A review from health experts in the United Kingdom has found no evidence that vitamin D deficiency is an independent risk factor for COVID-19. However, the country’s National Health Service recommend taking a daily vitamin D supplement to compensate for any lack of sunlight exposure during lockdown.

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.

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, 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.