Vitamin D, long known for its beneficial effect on bones, is increasingly being studied to see if it helps prevent colds and flus. Based on early research results, some doctors are recommending high doses to help stave off the upper respiratory infections, with some even speculating it could be a substitute to the annual flu shot. But while it's well established that vitamin D boosts the immune system, many scientists say so far there's insufficient evidence that taking it will help keep a cold or flu away.
Until recently, scientists have blamed the higher prevalence of flu cases during winter to the tendency of humans to congregate inside or the low humidity of cold weather, which makes viruses survive in the air longer. Increasingly, scientists are exploring another possible explanation: During the wintertime, we are outside less, resulting in lower vitamin D absorption from the sun.
"Unless you are out there in the sun all the time, which hardly anyone is doing in wintertime, it's impossible to get enough," says James R. Sabetta, director of infectious diseases at Greenwich Hospital in Greenwich, Conn., who has studied the effect of vitamin D on respiratory infections.
Vitamin D is naturally present in few foods, such as salmon and herring, so humans need to get it either from sunlight or dietary supplements. In a recent report, the Institute of Medicine tripled the amount of vitamin D recommended for most Americans to 600 international units a day. The IOM is an arm of the National Academy of Sciences, which sets government nutrient levels.
But, based in part on early evidence that vitamin D helps prevent upper respiratory infections, some scientists recommend daily levels of 2,000 to 4,000 IUs or even higher. The Vitamin D Council, a nonprofit scientist group supported by vitamin makers and other commercial interests, recommends 5,000 IUs daily—but warns that this dose doesn't replace the conventional recommendation of a flu shot. "I recommend vitamin D and a flu shot to cover all your bases," says the council's executive director John Cannell.
It's true that basic science shows that vitamin D boosts innate immunity, or the body's first line of defense against pathogens, says Pennsylvania State University scientist A. Catharine Ross, chairman of the IOM committee that made the latest recommendations. But the recommended increase was based on vitamin D's proven positive role in bone health, she says. The panel reviewed the studies and found "no strong evidence that supports the idea that increased levels of vitamin D are going to be protective" against upper respiratory infections, she adds.
Vitamin D boosts immunity by stimulating production of cathelicidin, an antimicrobial protein that serves as a "natural antibiotic" in the body, says Michael Zasloff, a professor of surgery and pediatrics at Georgetown University Medical Center in Washington, D.C.
But so far studies looking directly at whether higher vitamin D levels help prevent upper respiratory infections have had mixed results. A Japanese study of 167 schoolchildren first published online in March in the American Journal of Clinical Nutrition, found that vitamin D supplements helped reduced incidences of influenza A but not influenza B.
Influenza A and B are the two main types of flu viruses responsible for seasonal flu epidemics every year, according to the Centers for Disease Control and Prevention. Swine and bird flus are specific subtypes of influenza A.
In an observational study published in June, Dr. Sabetta and colleagues followed 195 people during winter and found that people with a blood serum concentration of 38 nanograms per milliliter of vitamin D had half the risk of getting an upper respiratory tract infection as those with levels below that threshold. The people with higher vitamin D levels hadn't gotten any more flu shots and weren't taking more of other vitamins than those with lower levels, according to the study. The study, however, didn't rule out the possibility that the group with higher vitamin D also had better overall nutrition.
"We're very confident and we think this is going to be verified" by other studies, says Dr. Sabetta, an associate clinical professor at Yale University School of Medicine.
A 167-person study by researchers at Winthrop University Hospital in Mineola, N.Y., puts something of a dent in the vitamin D claims. The study found people taking 2,000 IUs a day of vitamin D got about the same number of upper respiratory infections as a group who got a placebo during the three-month flu season.
If vitamin D has a small positive effect, a large randomized trial will be needed to see it, says author John Aloia, chief academic officer at Winthrop University Hospital. "We found no evidence that vitamin D was protective," he says. It's also possible that it's necessary to start taking vitamin D several months before flu season to build up levels in advance, he adds.
Vitamin D is generally safe, but in high doses scientists say it can pull calcium from bones, causing kidney problems and heart disease. The IOM report says vitamin D may be toxic if you take more than 10,000 IUs a day, and warns the risk of harm may begin to increase at 4,000 IUs daily.
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- Overview Influenza and vitamin D
- CLICK HERE for articles on Immunity and vitamin D
- More on Influenza from Vitamin D council - late 2010
- Review of vitamin D and immune system – abstract Sept 2010
- Vitamin D 3X better than placebo or vaccine at preventing flu in kids – March 2010
- Prevent colds and flu with vitamin D – Aug 2010
- Kids with 1200 IU vit D were 1/2 as likely to get influenza A
- Influenza of 1918 and vitamin D - July 2010
- Sunlight used to treat influenza - 1918 file