Vitamin D, Explained: A Huge Number of Americans Are Deficient in Vitamin D. What Does That Mean?

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 more than 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.

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