Vitamin D to fight COVID-19? Not so fast.

The Holy Grail of COVID-19 care is a safe, effective, inexpensive, widely available, and easily administered medicine to prevent or treat the infection.  

No magic bullet like this has been found for scourges like malaria, tuberculosis, or polio, but there’s always a market for people claiming that there’s a miraculous cure right under our noses.  

Remember hydroxychloroquine? Yes, there were anecdotes and observational studies, but, no, it flunked the gold standard for evidence, randomized controlled trials. Ditto with ivermectin. 

Now the man named “the most influential spreader of coronavirus misinformation online” is promoting another candidate for magic bullet: vitamin D.  

Joseph Mercola, who’s made a fortune selling dietary supplements on the web, claims in a September article in the right-wing Epoch Times that “there is simply no question that vitamin D optimization is a crucial component of COVID-19 prevention and treatment.” 

In 2021, the Food and Drug Administration warned him not to suggest that his vitamin D products could prevent or treat COVID-19. Since then, Mercola, an osteopathic physician who donated millions of dollars to the anti-vaccine movement before the pandemic, has gotten himself banned from YouTube in part because of his anti-COVID vaccine misinformation. 

Now he's back, touting four, partially overlapping meta-analyses of vitamin D and COVID-19 research that he thinks are so compelling for vitamin D preventing and treating the coronavirus infection that “we do not actually need randomized controlled trials.” Really?  

Two of Mercola’s meta-analyses looked at whether the risk of severe COVID-19 illness was associated with low vitamin D levels, a third examined whether vitamin D supplementation was associated with a lower risk of severe COVID-19 illness, and a fourth looked at both vitamin D supplementation and blood vitamin D levels and COVID-19. Together, they consist of a total of 89 observational studies and 2 randomized controlled trials.  

Observational studies follow individuals and record some of their health outcomes. They’re easier and cheaper to do than randomized controlled trials, but they can have so many different variables that it’s a challenge to figure out what causes what, if anything. That’s because they’re conducted in the real world and people in the real world are hugely complex with respect to their demographic characteristics, underlying medical conditions, and the medications they take.  

Observational studies can be useful, but they have been dead wrong in the past. For example, one suggested that large amounts of beta-carotene could prevent cancer until randomized controlled trials found that the nutrient didn’t and instead could cause lung cancer in smokers. Another suggested that hormone replacement therapy was an effective way of relieving menopausal symptoms until randomized controlled trials found that it could also increase the risk of coronary heart disease and breast cancer.  

The authors of Mercola’s four meta-analyses were candid about the quality of the observational studies they were analyzing. They variously pointed out that the studies present “a high risk of bias,” their quality was “medium to low,” the results may simply mean that vitamin D is a marker of good health, rather than being a cause of good health, and that the results should be interpreted “cautiously.” 

Randomized controlled trials, on the other hand, control who they study and test specific treatments under rigorous conditions to see if they’re more effective than a placebo, another treatment, or doing nothing at all. Randomization assigns patients to the treatment (e.g., Vitamin D) or a control group and essentially ensures that the two groups are very similar with respect to all other characteristics.  

Two of Mercola’s meta-analyses called for the evaluation of vitamin D in well-designed randomized trials. One said this was “urgent.”  

In fact, we do have some published randomized controlled trials and more are on their way. 

The British Medical Journal released, coincidentally on the day Mercola’s Vitamin D article appeared, the results of two huge randomized controlled trials testing whether vitamin D supplementation could prevent COVID-19 infections.  

Researchers in the United Kingdom randomly assigned over 6,000 volunteers to either daily doses of 3,200 IU or 800 IU of vitamin D or no supplementation. After 6 months, vitamin D takers were just as likely to catch COVID-19 as the no-supplementation group, regardless of dose.  

And researchers in Norway randomly assigned 34,601 subjects to take either cod liver oil containing 400 IU of vitamin D or a placebo oil every day. Over the next 6 months, the vitamin D did not protect participants from getting COVID-19. 

And whether giving vitamin D to hospitalized COVID-19 patients can prevent their illnesses from getting worse has been tested in three published randomized controlled trials.  

A study in Spain, Argentina, Guatemala, and Chile of 543 COVID patients randomly given at hospital admission either vitamin D or no vitamin D found that the vitamin did not shorten their hospital stay and did not prevent their admission to the ICU or reduce their death rates from COVID-19. 

Likewise, in a Brazilian study of 237 COVID patients, randomly given at admission either 200,000 IU of vitamin D or a placebo, the vitamin did not shorten their hospital stay or prevent their admission to ICU, or the need for ventilation or death from COVID-19. 

And in the smallest trial, a Spanish study in which 76 COVID-19 patients were randomly given either vitamin D or no vitamin D, the vitamin did lessen the risk of admission to the ICU from 50% to 2%, but did not prevent death from COVID. 

Summing up the data, the National Institutes of Health (NIH)  concludes that there is “insufficient evidence” to recommend for or against vitamin D  for the prevention or treatment of COVID-19. 

But this isn’t the final word. Two U.S. clinical trials are due to wrap up soon. The VIVID trial run by researchers at Harvard and elsewhere is testing in 2,024 subjects whether vitamin D supplementation can reduce the severity of COVID-19 infections. Note that this is four times as many subjects as even the largest of the existing treatment trials. And researchers at the University of Chicago and Rush University Medical Center are testing in 2,000 subjects whether vitamin D can prevent COVID-19 infections. Stay tuned. 

Getting enough vitamin D is certainly important for our general health. But until we know more, it would be folly to rely on it to prevent or treat COVID-19 infections when we have effective vaccines and treatments.