Vitamin D Deficiency and COVID-19: Is There a Connection?

When it comes to COVID-19, the world is eager for a cure—or even a quick fix to prevent infection, but there isn’t enough evidence to suggest that either has much to do with vitamin D, Yale Medicine physicians say.  

A number of recent studies have implied that vitamin D, which helps the body absorb calcium to strengthen bones, could play a role in COVID-19—from preventing infection to making the disease less severe.

But some of the reports are preliminary and have not been peer-reviewed, points out Kathleen Suozzi, MD, a Yale Medicine dermatologic surgeon. Dr. Suozzi says she worries that news coverage about the studies will cause people to sunbathe too much or take dangerous levels of vitamin D supplements. People get vitamin D from the sun, from eating foods that have it naturally or are fortified with it, and from supplements.

Too little vitamin D can lead to bone disorders in both adults and children; too much (from food or supplements) can be toxic and lead to heart and kidney problems, among others. Plus, trying to get too much of it from the sun puts people at risk for skin cancer.

“Healthy people can get adequate vitamin D from their diets and from reasonable amounts of intermittent sun exposure, like going for a walk outdoors. It is not necessary to sunbathe to get adequate levels of vitamin D,” Dr. Suozzi says, explaining that those who are most likely to be vitamin D deficient are older, have a chronic health condition, or have darker skin, which does not absorb UV rays as well as lighter skin. “As a dermatologist, my main concern heading into summer is that after being cooped up for months due to the pandemic, someone who is healthy and not vitamin D deficient reads pieces of these studies and goes out and bakes in the sun with the false hope that excess vitamin D will provide COVID-19 protection.”

Regarding the recent vitamin D and COVID-19 studies, Dr. Suozzi provided a closer look at one that came out of Northwestern University. It examined 10 countries with high COVID-19 mortality rates and noted that those countries have higher levels of vitamin D deficiency compared to countries that were not as severely affected by COVID-19.

“In this study, vitamin D levels were not measured in COVID-19 patients. Instead, they used historical vitamin D deficiency rates for those countries and correlated them with mortality rates from COVID-19,” says Dr. Suozzi. Elsewhere, a study from the University of Chicago focused on patients admitted to the hospital for COVID-19 and looked at their vitamin D levels from the past year, including which vitamin D deficient patients received a supplement. The study concluded that vitamin D deficiency that is not sufficiently treated is associated with COVID-19 risk.

“When considering the results of these studies, it is important to assess potential confounding variables,” says Dr. Suozzi, speaking of variables not accounted for in a study that may lead it to suggest correlations that don’t really exist. “Vitamin D deficiency is more common in the elderly population and in African American patients, and we also know COVID-19 morbidity and mortality is worse in these groups. In addition, vitamin D deficiency is associated with numerous chronic diseases, which we know also affect COVID-19 severity.”

In other words, there could be a connection, but we don’t yet have enough information to say that one causes the other, Dr. Suozzi adds.

So, while vitamin D may prove to play a role in COVID-19 severity, more and different types of studies are needed to understand if there is a causal connection. A more convincing study, Dr. Suozzi says, would be one that is prospective (meaning it looks at outcomes, like the development or cure of a disease, over a defined time period) and uses randomized controlled groups. “You could take patients hospitalized for COVID and test their vitamin D levels and then randomly assign them to a vitamin D supplement or no supplement and that could show us better if vitamin D plays a role,” she says.

Prior to COVID-19, research has examined vitamin D deficiency in the context of severe illness, particularly among patients in hospital intensive care units, notes Thomas Carpenter, MD, a Yale Medicine pediatric endocrinologist. “Vitamin D deficiency has also been implicated in asthma and other respiratory disorders,” he adds.

And since COVID-19 can cause serious respiratory symptoms, it’s no surprise that vitamin D has been discussed—but the connection between vitamin D deficiency and COVID-19 is not clear, Dr. Carpenter says.

“There has to be a lot of caution about this,” he says. “It’s reasonable that if someone is really deficient in their vitamin D levels, we supplement them to a normative level and maybe that could be an adjunct to help someone, but it is premature to say this is a panacea.”

While vitamin D is believed to play a role in the immune system, there is much that is not understood, Dr. Carpenter notes. “Some advocate for high-dose vitamin D therapy, saying it 'revs up' your immune system in a healthy, protective way,” he says. “But the data for that is pretty slim. And you could also make the argument that driving up the immune response could actually be harmful in COVID patients.”

Indeed, an overactive immune response is what some believe may cause persistent high fevers, severe respiratory problems, and lung damage in severe COVID-19 cases.

“There are just so many questions when it comes to COVID-19 that I certainly do not think it would be appropriate to take large doses of vitamin D as a preventive measure,” Dr. Carpenter says. “And if you put this in the context of how we have looked at all of the other illnesses that vitamin D has been purported to have an effect on, the most reasonable approach is to keep yourself at normal levels.”

According to the National Institutes of Health, normal levels of vitamin D, which can be measured by a blood test, are between 20 and 50 nanograms/milliliter. For most people, the recommended daily allowance (RDA) is 600 IU (international units) per day, or 800 for those over age 70. In their first year, babies who are exclusively or partially breastfed should be supplemented with 400 IU of vitamin D per day.

“For people who don’t get the correct daily amount from their diet, the problem is that there is no 600 IU supplement pill,” Dr. Carpenter says. “I tell patients to take a 1,000 IU pill five times a week to get the same amount.”

In the end, Dr. Carpenter says there is little harm in taking a supplement that is essentially equivalent to the 600 IU. But, if you are concerned about your vitamin D levels, talk to your physician, Dr. Suozzi says.

Click here to learn more about Yale’s research efforts and response to COVID-19.