Researchers around the world have reported anecdotal evidence that patients who develop the most serious cases of COVID-19 frequently have low levels of vitamin D. Does that mean your patients should take a supplement? Maybe.

Preliminary analyses indicate higher mortality rates in countries that had larger populations of individuals with vitamin D deficiency. In the U.K. and the U.S., for instance, the highest rates of death from SARS-CoV-2 infection have been reported in people of color. 

Humans primarily synthesize vitamin D from sunlight, though small quantities can be obtained from food sources, such as oily fish, red meat, egg yolks, and fortified milk and cereals. Individuals with darker skin more often have vitamin D deficiency, as higher melanin levels reduce the body’s ability to produce the vitamin from sunlight. Likewise, people living in long-term care facilities also frequently have low vitamin D as a result of lack of sun exposure. 

Suggestions of a benefit to boosting vitamin D levels have sparked at least eight new clinical studies that are currently in process. While no definitive answers are available yet, the evidence we do have supports an association.

Both U.K. and Irish studies that looked at vitamin D levels and COVID-19 in 20 European countries found that the higher the mean levels of vitamin D, the lower the number of cases and deaths per million. They specifically noted that vitamin D levels were “severely low” among the elderly in Spain, Italy, and Switzerland, populations devastated by the disease. Based in part on these studies, the U.K. government has recommended that people take vitamin D supplements during the pandemic.

A study of patients in Indonesia determined that the odds of dying from the novel coronavirus was 10 times higher in patients with vitamin D deficiency and seven times higher in patients with vitamin D insufficiency compared with patients with normal levels.

Researchers at Northwestern University in Chicago found that low levels of vitamin D were strongly associated with increased risk of cytokine storm, an uncontrolled over-response by the immune system that leads to destruction of healthy cells. Nonpeer-reviewed results of their study appeared on the preprint server medRxiv.

“This is what seems to kill a majority of COVID-19 patients, not the destruction of the lungs by the virus itself. It is the complications from the misdirected fire from the immune system,” said first author Ali Daneshkhah, PhD, a postdoctoral fellow at Northwestern.

As vitamin D has been shown to help regulate the immune system, the Northwestern researchers suggest that maintaining a healthy—not excessive—level of vitamin D could cut the risk of complications and death from COVID-19 in half.

So, should you recommend vitamin D for your patients?

“At best vitamin D deficiency will only be one of many factors involved in determining outcome of COVID-19, but it's a problem that could be corrected safely and cheaply,” said Adrian Martineau of Institute of Population Health Sciences, Barts and The London, Queen Mary University of London. “There is no downside to speak of, and good reason to think there might be a benefit.”

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