It's long been known that vitamin D helps protect our bones, but the question of whether taking vitamin D supplements helps guard immunity has been more controversial. An analysis published Wednesday suggests the sunshine vitamin can help reduce the risk of respiratory infections, including colds and flu — especially among people who don't get enough of the vitamin from diet or exposure to sunlight.
Researchers pooled data from 25 studies that included more than 10,000 participants. The studies looked at whether vitamin D supplements cut the number of infections.
"We found that overall there was a modest protective effect," says Dr. Adrian Martineau, a clinical professor of respiratory infection and immunity at Queen Mary University of London who led the research team. Overall, he says, vitamin D supplements seemed to reduce the risk of infection about 10 percent.
People who had been vitamin D deficient when they enrolled in the studies saw more benefit. Their risk of infection was cut in half, according to the findings.
"What we found is that those with the lowest vitamin D levels experienced the greatest benefit from supplementation," Martineau says.
Acute respiratory infections are responsible for millions of emergency department visits in the United States each year, says Carlos Camargo, a professor of epidemiology at Harvard Medical School and a senior author of the study. He says their findings support the notion that more foods should be fortified with vitamin D.
The U.S. has long fortified foods with vitamin D, including milk and other dairy products. Many food companies supplement orange juice products and cereals, too. And certain foods like sardines and other oily fish naturally contain significant levels of vitamin D. For instance, a serving of salmon can have almost a whole day's worth of vitamin D. And the body produces its own vitamin D when exposed to sunlight.
So not everyone is convinced that this study should lead us all to the supplement aisle.
According to recommendations from the Institute of Medicine, most adults need about 600 IU (international units) of vitamin D per day. Adults 70 years old and older are advised to increase their intake to 800 IUs per day.
Many Americans don't routinely get this amount from their diets, so a multivitamin can help make up the shortfall. "The standard multivitamin has about 400 IUs," says Steven Abrams, a professor of pediatrics at the Dell Medical School at the University of Texas at Austin. He was a member of the Institute of Medicine panel that wrote the vitamin D recommendations for children and adults.
Abrams says the guidelines, issued in 2010, were based on evidence that vitamin D helps protect bone health. "Now the controversy is whether there's enough evidence about other diseases to make [new] recommendations."
Over the last 10 years, a number of studies have suggested that the sunshine vitamin can help prevent disease. That has led people to think that higher doses of supplements are better. But Abrams says he's not convinced there's a benefit of taking a supplement for people who are not deficient. "It needs further studies to confirm."
Abrams says the importance of the new study is that it's a summary of 25 controlled trials. "And it shows that people with very low vitamin D [levels] do better when they're given supplements." He says this is not too surprising. "If you're deficient, getting an adequate amount will make a difference."
In other words, if you're getting the recommended 600 IUs of vitamin D from your diet, a supplement may not lead to any further benefit. But the growing interest in vitamin D has lots of people curious about their levels.
"Many of our patients come in and say: I want to be tested for vitamin D," says Monique Tello, a primary care doctor at Massachusetts General in Boston who has written about vitamin D levels. A lot of people suspect they're deficient, but she says determining the right level can be tricky.
The IOM report estimated that a vitamin D blood level of 20 ng/mL or higher was adequate for good bone health, and that anything below that was deficient.
But a 2011 guideline from the Endocrine Society says that people should consider a blood level of 30 ng/mL as the minimum. Contrast that with a study published last year in The New England Journal of Medicine, which says that both the IOM panel's and the Endocrine Society's cutoffs are too high, and that 12.5 ng/mL is an appropriate definition for deficiency. That was based on national survey data finding that less than 6 percent of Americans had vitamin D levels lower than 12.5.
And in 2014, the U.S. Preventive Services Task Force said there's not enough evidence of benefit for all people to be routinely tested for vitamin D deficiency.
Certain groups of people are at higher risk of vitamin D deficiency, including people with digestive disorders such as celiac disease and people who cover up most of their skin or get very little exposure to the sun. And pregnant and nursing women, as well as women with osteopenia or osteoporosis, often need more vitamin D to maintain bone health.
