the skin and ignite vitamin D synthesis.

We also get small amounts of vitamin D from certain foods, like fortified dairy products, cereal, and fatty fish.

Typically, a person can get a sufficient amount of vitamin D just by living their life — as opposed to taking vitamin D supplements. This is true unless you have a condition that makes synthesizing vitamin D difficult.

“Most people don't need to take a supplement to avoid vitamin D deficiency,” Manson advises. “That is avoided and prevented in the vast majority of the population through time outdoors.”

Beyond Covid-19, there are important reasons to avoid becoming vitamin D deficient. According to the Cleveland Clinic, getting enough vitamin D (about 30 nanograms per milliliter) helps stave off heart disease, diabetes, some cancers like breast cancer, and multiple sclerosis. When vitamin D is deficient (less than 20 nanograms per milliliter), bone and cardiometabolic health can be threatened.

Who may want to consider adding vitamin D supplements to their routine during winter months are the elderly (skin's ability to produce vitamin D declines with age) or those living in nursing homes who may be without regular sun exposure. Some populations — including African Americans, Hispanics, and obese adults — are also at an elevated risk of low vitamin D levels, especially during the wintertime.

What's next? — There is some research that suggests vitamin D supplements can protect against lung infections. In 2017 scientists analyzed 25 different randomized controlled trials in an effort to see if taking vitamin D supplements influenced the risk of acute respiratory illness infection. This meta-analysis was published in the British Medical Journal.

The team found that vitamin D supplementation significantly reduced the risk of becoming infected with a respiratory illness.

But until randomized controlled trials are completed — which could take up to a year, according to Backman — we won't be able to "prove" vitamin D makes a meaningful difference when it comes to Covid-19.

Manson and her team at Brigham & Women's Hospital are currently planning a randomized clinical trial to test how vitamin D supplementation may influence Covid-19 outcomes.

Ultimately, if vitamin D does prove to help in the fight against Covid-19, it could be a relatively cheap and low-risk intervention, scientists say. But while some early studies might argue that it plays a role in coronavirus cases, it is too soon to say whether or not that's actually true.

To date, the best ways to protect yourself from Covid-19 are washing your hands, putting six feet between yourself and others, covering your mouth and nose with a cloth face cover, and cleaning frequently touched surfaces.

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