A preliminary, unpublished study speculates that vitamin D deficiency may have something to do with poor COVID-19 outcomes. Its authors suggest people may benefit from ensuring they are getting enough vitamin D. However, there are serious concerns about the research.

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A new study wonders whether vitamin D deficiency might play a role in COVID-19 outcomes.

Vitamin D is one of the nutrients that are crucial to human health, on the whole.

The human body naturally synthesizes this vitamin — in fact, a group of substances — through adequate exposure to sunlight.

Only a few foods, such as egg yolks, can be a source of vitamin D, which means that people who have little access to sunlight may experience vitamin D insufficiency or deficiency.

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To prevent or address a lack of vitamin D, people can take vitamin D supplements under the guidance of a healthcare professional.

Inadequate levels of vitamin D can cause problems in the bones, as well as issues such as hair loss and joint pain.

Now, three researchers from the United Kingdom have put forward another idea: Could science link insufficient vitamin D with COVID-19 and, more specifically, COVID-19 deaths?

Dr. Petre Cristian Ilie, from The Queen Elizabeth Hospital Foundation Trust in King’s Lynn, Dr. Simina Ștefănescu, from the University of East Anglia in Norwich, and Lee Smith, Ph.D., from Anglia Ruskin University in East Anglia present their preliminary study online in preprint form.

In their study, the researchers first identified the mean levels of vitamin D for the inhabitants of 20 European countries.

They then looked at whether they could find any associations between these and data around the number of COVID-19 cases in each country, as well as the number of COVID-19 deaths.

The research team reports that, according to their observations, “the mean level of vitamin D in each country was strongly associated” both with a higher number of COVID-19 cases and with higher mortality due to the disease.

The investigators claim that countries such as Spain and Italy, which have among the highest numbers of COVID-19 cases in Europe, demonstrate low mean levels of vitamin D in blood in their populations.

Based on this association, the researchers hypothesize that vitamin D may help protect against contracting SARS-CoV-2 or experiencing severe outcomes after developing COVID-19.

In their online paper, they write: “In conclusion, we found significant relationships between vitamin D levels and the number COVID–19 cases and especially the mortality caused by this infection. The most vulnerable group of population for COVID–19 is also the one that has the most deficit in Vitamin D.”

They continue:

“Vitamin D has already been shown to protect against acute respiratory infections, and it was shown to be safe. We believe that we can advise vitamin D supplementation to protect against COVID–19 infection.”

There are several fundamental concerns about the methods and conclusions in this paper, however. For instance, it is unclear how the researchers derived the average vitamin D level for each country.

The authors reference a paper in the European Journal of Endocrinology as their source of average vitamin D levels. Yet this paper has several values for some countries. It is also unclear if the authors calculated an average or selected one of the values. Crucial details about the methodology are missing.

Older people are at risk of having low vitamin D, for instance, due to reduced dietary intake of vitamin D and less time spent outdoors. They are also at greater risk of dying from COVID-19.

This study does not demonstrate a link between these two facts.

Older people may be dying of COVID-19 due to higher rates of other health conditions that are independent of their vitamin D levels.

That is not to say that vitamin D does not play a role in how likely a person is to contract the SARS-CoV-2 virus or die from COVID-19. More studies are needed to explore a possible link in greater depth.

Indeed, a team at The Centre for Evidence-Based Medicine at the University of Oxford in the U.K. published a Rapid Review on vitamin D as a method to prevent or treat COVID-19 on May 1, 2020.

“We found no clinical evidence on vitamin D in COVID-19,” they write. “There was no evidence related to vitamin D deficiency predisposing to COVID-19, nor were there studies of supplementation for preventing or treating COVID-19.”

The authors of the current research note, themselves, that their study faced limitations, including the fact they had no clear way of verifying the true number of COVID-19 cases in each of the countries.

So, while it may be tempting to turn to an easily obtainable dietary supplement to help us keep SARS-CoV-2 at bay, it may be safer to hold off on the enthusiasm for now.

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