"People [at higher risk] should get tested," Tello says. She says when a patient's blood screening test shows levels of vitamin D between 20 and 32 ng/ml, "I recommend that they take between 1,000 and 2,000 international units of vitamin D daily, indefinitely."
Tello says that if a patient has a blood level below 20 ng/ml, then she usually recommends they temporarily take a higher dose of supplements.
The Institute of Medicine says that adults shouldn't take more than 4,000 IU a day.
STEVE INSKEEP, HOST:
A new study finds vitamin D can help reduce the risk of colds and flu. Researchers say it's some of the best evidence yet that vitamin D may have benefits beyond keeping our bones strong. So who knows? Maybe we need some in my family. But the question is should we take supplements to get more of it and how much? NPR's Allison Aubrey reports.
ALLISON AUBREY, BYLINE: It's long been known that vitamin D, which we get from the sunshine and from our diets, helps protect our bones. But in recent years, researchers have studied whether vitamin D may also boost immunity and cut the risk of sickness. Adrian Martineua of Queen Mary University of London says there are now 25 studies that include more than 10,000 participants that have tested whether vitamin D supplements can help protect against colds and the flu. And when he and his collaborators at Harvard Medical School put all the data together and analyzed it, they found taking extra vitamin D did seem to be beneficial.
ADRIAN MARTINEAU: The headline finding, the first thing we found, was that overall there was a modest protective effect of vitamin D against respiratory infections in the population as a whole.
AUBREY: Supplements cut the risk of colds and flu overall about 10 percent. But when they focused on people who were known to have very low levels of vitamin D, they found that, for them, taking a daily or weekly supplement had a bigger benefit.
MARTINEAU: What we found was that those with the lowest vitamin D levels, you can take a vitamin D supplement, you can reduce your risk of a respiratory infection by around 50 percent. It's quite a large effect.
AUBREY: Now, just after I interviewed Dr. Martineau yesterday, I thought to myself, well, it's cold, it's dark, and I haven't seen much of the sun. And everyone around me, including my editors, seems to be fighting off bad viruses. Maybe I should stop at the drugstore and pick up some vitamin D. But then I spoke to Steve Abrams. He's a physician and professor of pediatrics at the Dell Medical School at The University of Texas at Austin. He was part of an expert panel that wrote guidelines several years back on how much vitamin D children and adults in the U.S. should get.
STEVEN ABRAMS: The recommendation for most adults is that they have 600 units per day of vitamin D intake.
AUBREY: My first question to him was what dose of a supplement he takes.
ABRAMS: I don't actually take a supplement.
AUBREY: Abrams says, living in Texas, he sees a fair amount of sun, and he gets vitamin D from his diet.
ABRAMS: A glass of milk, an eight-ounce glass of milk for example, will provide a little over 100 units per day.
AUBREY: That's a chunk of what you need. And since milk and other dairy products - as well as many orange juices and cereals - are fortified, Abrams says these can all be good sources, too. The other excellent source is oily fish. One serving of salmon can provide almost a whole day's worth of vitamin D. Abrams says given that many Americans don't eat much fish and don't get enough sunshine, taking a multivitamin that contains vitamin D is a reasonable option.
ABRAMS: The standard multivitamins usually have 400 units per day.
AUBREY: So that's enough for many people but not for everyone. Monique Tello is a primary care doctor at Massachusetts General in Boston. She says lots of patients come in asking to be tested, and certain people are at higher risk of not getting enough vitamin D. This includes many postmenopausal women, those who cover their skin and people with conditions such as celiac disease.
MONIQUE TELLO: Those people should get tested. You know, we should have an idea of where their level's at and just make sure that it's acceptable.
AUBREY: Tello says when a screening test shows insufficient levels...
TELLO: I recommend that they take between 1,000 and 2,000 international units of vitamin D daily indefinitely.
AUBREY: And for people with a severe deficiency, Tello says there are cases when much higher doses are recommended - at least in the short term. Allison Aubrey, NPR News.
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