The study retrospectively examined the relationship between vitamin D levels and likelihood of testing positive for COVID-19 in 3,000 patients at UChicago Medicine who had had their vitamin D levels tested within 14 days before a COVID-19 test. While levels of 30 ng/ml or more are usually considered sufficient, Black individuals who had levels of 30 to 40 ng/ml had a 2.64 times higher risk of testing positive for COVID-19 than people with levels of 40 ng/ml or greater. Statistically significant associations between vitamin D levels and COVID-19 risk were not found in white people.
This research is an expansion of an earlier study, which found that vitamin D deficiency—less than 20 ng/ml—may raise the risk of testing positive for COVID-19. The current study found the same, noting that individuals with a vitamin D deficiency had a 7.2% chance of testing positive for the virus. A press release about the study notes that separate research found that over 80% of patients diagnosed with COVID-19 were vitamin D deficient.
"These new results tell us that having vitamin D levels above those normally considered sufficient is associated with decreased risk of testing positive for COVID-19, at least in Black individuals," said David Meltzer, M.D., Ph.D., Chief of Hospital Medicine at UChicago Medicine and lead author of the study, in the press release. "This supports arguments for designing clinical trials that can test whether or not vitamin D may be a viable intervention to lower the risk of the disease, especially in persons of color."
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Dr. Meltzer raised a question: What does it mean to be vitamin D sufficient? “There’s a lot of literature on vitamin D,” he says in the press release. “Most of it has been focused on bone health, which is where the current standards for sufficient vitamin D levels come from. But there’s also some evidence that vitamin D might improve immune function and decrease inflammation. So far, the data has been relatively inconclusive. Based on these results, we think that earlier studies may have given doses that were too low to have much of an effect on the immune system, even if they were sufficient for bone health. It may be that different levels of vitamin D are adequate for different functions.”Currently, the adult recommended dietary allowance for vitamin D is 600 to 800 international unites (IUs) per day, Dr. Meltzer states. While some individuals—Dr. Meltzer points to lifeguards and surfers—may have more than sufficient vitamin D levels, roughly half of the world’s population has levels below 30 ng/ml, particularly people with darker skin. And it is important to take this into account or to have vitamin D levels tested before upping vitamin D intake: Excessive consumption of vitamin D supplements is associated with hypercalcemia, a condition in which calcium builds up in the bloodstream and causes nausea and weakness. Hypercalcemia, left unchecked, can lead to bone pain and kidney stones. Dr. Meltzer cites the National Academy of Medicine, which says that taking up to 4,000 IUs per day is safe for the vast majority of people; risk of hypercalcemia increases at levels over 10,000 IUs per day.
One of the limitations of this study is that it is observational, so while researchers can see an association, they cannot determine causation—whether or not vitamin D directly affects COVID-19 risk. As such, a team at the University of Chicago and Rush University is conducting two studies to assess whether or not taking a daily vitamin D supplement can help prevent COVID-19 or decrease the severity of its symptoms.
