Vitamin D is vital for maintaining healthy bones and muscles. There is also some evidence that it may help protect against viral respiratory infections and play a regulatory role in the body’s immune response.
This has led to a proposal that taking vitamin D supplements could help prevent or even treat COVID-19, the primarily respiratory disease caused by the virus SARS-CoV-2.
While no clinical trials have tested the vitamin’s efficacy as a treatment or a preventive measure, several studies have found an association between low levels of vitamin D and COVID-19.
However, a review of five of these studies by the U.K.’s National Institute for Health and Care Excellence (NICE), which compiles guidelines on best practices, concludes that the studies provide no evidence that vitamin D levels influence the risk of getting COVID-19 or dying as a result.
None of the five studies had been designed to investigate the effects of the supplements on the risk of the infection or as a treatment for the disease, the authors point out. Either investigation would have required an intervention study, such as a randomized controlled trial.
The existing studies, therefore, provide no insights into the vitamin’s efficacy, appropriate doses, or possible adverse effects as a means of treating or preventing COVID-19.
Moreover, only one study in the review accounted for confounding factors, which may provide alternative explanations for the observed associations between vitamin D status and COVID-19.
Higher body mass index (BMI), older age, and socioeconomic deprivation, for example, are all factors that could affect both the risk of COVID-19 and levels of the vitamin.
This makes it impossible to draw any firm conclusion about whether having a lack of the vitamin increases the risk of contracting the virus that causes COVID-19 or dying as a result of the disease.
One of the studies, reported by Medical News Today, found an association between average levels of vitamin D and numbers of COVID-19 cases and deaths by country. But the research had limitations — it did not, for example, account for the proportions of older people in these populations.
The only study reviewed by NICE that had adjusted for potential confounding factors — including preexisting illnesses, sociodemographic factors, ethnicity, and BMI — found no independent association between COVID-19 and vitamin D levels.
As the authors of the review observe:
“There is no evidence to support taking vitamin D supplements to specifically prevent or treat COVID-19. However, all people should continue to follow U.K. government advice on daily vitamin D supplementation to maintain bone and muscle health during the COVID-19 pandemic.”
The review focused exclusively on studies that had been peer reviewed and published in scientific journals.
“Vitamin D enthusiasts will point to a large number of other studies, published on preprint websites, that were not included in this review,” says Prof. Adrian Martineau, a clinical professor of respiratory infection and immunity at Queen Mary University of London, who was not involved in the NICE review.
“However, these studies have not yet undergone peer review, so their findings should not be relied on to guide clinical practice or public health policy.”
Only about 10% of the vitamin D in our bodies originates from food. The remainder is synthesized in the skin through exposure to ultraviolet light in sunlight.
Research suggests that the U.K. population has one of the highest levels of vitamin D deficiency in Europe.
The country’s National Health Service, therefore, recommend that people take a daily supplement containing 10 micrograms of vitamin D during the autumn and winter months, when sunlight exposure may be limited.
But its latest advice says that people should consider taking a supplement during the summer months, as well, if they are spending more of their time indoors as a result of the pandemic.
The aim of this recommendation is to support bone and muscle health, rather than to protect against COVID-19.
In a preprint review, The Royal Society, in London, has called for more research into the possibility that vitamin D deficiency predisposes people to COVID-19.
The review cites several lines of evidence to suggest that people with the deficiency may be more susceptible to the disease:
“Vitamin D deficiency is associated with an increased risk of both respiratory viral infections and inflammatory conditions. Vitamin D has an important regulatory role in the human immune system, so a deficiency of vitamin D is likely to cause immune dysregulation, which may reduce the first line of our defense against COVID-19. It is therefore biologically plausible that vitamin D deficiency may contribute to susceptibility to COVID-19 infection. However, there is no direct causal link yet between vitamin D deficiency and increased susceptibility to COVID-19.”
The authors point out that vitamin D deficiency is more common among older people, those who are Black or of Asian origin, and those who have obesity. These groups of people are also known to have an increased risk of developing severe COVID-19.
Nonetheless, the scientists emphasize that “Correlations are not the same as causality.”
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