- Forget the apple. The largest study of its kind to date shows that vitamin D each and every day is what will keep the doctor away when it comes to the common cold or the flu.
The finding is based on an assessment of vitamin D levels, nutritional habits and respiratory infection rates among nearly 19,000 American men and women.
"We don't want to jump ahead of ourselves," said study author Dr. Adit Ginde, an assistant professor of surgery in the division of emergency medicine at the University of Colorado Denver School of Medicine. "But our study provides support that lower levels of vitamin D are associated with an increased risk for respiratory infections, such as the common cold and the flu. And people who have pre-existing respiratory disease -- like asthma an emphysema -- appear to be at an increased risk for this association."
Ginde's team, from Harvard Medical School and Children's Hospital Boston, reports its findings in the Feb. 23 issue of the Archives of Internal Medicine.
Vitamin D can be found in such foods as canned tuna, cereal and fortified milk or juice, according to the American Dietetic Association (ADA). The body can also be triggered to naturally produce vitamin D after adequate exposure to sunlight.
In addition to its well-established role as a calcium builder and bone fortifier, vitamin D has recently been touted as having a protective role against both colon cancer and multiple sclerosis, the ADA noted.
And in December, a review of studies conducted by researchers at the Mid-America Heart Institute in Kansas City suggested that those with vitamin D deficiency -- a designation estimated to include about half of American adults and nearly one in three children -- might face an increased risk for heart attack and stroke.
To gauge the specific relationship between vitamin D and respiratory risk, Ginde's team analyzed data from the Third National Health and Nutrition Examination Survey, collected from 1988 to 1994.
Participants were aged 12 and up -- with an average age of 38 -- and three-quarters were white. All completed nutritional and health surveys and had physical examinations. Blood samples were taken to measure levels of 25-hydroxyvitamin D, considered to be the optimal measure of vitamin D status.
The researchers found that those with less than 10 nanograms of vitamin D per milliliter of blood, considered low, were nearly 40 percent more likely to have had a respiratory infection than those with vitamin D levels of 30 ng or higher. The finding was consistent across all races and ages.
In particular, people who had a history of asthma or some form of chronic obstructive pulmonary disease (COPD) were even more likely to suffer from vitamin D deficiencies.
Asthma patients with the lowest vitamin D levels had five times the risk for respiratory infection, and vitamin D-deficient COPD patients had twice the risk.
"We still need to do the clinical trials that we already have planned to definitely say whether supplementation with vitamin D would actually reduce the risk we found," Ginde cautioned. "But I think we can say that most Americans probably do need more vitamin D for its effects on bone health, as well as for its general benefits with respect to the immune system."
Lona Sandon, an assistant professor of clinical nutrition at the University of Texas Southwestern Medical Center and a spokeswoman for the American Dietetic Association, said that evidence of a vitamin D-immune system connection seems "pretty strong."
"There does seem to be a link because, when we're not getting enough vitamin D, our immune system appears not to function at its best," she said.
Sandon noted, however, that getting enough vitamin D from food alone can be difficult.
"The best sources are salmon with the bones, or three cups a day of milk," she said. "But not many people get that. So I would say, get outside and expose some skin to the sun. Dermatologists don't always like that advice because they're concerned with skin cancer, but just 15 minutes a day at the sun's peak -- roughly 11 to 1 -- does the trick."
"However," Sandon added, "if you live north of Atlanta, in the middle of winter it's hard to get enough vitamin D that way, even if you're out in the middle of the day. So that leaves us with supplements in some form. And in that case, a general multivitamin typically has about 200 Ius [international units] of vitamin D, which is considered adequate."
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