There is some encouraging evidence regarding the positive influence of vitamin D on COVID-19. But is it convincing? Is it causal? As always, it is worth looking at the totality of the reliable evidence.
In this systematic review and meta-analysis, the researchers analyze the association between vitamin D deficiency and COVID-19 severity. They conducted an analysis of the prevalence of vitamin D deficiency and insufficiency in people with the disease. Five online databases—Embase, PubMed, Scopus, Web of Science, ScienceDirect and pre-print Medrevix were searched. The inclusion criteria were observational studies measuring serum vitamin D in adult and elderly subjects with COVID-19. The main outcome was the prevalence of vitamin D deficiency in severe cases of COVID-19.
The researchers identified 1542 articles and 27 met their inclusion criteria. The results show that
- vitamin D deficiency was not associated with a higher chance of infection by COVID-19,
- severe cases of COVID-19 present 64% more vitamin D deficiency compared with mild cases,
- vitamin D concentration insufficiency increased hospitalization and mortality rates,
- There was a positive association between vitamin D deficiency and the severity of the disease.
The authors concluded that the results of the meta-analysis confirm the high prevalence of vitamin D deficiency in people with COVID-19, especially the elderly. We should add that vitamin D deficiency was not associated with COVID-19 infection. However, we observed a positive association between vitamin D deficiency and the severity of the disease. From this perspective, evaluating blood vitamin D levels could be considered in the clinical practice of health professionals. Moreover, vitamin D supplementation could be considered in patients with vitamin D deficiency and insufficiency, if they have COVID-19. However, there is no support for supplementation among groups with normal blood vitamin D values with the aim of prevention, prophylaxis or reducing the severity of the disease.
These are interesting findings, no doubt. They relate to associations, as the authors repeatedly stress in the text of the paper. They do not, however, signify cause and effect relationships. The principal outcome of this research should be a hypothesis that subsequently needs testing in clinical trials.
So, why on earth did the authors chose that seriously misleading title of their paper? It clearly implies a causal effect; and this can only be verified by conducting clinical trials. One such study has been published (as discussed here) and it concluded that administration of calcifediol may improve the clinical outcome of subjects requiring hospitalization for COVID-19.
My conclusion: it seems well worth conducting more and more rigorous clinical trials.