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.”