The link with COVID-19


It's estimated as much as 40% of the U.S. population doesn't get enough vitamin D and as many as 1 billion people worldwide have deficient levels.

Early in the pandemic, researchers noticed the overlap between populations that were at high risk of severe illness from COVID-19 and those likely to have vitamin D deficiency, in particular people who are overweight, elderly and those with darker skin.

It sparked a rush of commentary and academic articles on whether boosting vitamin D levels could help shield certain vulnerable people from coronavirus infection.

There are now quite a few observational studies and large reviews of the available evidence that show low vitamin D levels are associated with higher risks of contracting COVID-19 or with becoming seriously ill.

"What is clear from a number of studies is that there's a strong relationship in terms of your levels prior to infection," said Dr. Shad Marvasti, a professor of family and preventive medicine at the University of Arizona College of Medicine in Phoenix.

Low levels of vitamin D are associated with an increase in cytokines — "cell to cell chemical messengers that are responsible for inflammation" — and lower levels of protective immune cells, Marvasti said.

A study of 489 patients published in JAMA Network Open in September found "the relative risk of testing positive for COVID-19 was 1.77 times greater" for patients who were likely vitamin D deficient compared with those with sufficient levels.

"That was really very striking," said Dr. David Meltzer at the University of Chicago, who was the lead author of that study. "I started taking it and telling all my family and friends."

In another recent study, Meltzer has also found that Black individuals with high levels of vitamin D were less likely to test positive than those who had levels traditionally considered sufficient.

Another small study of patients hospitalized for COVID-19 in Spain found over 80% had vitamin D deficiency, compared with 47% of the general population; however, it did not find any relationship between vitamin D levels and the severity of disease.

"If I had money on it, I would bet that it's more likely that vitamin D is helpful than not in COVID, but I don't know for sure," Meltzer said.