In previous editorials I have expressed doubts about the safety and efficacy of using high-dose vitamin D for the prevention and treatment of various skeletal and non-skeletal diseases. Results from randomized controlled trials suggest that large doses of vitamin D (such as 4,000 IU per day or more) are generally not more effective, and may even be less effective, than moderate doses (such as 800 IU per day). In addition, long-term use of high-dose vitamin D may increase the risk of developing kidney stones and (at least according to animal studies) atherosclerosis.

It has been hypothesized that vitamin D could help prevent or decrease the severity of COVID-19 infection by regulating the renin-angiotensin system and by enhancing innate or adaptive immunity. In vitro studies suggest that vitamin D may also decrease the severity of the cytokine storm, which contributes to morbidity and mortality in patients with severe COVID-19 disease. In observational studies, COVID-19 patients with lower serum concentrations of 25-hydroxyvitamin D had worse outcomes, when compared with patients with higher 25-hydroxyvitamin D levels. This latter finding does not necessarily suggest that vitamin D sufficiency confers protection against COVID-19. That is because 25-hydroxyvitamin D levels decline in response to inflammation, so a low of 25-hydroxyvitamin D level may be an effect, rather than a cause of more severe disease. Nevertheless, because vitamin D is inexpensive, usually safe for short-term use even in relatively high doses, and widely available throughout the world, its potential as a treatment for COVID-19 deserves serious consideration. At the time of this writing, two clinical trials have been published, and the results are very encouraging.