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Notebook Medicine

The vitamin D factor

In this 2012 photo, a medical technician fills test tubes with serum and a reagent used to measure the levels of vitamin D in a person’s blood. ( Chicago Tribune)


The vitamin D factor

Research shows vitamin D sometimes prevents the common cold. Could it help ward off the new coronavirus?

The research world has mounted an electrifying response to the new coronavirus: Those playing a role include vaccine developers, drug researchers, gene sequencing labs, and the fields of public health and artificial intelligence. In the shadows of this effort is a less glamorous role—that of checking the archives of past research for clues to better treating and preventing COVID-19.

WORLD reader Dr. John Umhau thus calls our attention to a 2017 paper in The BMJ (formerly British Medical Journal). In it, a team performed a meta-analysis—basically, a study of other studies—investigating a common compound and its effect on immunity against respiratory infections like the common cold. The meta-analysis didn’t focus specifically on coronaviruses (a family of viruses that can also cause colds), but it may still apply to them. The drug in question? A humble compound we associate more with milk and bone health: vitamin D.

The meta-analysis combined 25 separate studies and went about it the hard way: Researchers obtained and reanalyzed individual participant data for all 25 studies, tallying up almost 11,000 study participants in what one of The BMJ’s peer reviewers called a Herculean effort. The paper is free to the public, but I’ll summarize it here: The results show that taking a moderate dose of vitamin D daily or weekly (but not more irregular, large doses) helps to protect users from respiratory infections. 

Unsurprisingly, vitamin D supplements were most helpful for study participants who started out deficient in the vitamin, but the supplements may help even those who aren’t. Equally important, people appeared to tolerate the vitamin well: The main problems linked to vitamin D supplementation were high blood calcium levels (in 1 in 200 participants) and kidney stones (in 1 in 500).

Time for a caveat. A figure called the “number needed to treat” (NNT) helps us understand how effective a treatment is. For daily or weekly vitamin D users in the meta-analysis, the overall NNT was 20, indicating a measurable but limited effect. Translating the research lingo into English, this figure means that 1 in 20 people taking vitamin D supplements could expect to avoid a cold because of it. But for study participants with “profound” vitamin D deficiency, the NNT was 4: Every fourth person taking the supplements likely dodged a cold.

So, the big question: Do I, as a doctor, advocate vitamin D supplements based on this research? In moderate doses, yes—especially for those at risk of vitamin D deficiency. (By this point in winter, that’s probably most of us.)

What’s a moderate dose? As the Mayo Clinic notes, “The recommended daily amount of vitamin D is 400 international units (IU) for children up to age 12 months, 600 IU for ages 1 to 70 years, and 800 IU for people over 70 years.” Mayo also offers useful information on potential drug interactions (that’s important to know if you’re on other medications) and cautions that over 4,000 IU per day of vitamin D increases the risk of harmful effects. For those who want to be certain they’re getting enough vitamin D, bloodwork can check whether they’ve hit the recommended level of 50 nanograms per milliliter.

None of this substitutes for the usual advice: Wash your hands frequently, don’t touch your face needlessly, and avoid being around sick people more than necessary. But given how cheap and safe vitamin D is, I’d recommend adding it as well.

In summary, while the research doesn’t yet prove vitamin D is effective against the new coronavirus strain, it does show the vitamin helps protect against common respiratory infections. With the coronavirus and other bugs going around, I plan to start taking vitamin D supplements myself, at least during the winter months.


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  • Heidi B
    Posted: Tue, 03/10/2020 02:34 pm

    What about simply getting more sunlight instead of taking vit. D pills? I assume that would eliminate side effects (high blood calcium levels and kidney stones), while also giving the other numerous benefits of being outside.

  • AlanE
    Posted: Tue, 03/10/2020 02:59 pm

    What Heidi said. Plus, spending more time outdoors and less time in crowded spaces indoors helps reduce the transmission vectors.

  •  fiddlexela's picture
    Posted: Tue, 03/10/2020 03:42 pm

    A medical study back in the early 2000s looked at health outcomes of identical twins as relates to VitD levels-- they were looking at cancer outcomes, which were noticeably different depending on residential latitude of the twin.  Overall resistance to respiratory ailments as well as lessening of depressive symptoms were also observed at that time. The study demonstrated that for our bodies to make enough VitD for these benefits from sunlight alone requires daily sun time equivalent to 30 minutes of total body exposure outside at the equator at noon.  So for most of us in the US outside of summer months, not really achievable! Esp not here in the PNW.  Supplementation super helpful for most people at least some of the year; occasional monitoring helps keep levels safe enough to avoid the mentioned side effects.