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“What is this nonsense?” Erica Rice, a social worker from California, remembers thinking while she watched the rambling video her aunt had shared on Facebook. A middle-aged woman in Hunstville, Alabama, stood on her porch telling viewers they needed to get outside and lay in the sun to prevent “the COVID.” 

It was April 2020, early in the pandemic—before masks became like a second skin—and Rice assumed the minutes-long clip was just one of the many virus-conspiracy theories plaguing the internet. But telling people to soak in ambient light, breathe fresh air, and keep their immunity high seemed a lot more benign than advising them to guzzle bleach, snort cocaine, or even sit in a tanning bed. Besides, her aunt, an astute, college-educated woman, had posted the idea on her feed. Maybe there was something to it?

The video carried an easy take-home message that gained popularity as COVID-19 rates skyrocketed in the US, despite the lack of scientific backing. Angelique Campen, an emergency medicine physician at Ronald Reagan UCLA Medical Center, isn’t surprised by how much misinformation proliferated during the pandemic—especially given the poor insight among health care professionals. “Doctors are used to knowing the science and knowing what to do,” she says. “But this has been a time where you really didn’t know what treatments worked. People were grasping at straws and holding on to the ones that fit. It was trial and error.” 

And while sunbathing has largely been debunked as a COVID cure (the world has monoclonal antibodies for that now, and vaccines for prevention), medical experts are paying closer attention to the notion that UV rays could play a hand—or at least a pinkie—in fortifying the human body against disease. For decades, researchers have looked at vitamin D’s role in illnesses like bone disorders, diabetes, cancer, depression, and autoimmune diseases with some positive results. But understanding the complex nutrient has proven challenging and messy, particularly in highly melanated individuals, who often test low for it. Now, with more than 47 million coronavirus cases logged in the US and variants emerging rapidly, there’s a new wave of urgency to settle the hype around the “sunshine supplement.”

How vitamin D works in your body

Vitamin D is best known for building strong bones, but it also plays a pivotal role in the immune and respiratory systems, the main targets of SARS-CoV-2 and its developing variants. Only a handful of foods are naturally rich in the nutrient—oily fish, egg yolks, mushrooms, red meat, and liver—which is why it’s added to many cereals, milks, and juices post-process. 

But it’s called the sunshine supplement for a reason: The body can produce its own supply, but it needs solar power to do so. 

Your skin already contains a precursor of vitamin D called 7-dehydrocholesterol, but only the sort of ultraviolet B radiation found in the sun’s rays can kick off a multi-step process towards turning that precursor into a functional nutrient. While the body can’t use these so-called previtamin as-is, it’s a good predictor of how much active ingredient you can make. In fact, many doctors use one of the previtamins, 25-hydroxyvitamin D, as a baseline when checking blood vitamin D levels.  

Here’s how the process works. When triggered by solar radiation, the vitamin D binding protein (DBP) in your blood plasma grabs 7-dehydrocholesterol and carries it to your liver and kidney, where it’s chemically reshaped into an active form of vitamin D, or calcitriol. The homespun nutrient then gets recruited by the immune system to run the power switch on a number of anti-bacterial and anti-inflammatory defenses (though the exact mechanisms by which it does so remain a mystery). 

[Related: Is it possible to boost your immune system?]

But it’s not as simple as sunshine in, vitamins out: In people with certain racial backgrounds, especially those of African origin, the binding protein may latch on too tightly to the precursor, which means it can’t produce and release vitamin D as needed. Compounding this is the fact that the pigment, which darkens skin and hair, acts like a natural sunscreen by absorbing UV B radiation. Consequently, this slows down the initial binding process and the conversion of the vitamin D precursor. 

As a result, Black and Hispanic individuals are more likely to be more deficient in vitamin D than their white peers. An extra dose of sunshine can help, but darker-skinned people may need anywhere from 30 minutes to three hours more time in the daylight to produce the same amount of the stuff as white folks. Many clinicians suggest supplements for necessary back-up, even with little evidence of benefits against a virus that’s disproportionately affecting Blacks and Hispanics.

The vitamin D and COVID-19 connection

While low vitamin D levels can’t be blamed for the many racial inequities in COVID mortalities, some medical experts are now wondering if the nutrient could be a fringe factor. Studies established correlations between vitamin D deficiency and higher risks of developing immune system disorders like multiple sclerosis, arthritis, diabetes and respiratory infections long before the pandemic. There’s also evidence of supplements helping lab animals and patients with heart disease markers regain their health.

The link between vitamin D and coronavirus first appeared in a paper by Northwestern University researchers in April 2020 (the results still have to undergo peer review). They looked at publicly available hospitalization, recovery, and mortality rates along with reported pre-pandemic vitamin D levels from 10 countries, including the US. The authors noticed that elderly patients with low concentrations of vitamin D in their blood had higher COVID-19-related mortality in six of the countries in the sample. In the end, the authors suggested that the deficiency could be a potential risk factor for severe COVID-19 infection.

A slew of other COVID-centered data on vitamin D followed later in 2020. David Meltzer, the chief of hospital medicine at the University of Chicago Medicine, also conducted a retrospective analysis of vitamin D levels in 489 hospitalized patients a year prior to COVID. His results, published in The Journal of the American Medical Association in September of 2020, revealed that patients with low vitamin D levels were 77 percent more likely to test positive for COVID-19. 

