UrbanUpdate

In 2020, as COVID-19 spread rapidly throughout the U.S., Dr. David Meltzer found himself curious about vitamin D. Most doctors who study the vitamin know about its importance to overall bone health. But Meltzer, who is the chief of hospital medicine at the University of Chicago Medicine, began reading medical literature linking vitamin D to a decreased risk of viral respiratory tract infections, and became curious whether the level of vitamin D in someone’s blood would have any association with an increased or decreased risk of contracting COVID-19.

With a research team, he looked at 489 patients at UChicago Medicine whose blood levels of vitamin D had been measured within a year of being tested for COVID-19. “We found that people who were vitamin D deficient were almost two times as likely to test positive for COVID,” he told GQ. “I started taking vitamin D shortly after I started this work, and I haven’t stopped.”

He published his results in September 2020. Meltzer is careful to note that correlation is not evidence of causation—that vitamin D does not necessarily prevent COVID.

Still, most Americans would do well to take note: It’s estimated that around half of us could be considered vitamin D deficient. Here’s what that means.

Soak Up the Sun

You might know of vitamin D in the context of days at the beach. Otherwise known as the sunshine vitamin, our skin produces vitamin D when it’s exposed to the sun’s ultraviolet B radiation.

We need vitamin D for healthy bones, because it’s the vitamin that helps our body absorb calcium. After vitamin D is taken in, it's metabolized in the liver and the kidneys, travels to the intestines, and tells it to increase the efficiency of calcium absorption. This interplay ensures that there’s enough blood calcium in the body to be deposited into your bones. Without it, muscle weakness and bone pain can occur. Vitamin D levels are also linked to immune function, specifically when it comes to supporting the work that macrophages, T cells, and B cells do to clear the body of infection.

The problem, however, is that vitamin D is not readily found in many foods. Unless you’re eating a lot of salmon, cod liver oil, or tons of sun-dried mushrooms, you’re mostly getting the vitamin from foods and drinks that have been fortified: Think of the cartons of milk or jugs of orange juice that advertise they have vitamin D.

D for Deficiency?

What constitutes a deficiency in vitamin D is a matter of some debate. It’s usually defined as having below 20 nanograms per milliliter of vitamin D in the blood. But about a decade ago two schools of thought emerged. The National Academy of Medicine determined that having more than 20 nanograms was sufficient, while the Endocrine Society concluded that Americans should have at least 30 nanograms per milliliter. If the latter baseline is taken into account, it’s estimated that roughly one billion people worldwide are deficient in vitamin D.

As far back as the 1920s it was known that a severe lack of vitamin D led to rickets in children and soft bones in adults. The Cleveland Clinic says that maintaining sufficient vitamin D levels can help protect against heart disease and high blood pressure, diabetes, infections, and immune system disorders. Low levels of vitamin D are associated with fractures and bone loss. Also important: Dark skin makes getting enough harder, as the more melanin a person has in their skin, the more sunlight exposure is needed to get a sufficient level of the vitamin.

Many Americans, however, tend to have levels above 20 nanograms. And just last November, a study published in Nature defined adults with more than 20 nanograms as “vitamin D-replete,” noting that supplementation over that level didn’t seem to have many positive effects.

The Baseline

If you’re deficient in vitamin D, that is absolutely something you should seek to correct, and the only way to know is from a blood test from your doctor. My own primary care physician first ordered a vitamin D test for me in 2018, which found I had a level of only 21 nanograms—on the low side no matter how you slice it. 

Now here’s the good part: It’s rather easy to correct. For the past four years, I’ve taken at least 2,000 units of vitamin D per day by pill. Units, in this case, means international units, IUs. Every 100 IUs of vitamin D raises the blood level by 1 nanogram per milliliter. The Endocrine Society says that taking up to 10,000 IUs a day is safe, but most adults can become vitamin D sufficient by taking 2,000 to 4,000 IUs daily. Bear in mind that vitamin D doesn’t boost the immune system to superhuman levels or turn your bones unbreakable. Think of it as the gas tank in your car. What vitamin D can do is to ensure the tank is filled up and everything is working correctly.

And while vitamin D supplementation hasn’t been shown to be preventive of COVID-19, one study published in 2020 found that more than 80% of people diagnosed with COVID-19 had deficient levels of vitamin D. Another study published the same year found that vitamin D sufficiency can reduce by 54% a person’s risk of getting COVID-19. Just last year, Meltzer conducted another study showing that having vitamin D levels above those considered sufficient may lower the risk of infection. He’s currently conducting more research into the association between vitamin D levels and COVID-19. The trial, funded by the National Institutes of Health, is currently enrolling participants.

Overall, Meltzer says he doesn’t know if vitamin D decreases the risk of contracting or combatting COVID. “Do I believe it does? I think it’s a strong possibility,” he says. “But the downside of taking significant doses seems very low for the vast majority of people, so I think it’s something that everyone should consider taking.”

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