Does vitamin D protect against COVID-19?

We’ve been reading a lot lately about vitamin D supplementation. Some doctors suspect vitamin D could protect against COVID-19 since vitamin D can also have a positive effect on influenza. Others have suggested that vitamin D deficiency may play a role in the excess of deaths from COVID-19 among people of color in the United States and the United Kingdom. But could vitamin D supplements be the next hydroxychloroquine?

While lots of doctors seem to think vitamin D supplements are a good idea, many other sources have indeed put vitamin D and hydroxychloroquine in the same category. We think this is a false equivalence. Hydroxychloroquine is an unproven and even dangerous idea for self-medication that the FDA pulled from emergency use. Vitamin D is a necessary micronutrient in which many people are deficient.

We don’t yet know if vitamin D could really protect against COVID-19, but we looked into the studies and collected the best information out there. In the end, here’s what we recommend for finding the right dose of vitamin D as COVID-19 infections start to surge again:

1. If you can, ask your physician to measure your vitamin D blood levels. Then, ask whether they recommend taking a vitamin D supplement. If it’s not possible to go to the doctor for a blood test, schedule a telehealth appointment to find out what your doctor recommends.
2. If you already knew you were vitamin D deficient or borderline deficient, take your vitamins!
3. If you don’t know your own vitamin D blood level and have been quarantining indoors, order a vitamin D supplement. Start with a low dose until you can talk with your doctor. Most medical professionals recommend taking 1-2 tabs of 1,000 IU per day.

Note: We’d be skeptical of anyone who recommends taking more than 4,000 IU of vitamin D. While you may find some experts recommending 10,000 IU of vitamin D, you can also find doctors who warn against it. You should always talk to your doctor before making any changes to your medications.

What’s a normal vitamin D level?

Regardless of the current COVID-19 epidemic, supplementation with vitamin D is recommended by many doctors, especially—but not only—for people whose skin contains more melanin. That’s because vitamin D deficiencies can make people more susceptible to the flu and other viruses. In children, vitamin D deficiency can lead to rickets. In adults, vitamin D deficiencies have been linked to bone disorders like osteoporosis.

What is less well known is that vitamin D also plays a role in our immune system, both in B-cell proliferation, the type that makes antibodies, as well as in T cells, which offer cellular immunity often implicated in coronavirus response. Vitamin D hence protects against infections: People with lower vitamin D levels are more likely to have upper respiratory tract infections.

We should all increase our vitamin D levels and take supplements then, right? Well, not quite. Doctors actually disagree about how much vitamin D is the ‘right amount’ to have in your blood.

Some of the literature says that only under 10 ng/ml is deficient, while others say anything less than 25 or 30 ng/ml is too little. For each report of deficiency below, we need to remember what level different doctors use to define deficiency. Similarly, doctors disagree on how much vitamin D is too much.

Just to confuse everyone, there are two sets of units used, ng/ml (or ng/mL) and nmol/Liter. Here are the levels used in the US clinical primary care for adults:

Supplementary vitamin D is usually sold in international units, IU, or in terms of µg a day.

The good news is, the range between deficiency and toxicity is quite high, so it is not so easy to get too much vitamin D. But unless you’ve had your vitamin D measured with a blood test, you can’t know what your level is and therefore what dosage you should take.

Why do people have different levels of vitamin D?

Vitamin D is a micronutrient that we cannot routinely get from natural food (except fortified food such as milk, egg yolks, and some oily fish). Rather, most of our vitamin D is made in our skin cells when we expose ourselves to sunlight.

Before you go out in the sun for hours, though, there’s a catch. Prolonged exposure to the sun can lead to sunburns. Those sunburns, particularly when they occur in childhood, can later lead to skin cancers. That’s why we now recommend everyone use sun protection when outside. That’s good news for skin cancer risk, but wearing sunscreen also blocks the natural production of vitamin D.

Melanin and vitamin D

Melanin is the dark pigment in our skin cells that determines our skin color and how much vitamin D our bodies naturally produce. Melanin absorbs the harmful rays of the sun and thus protects against skin cancer.

Before colonization and migration, people who lived in hot climates like central Africa, India, and Australia, developed more melanin in their skin. The sun below the equator is so strong that their skin needed to be protected from its harmful effects–but due to the strong sun, they could still make enough vitamin D even when most of the sunlight was blocked.

By contrast, in areas with little sun, like England, Ireland, and Scandinavia, skin tone tended to be lighter, with far less melanin. That maximized the production of vitamin D even in the presence of very little sunlight.

Over history, people have moved around the world due to colonization and migration. Today, many people of African or South Indian descent live in cloudy Michigan, and people of Scottish-Irish heritage live in sunny Australia. As a result, Australia has the highest incidence of skin cancer in the world. On the other hand, African Americans living in cloudy Michigan can end up being highly vitamin D deficient.

Deficiencies in the United States and elsewhere

Vitamin D levels on the whole have decreased in the US between 1988 and 2004, from 45% to 23% of the population with levels above 30 ng/ml. That’s probably because we now understand the link between skin cancer and sun protection, so we wear more sunscreen and are careful in the sun. In those years, white people were found to have the highest levels of vitamin D. Black Americans, however, had the lowest levels of vitamin D, and Mexican Americans an intermediate level of vitamin D.