“The are multiple streams of evidence that suggest that vitamin D is part of an extremely complicated system, or at least a complex system that has variability by race.”

David Meltzer, University of Chicago Medicine

Others looked at the impact of supplements in fighting the disease. One meta-analysis in The Lancet looked at whether treating people with vitamin D impacted the rate of respiratory tract viral illnesses in 25 randomized double-blind trials. The findings showed that participants with low vitamin D levels who received daily supplements saw a 70-percent reduction in the infections. But a randomized clinical trial published around the same time in Brazil didn’t find any positive or negative outcomes of treating hospitalized COVID patients with vitamin D supplements.  

Monica Gandhi, an infectious diseases doctor and professor of medicine at University of California-San Francisco, notes that much of this published work is observational, which means it looks for trends but doesn’t directly measure the effect of a treatment. What’s more, with small sizes and no control and experimental groups, she says, it’s almost impossible to establish a scientifically sound conclusion. 

Why melanin further colors the question

A few studies published in the past two years have tried to break down vitamin D-COVID impacts by race. That’s where it gets tricky.

CDC data collected over the course of the pandemic holds that non-Hispanic American Indians, non-Hispanic Blacks, and Hispanic or Latinos in the US were roughly three times more likely to be hospitalized from COVID than their Non-Hispanic Asian, Pacific Islander, or white peers. Campen, of Ronald Reagan UCLA Medical Center, notes that, anecdotally, she saw patients of color suffering from worse symptoms—but she’s not yet convinced that vitamin D deficiency is the reason. After all, minorities are more likely to share rooms in multi-generational households and work essential jobs, which increases their exposure to SARS-CoV-2. Underlying health conditions such as Type 2 diabetes can also increase the risk of severe infection. 

Still, Campen thinks additional effort should be put into understanding how different bodies handle COVID-19, beyond the socioeconomic factors. Meltzer agrees—though he cautions that it’s difficult to tease out individual variables while investigating the virus. Another retrospective study he published in March 2021 suggests that Black people with vitamin D levels lower than 40 nanograms per milliliter may be more susceptible to COVID than white individuals. 

“There are multiple streams of evidence that suggest that vitamin D is part of an extremely complicated system, or at least a complex system that has variability by race,” Meltzer says. “But we absolutely would never want to argue that the differences we see in COVID risks or outcomes by race are some product of a particular biological mechanism. It’s clear that race is way bigger than vitamin D.”

“That’s not to say that vitamin D is not part of what we see by race or that we may have different needs because of the backgrounds that we come from,” Meltzer continues. “Research that tries to understand those needs could potentially be an important part of addressing these challenges.” 

In the context of other at-home COVID treatments

Unfounded coronavirus treatments have put health care professionals on edge throughout the pandemic. But the disease has presented a moving target for researchers and people desperate for solutions at home. Some COVID-19 studies (more than 100, according to Retraction Watch) have been pulled from publications due to questionable data or results. The most prominent one was a June 2020 paper in the Lancet, which claimed the malaria drug hydroxychloroquine could severely harm hospitalized COVID-19 patients. The FDA ultimately revoked the use of the treatment, but only because of poor efficacy and other safety concerns.

[Related: How to tell when the COVID pandemic is nearing its end]

While far more benign, vitamin D shouldn’t be billed as a miracle drug against COVID-19 either. “Vitamin D is really complicated,” Meltzer says. “There’s meaningful racial diversity; there’s exposure through both diet and environment.” Combine that with the fact that supplements are largely unregulated in the US, and the intricacies of human physiology—the many pathways for ingestion, binding, and production—and you have a maze of medical quandaries that still need to be mapped out.

But there hasn’t been a big push to dig deeper, despite the loose correlation between vitamin D supplementation and severe outcomes of COVID and other diseases. The nutrient is cheap, accessible, and unpatentable, which makes it hard to raise capital and interest for a randomized study involving tens of thousands of patients, both Campen and Gandhi say.

Nonetheless, neither of them sees a problem with people taking vitamin D in a bid to outlast the pandemic (it’s harmless unless taken in extreme quantities). In fact, Gandhi says it’s good for individuals to keep their levels up, given the many benefits that calciferol carries for the body. Interestingly about 50 percent of the world’s population is vitamin D-deficient to varying degrees. But that shouldn’t be a substitute for proven COVID defenses, like getting vaccinated and wearing masks in public.

Another positive aspect of the “sunshine supplement” and COVID debate is that it brings the lack of medical knowledge on Black and Hispanic communities into focus. There needs to be more research on how social inequities like health care access and workplace safety affect a person’s immunity—and how they compound with genetic and molecular differences across racial backgrounds. The vitamin D binding protein is a stark example.

Understanding the inner workings of a novel virus in a space as unstable as the human body is an enormous feat. The past two years have been devoted to that—maybe now doctors will have an opening to tackle the other eyebrow-raising ideas on Facebook feeds.

Editor’s Note (November 23, 2021): An earlier version of the story neglected to mention at the outset that monoclonal antibodies and vaccines are the leading methods for treating and preventing COVID-19.

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