Studies investigating the role of vitamin D in preventing or treating COVID-19 have drawn conflicting conclusions. But should a lack of evidence stop us from topping up our vitamin D levels as the Northern Hemisphere heads toward winter?

Most people know vitamin D as an essential vitamin for healthy bones and teeth. But researchers have attributed a host of other functions to the vitamin, and one of these is supporting the immune system.

A systematic review and meta-analysis from 2017 in BMJ drew on data from 25 randomized controlled trials to look at whether taking a vitamin D supplement could prevent acute respiratory tract infections.

The international research consortium, led by Prof. Adrian R. Martineau, from the Centre for Primary Care and Public Health and the Asthma UK Centre for Applied Research, at Queen Mary University of London, in the United Kingdom, looked at data from nearly 11,000 study participants.

Prof. Martineau and colleagues concluded that “Vitamin D supplementation was safe and it protected against acute respiratory tract infection overall.”

But does vitamin D have a part to play in COVID-19? By now, a number of studies have looked for links between the vitamin and the condition, and their findings have conflicted.

In this Special Feature, we investigate why some experts have suggested a link between COVID-19 and vitamin D, and we dig deep to explore how convincing the evidence from the latest studies really is.

We also discuss whether taking a vitamin D supplement can have realistic benefits, particularly for those in communities that have been hit the hardest by COVID-19.

Why vitamin D?

A number of experts have cited the 2017 study as circumstantial evidence that vitamin D may have a protective effect against COVID-19.

The common thread is that they highlight that adequate vitamin D levels may help our immune systems fight off the SARS-CoV-2 virus, as with other viruses that cause upper respiratory infections. People with vitamin D deficiency may, therefore, not be able to do this as effectively.

One aspect of this is that it provides an elegant excuse about why people from marginalized racial and ethnic groups have been disproportionately affected by COVID-19, as some scientists have suggested.

There is already evidence to suggest that people with darker skin tones who live in Northern latitudes have inadequate vitamin D levels.

To make vitamin D, our bodies convert a metabolite of cholesterol in our skin cells into an inactive form of vitamin D when we are exposed to sunlight, specifically to ultraviolet B (UVB) light. This inactive form then undergoes further chemical modification in the liver and kidneys.

The pigment melanin that gives our skin its color stops UVB light from reaching the cells. Hence, the darker a person’s skin, the more UVB light they need to make adequate levels of vitamin D from sunshine alone.

A study in the American Journal of Clinical Nutrition found that 17.5% of Black study participants in the United States were classed as being at risk of vitamin D deficiency, a figure nearly 8.5 times greater than the percentage of their white counterparts who were at risk of the deficiency.

Data from the past few months have shown that in the U.S. and the U.K., Black people are more likely to die if they have COVID-19 than white people.

Given the relationship between vitamin D and respiratory infections, it is perhaps not unsurprising that many people have suggested a tentative link between the vitamin and the disease.

So, let’s look at the studies that have sought to investigate this link in more detail.

Evidence so far

Back in June, the National Institute for Health and Care Excellence, in the U.K., reported that “There is no evidence to support taking vitamin D supplements to specifically prevent or treat COVID‑19.”

The organization based their statement on data from a number of published studies, all of which they deemed to contain a “very low quality of evidence.”

In August, a research team from the University of Glasgow, in the U.K., looked at the vitamin D levels of 341,484 participants in the U.K. Biobank health data repository. Of these, 656 had been to the hospital with COVID-19, and 203 had died.

Once the authors accounted for confounding factors, they concluded that there was no link between vitamin D levels and the likelihood of needing hospitalization for COVID-19 or dying from the disease.

The main limitation, the team noted, was that the vitamin D measurements had been taken roughly 10 years earlier.

Also in August, researchers in Spain reported the results of a small clinical study looking at intensive care unit (ICU) admissions and vitamin D supplementation.

The team gave one group of patients a supplementary high dose of calcifediol, a precursor molecule to vitamin D, in addition to a range of drugs to treat COVID-19. The other group did not receive calcifediol.

“Of [the] 50 patients treated with calcifediol, one required admission to the ICU (2%), while of [the] 26 untreated patients, 13 required admission (50%),” the researchers reported.

While these numbers seem impressive, the study was small and has several limitations. One is that the vitamin D levels of the participants were not measured before and during the study. There were also differences in confounding factors, such as other health conditions, between the two groups.

In addition, the study was open label, so both the researchers and the participants knew who had received vitamin D, which leaves room for bias.

Medical reference: Medical News Today