Vitamin D supplement sales have soared amid the pandemic as people try to curb their risk of contracting the novel coronavirus—but some experts are urging caution, noting that not enough research has been done to establish a definitive relationship between taking the supplement and fending off Covid-19.

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What the research says about vitamin D and Covid-19

According to Sabyasachi Sen, a professor of endocrinology and medicine at George Washington University (GW), deficiencies of vitamin D are "not rare" and are especially common in older adults, obese people, and people with darker skin—some of the same populations most affected by Covid-19.

While vitamin D is known for protecting bone health, it also helps with the immune system, Sen said. It's believed that vitamin D improves the function of certain cells, including T cells, which fight off pathogens and can assist in modulating inflammatory responses.

In addition, Sen continued, research has found vitamin D deficiencies have been associated with an increased risk of infection. "Now, what is unknown is whether it's a cause and effect rather than an association," he said.

According to the Washington Post, researchers studying the relationship between vitamin D and Covid-19 outcomes are interested in precisely that question: whether there's a cause and effect relationship, or merely an association.

For example, one study, published in JAMA Network Open, looked at the health records of 489 people in Chicago and found patients with a vitamin D deficiency in the year prior to testing for Covid-19 were 77% more likely to test positive for the disease than those with normal vitamin D levels. Taking a converse approach, another study looked at a small group of Covid-19 patients in Italy who had been hospitalized with acute respiratory failure and found that 81% of them had a vitamin D deficiency.

Meanwhile, an experimental study in France at a nursing home with 66 people found that taking vitamin D supplements was "associated with less severe Covid-19 and a better survival rate." Similarly, a study in South Korea of 200 people found that a deficiency of vitamin D could "decrease the immune defenses against Covid-19 and cause progression to severe disease." And a small study in Spain involving 76 hospitalized Covid-19 patients found that those treated with calcifediol—an activated version of vitamin D, distinct from the over-the-counter supplement—seemed to curb the severity of the disease.

On the flipside, however, a recent paper considered by the National Institute for Health and Care Excellence in the United Kingdom looked at vitamin D levels from up to 14 years ago and didn't find any correlation between vitamin D levels and Covid-19 mortality. And while the lead author of that study has in other papers called for further research on the link between vitamin D and Covid-19 outcomes, the researchers concluded, "For now, recommendations for vitamin D supplementation to lessen Covid-19 risks appear premature and, although they may cause little harm, they could provide false reassurance leading to changes in behaviour that increase risk of infections."

Similarly, a double-blind randomized controlled trial of 240 patients in Brazil, which has yet to be peer-reviewed, found that one large dose of vitamin D didn't reduce hospital stay length or mortality rates among patients with a severe case of Covid-19 compared with those in a placebo group.

Correlation—not necessarily causation

"We do know that people who have lower blood levels of vitamin D tend to have a higher risk of being infected with Covid-19 and having severe Covid-19 illness," JoAnn Manson, chief of the Division of Preventive Medicine at Brigham and Women's Hospital, said. "But as we say in epidemiology, 'Correlation doesn't prove causation.' We don't know for sure that the low vitamin D level is causing an increased risk of Covid-19."

According to Natasha Chida, an infectious disease expert and assistant professor of medicine at Johns Hopkins University, people who have a vitamin D deficiency typically have other health factors that could affect how likely they are to develop a severe case of Covid-19—and people who do develop diseases such as Covid-19 often experience a drop in vitamin D levels.

"Unless you take into account all those factors and separate all those out and look at just vitamin D … it's really hard to make any inferences about what vitamin D is doing here," she said.

Chida added that there's "some biologic plausibility" that vitamin D could help Covid-19 patients. "It's just that despite years of research into the use of vitamin D in respiratory tract infections, there still hasn't really been a clear, slam-dunk answer that there's a benefit."

Research into the relationship is ongoing, however. According to the Washington Times, about 70 clinical trials assessing vitamin D and Covid-19 have been filed in the U.S. National Library of Medicine database.

Should you take vitamin D supplements?

As of now, experts say people who know they have vitamin D deficiencies should continue their treatment, and those thinking about taking supplements should talk to their health care provider first, given there's no firm evidence that vitamin D supplements curb people's risk of infection or serious Covid-19 illness.

"People should be wary of taking mega doses of vitamins or unproven interventions specifically for Covid-19, because we don't have good quality data yet to suggest that this is of any help," Hana Akselrod, an infectious disease specialist at GW, said.

Instead of supplements, people can add more vitamin D to their lives by being outdoors for 15 or 20 minutes a day, Akselrod added. And some foods, such fatty fish or fortified dairy products, could also improve vitamin D levels, Manson said.

"There are all of the positive confluences around nutrition and outdoor exercise that aren't just limited to the number of how many units of vitamin D you get every day," Akselrod said. "And on top of that, people absolutely need to continue all the other safety precautions, like masking and safe distancing and avoiding gatherings, because we're in the most dangerous phase of the pandemic yet" (Chiu, Washington Post, 1/11; Busby, The Guardian, 1/10).