A person holding a vitamin D capsule.

Vitamin D has been promoted as a cure-all. You may have seen headlines claiming that taking vitamin D can help prevent or even treat COVID-19, but there’s no solid science to support that yet. A paper recently published  in BMJ Nutrition, Prevention & Health indicated that while everyone should strive to get enough of the vitamin, there’s still a dearth of research showing a beneficial effect on COVID-19.

But there's a connection between vitamin D levels and the risk of respiratory infections in general. The vitamin plays many roles throughout the body. “It supports a range of antiviral responses,” says Adrian Martineau, Ph.D., a clinical professor of respiratory infection and immunity at Queen Mary University of London. It boosts the ability of lung cells to fight bacteria and viruses, among other things, he says.

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Martineau was the lead author of a 2017 analysis of 25 studies looking at the vitamin and respiratory illness. Published in BMJ, it involved almost 11,000 people of all ages, and concluded that taking a D supplement (anywhere from less than 800 to more than 2,000 IU daily) reduced the risk of having at least one respiratory tract infection. Those who were very deficient in the vitamin (defined in this study as having blood levels below 25 nmol/L) saw the most benefit.

This anti-inflammatory vitamin also puts the brakes on your immune system. “That might seem like a bad thing, but not all immune responses are helpful when your body mounts them, which is particularly well-illustrated in COVID,” Martineau says. In many cases, severe COVID-19 symptoms result from the body’s overly exuberant response to the virus (what’s called a cytokine storm). The result is that the immune system attacks “friendly” tissues instead of targeting just the virus.

The Risks of Very Low Levels

Martineau’s findings match those of many other studies showing the benefit of raising low levels of vitamin D for a variety of health conditions. It’s well-known that having too little of it weakens bones, and some studies suggest there may be a link between a deficiency and a higher risk of cancer, heart attacks, strokes, and more.

“There’s no question that additional vitamin D is helpful if someone is low or deficient,” says F. Michael Gloth III, M.D., an associate professor in the division of geriatric medicine at Johns Hopkins University’s medical school. “But no trial has shown any benefit for giving vitamin D in any population that’s already getting enough.”

In 2018, long-awaited results from a study that looked at the effects of vitamin D and fish oil pills in more than 25,000 people ages 50 and older were published in The New England Journal of Medicine. Known as the VITAL trial, it found that taking 2,000 IU of vitamin D daily didn’t cut cancer or cardiovascular risks compared with a placebo. But few of the people in the study had low blood levels of vitamin D.

Still, some research questions how helpful it is to raise low vitamin D levels. For example, doctors commonly recommend that older adults take vitamin D pills to help prevent falls and fractures. But a 2018 analysis of 81 studies, published in The Lancet Diabetes & Endocrinology, failed to support this, although only a few of the trials included people with really low levels.

Moreover, too much vitamin D may actually contribute to fractures. A 2019 study published in JAMA found that people who took 4,000 or 10,000 IU a day saw a reduction in bone density compared with those who took 400 IU. (But other research shows that taking vitamin D with calcium may lead to slight reductions in fracture risk.)

What Tests Can Reveal

The most common way to measure vitamin D levels is with a blood test for 25(OH)D [25-­hydroxyvitamin D], but it’s not perfect. “There are many different versions of the test, and the results can vary,” says Mark Moyad, M.D., director of complementary and alternative medicine at the University of Michigan Medical Center, who specializes in studying vitamins, minerals, and supplements. You can get different results from different labs and even after multiple tests at the same lab.

“Many of us in geriatrics, for better or worse, do screen and do treat” vitamin D deficiencies, says Veronica Rivera, M.D., an assistant professor of geriatrics and palliative medicine at the Icahn School of Medicine at Mount Sinai. She admits that the evidence about testing and treatment is unclear. “If I’m doing yearly labs on someone, I may add it in. If they’re having falls, I may check it. If someone has osteoporosis or osteopenia, I would definitely screen,” she says. “The evidence is still conflicting, but I think the safe approach is to keep everyone at sufficient levels and to make it easy.” 

Another confounding factor is that “normal” D levels may differ depending on skin color. We make vitamin D when our skin is exposed to sunlight. Darker skin makes it harder to synthesize the vitamin, leading to lower levels, but researchers are still trying to understand the health implications of that and the need for supplements.

“The vitamin D test may also be exposing an existing health disparity,” Moyad says. In the VITAL trial, he notes, Black people had the lowest D levels and higher rates of hypertension, obesity, and diabetes. It may be that those conditions contribute to low levels of the vitamin. And in general, people of color don’t get the same quality of healthcare as white people. “When patients, regardless of race or ethnicity, have better access and equitable opportunities to improve their health,” Moyad says, “their vitamin D levels can also increase without initially or only relying on supplements.”

Deciding on Supplements

Ultimately, whether to get tested or take a supplement and how to do it comes down to having a discussion with your doctor.

The National Academy of Medicine recommends 600 IU of vitamin D a day up to age 70; 800 IU daily after that. “The magic number is probably between 800 and 2,000 IU a day,” Gloth says.

“No matter your age, you should know what your blood level is,” says Christina Barth, R.D.N., a lecturer in nutrition at Arizona State University. If it’s not optimal (50 to 80 nmol/L), turn to food first and then a supplement if necessary, she says. Choose vitamin D2 or D3 for the best absorption.

To help you and your doctor decide whether you need a supplement, consider the following factors:

• How much sun do you get? Just 15 to 20 minutes a day (on your face, arms, legs, or back, without sunscreen) can give you a healthy dose of vitamin D. But if you've been confined indoors, the way many people have been this year, you may not be able to rely on the sun for your D. You also may need longer sun exposure to produce vitamin D in the winter or if you have darker skin. But more time in the sun means more exposure to UV rays, which can raise skin cancer risk.

• What’s your diet like? Many foods are fortified with vitamin D, but it may still be challenging to get enough from food alone. Cow’s milk and plant milks are fortified with it, as are some juices and cereals (all contain about 100 IU per cup). Fatty fish (450 IU per 3 ounces) and egg yolks (41 IU) also have D. Mushrooms naturally increase their D levels when they’re exposed to UV light (366 IU per half-cup).

• How old are you? About 80 percent of older adults don’t get enough D in their diet, and with age, skin becomes less able to make the conversion.

• Do you smoke? That danger­ous habit depletes many vitamins and can limit your body’s ability to make D.

• Are you obese? People who are carrying extra weight have lower levels of the vitamin. Losing weight may boost D counts.

• Are you physically active? Blood levels of vitamin D may increase with more activity.

• How’s your gut? People with bowel disease or metabolic problems that affect nutrient absorption may run low on the vitamin.

Editor’s Note: This article also appeared in the October 2020 issue of Consumer Reports On Health