Before 2001, more than 50% of white Americans had optimum levels of vitamin D (>30 ng/ml). In 2001-2004, as many as 30% of black Americans had a level below 10 ng/ml. Only 4% of white people had those low levels.

From 2001-2004, only 2% of Black Americans had optimum levels (>30 ng/ml). In 2018, another study reported that nearly half of Americans were vitamin D deficient (<20 ng/ml). Confirming earlier studies, race was a significant factor. Around 80% of black Americans were found to be vitamin D deficient.

A recent large study in Ireland reported that more than 50% of frail elderly in Ireland are vitamin D deficient (<25 ng/ml) in the winter. Large European studies have found that overall 13% of Europeans are vitamin D deficient. When the studies account for race, however, 40% of black and brown Europeans are vitamin D deficient.

In addition to years of increasing vitamin D deficiency rates, the COVID-19 pandemic has meant park closures. Some of us, particularly the elderly, have been going out in the sun even less than usual. Though we haven’t (yet) found studies to prove it, our vitamin D may now be lower than ever.

Is there a link between vitamin D, race, and COVID-19?

Early in the COVID-19 pandemic, British doctors began testing the relationship between vitamin D and COVID-19 in 300,000 UK Biobank patients. When they examined the first 500 cases within the biobank, researchers found a small but significant effect of vitamin D on COVID-19 infections. When the researchers corrected for race, that effect went away.

Now, to be clear, vitamin D doesn’t appear to be the only (or even the most significant) reason people of color are at higher risk of contracting or dying from COVID-19. Vitamin D is not the major reason for racial differences in the number of COVID-19 cases in the UK (and probably not in the US, either).

The UK Biobank study looked at only 500 people and only at whether or not they tested positive for COVID-19, not how severely. So this study doesn’t address whether, once infected, vitamin D deficiency increases one’s risk of becoming more severely ill or dying of COVID-19.

Several studies, mostly not yet peer reviewed and only published in a pre-publication server, have recently claimed that vitamin D insufficiency is common, in particular among ICU patients. One very small study in the US found that most young patients admitted to the ICU with COVID-19 also had vitamin D deficiencies.

A caveat—we want to be careful here and not draw conclusions too quickly. We’ve known for a long time that the risk of COVID-19 ICU admission is higher for black people, and those with obesity, the elderly, and people with underlying health concerns. Those same people are also more likely to be vitamin D deficient. Now, it could be that in a year’s time we’ll say, “Oh! It’s because those people were vitamin D deficient!”

But it’s also possible that we’ll say, “It was the other conditions that made them more likely to contract and die from COVID-19 at higher rates. The vitamin D deficiency was just a coincidence.” In science, correlation does not equal causation. This may be a case where other comorbidities lead to hospitalization, and those people also happen to be deficient in vitamin D. We have to be careful before drawing a conclusion like this too quickly.

However, a larger, better controlled study of 263 patients in Belgium also corrected for age and gender and found that vitamin D deficiency increased COVID-19 risk, but only in men.

Supplementation recommendations during the COVID-19 pandemic

Based on all these considerations, a study in the Irish Medical Journal made a strong recommendation for the general adult population to supplement with vitamin D. But on the other hand, the same journal that published the findings had strong objections to this blanket recommendation. A group of scientists have recently recommended against supplementing with more than 4,000 IU of vitamin D per day.

None of these studies makes the distinction between vitamin D2 and D3. Vitamin D3 is more effective at raising overall blood levels of vitamin D.

Another issue not addressed here: once you are vitamin D deficient, it can take a year to get to good blood levels. That’s true even if you take a large supplement of 4000 IU/day. Getting ‘back to normal levels’ takes even longer if you only take the recommended 400 IU/day.

So let’s summarize what we know and what we don’t know.

• Many people in the US and the world are vitamin D deficient.
• Because there is no consensus of what constitutes deficiency or optimal levels, it’s unclear how many people are vitamin D deficient, but 42% of the US population is the most commonly cited number.
• Since the sun is the best way to get vitamin D, and many people are outside less during lockdown, it’s possible that more people are deficient right now.
• Increasingly, the data shows that people with severe COVID-19 are also likely to have vitamin D deficiency. But whether this is correlation is spurious or causal is still unclear.
• It’s certain that vitamin D levels of  <60 ng/ml are safe. This is a lower boundary, however, as many doctors say even more is also safe.
• A blood level of more than 150 ng/ml of vitamin D in blood is toxic.
• Most internationally recognized organizations recommend 10 µg/day (400 IU) for those not exposed to sun at all, and 20 µg/ day (800 IU) for frail elderly, i.e. those particularly at risk from COVID-19.
• There are growing concerns that these recommendations are too low.
• There is a “general agreement” that 4000 IU/day is the upper safe limit to take.
• Many but not all experts agree with a consensus that “…consuming 1,000–4,000 IU (25–100 mcg) of vitamin D daily should be ideal for most people to reach healthy vitamin D blood levels”.

Taking moderate levels of vitamin D is cheap and relatively safe. Vitamin D deficiencies are common in the United States, so taking a supplement is generally recommended. It’s possible, but the jury is still out on whether vitamin D deficiency increases vulnerability to COVID-19 infection or severe disease, and whether or not taking vitamin D in reasonable amounts will help you ward off COVID-19. For now, we’ll keep taking our vitamins.