The study analyzed data from the Third National Health and Nutrition Examination Survey (NHANES III), conducted by the National Center for Health Statistics. Participants were interviewed in their homes regarding their health and nutrition, and most participants also received a physical examination that included collection of blood and other samples for laboratory analysis. The research team analyzed blood levels of 25-hydroxyvitamin D (25OHD) -- the best measure of vitamin D status -- from almost 19,000 adult and adolescent NHANES III participants, selected to be representative of the overall U.S. population.

Study participants with the lowest vitamin D blood levels -- less than 10 nanogram per milliliter of blood -- were about 40 percent more likely to report having a recent respiratory infection than were those with vitamin D levels of 30 or higher. The association was present in all seasons and even stronger among participants with a history of asthma or chronic obstructive pulmonary disease (COPD), including emphysema. Asthma patients with the lowest vitamin D levels were five times more likely to have had a recent respiratory infection; while among COPD patients, respiratory infections were twice as common among those with vitamin D deficiency.

"A respiratory infection in someone with otherwise healthy lungs usually causes a few days of relatively mild symptoms," Carlos Camargo of the MGH Department of Emergency Medicine and senior author of the study, said.

"But respiratory infections in individuals with an underlying lung disease can cause serious attacks of asthma or COPD that may require urgent office visits, emergency department visits or hospitalizations. So the impact of preventing infections in these patients could be very large," said.

The authors stressed that the study's results need to be confirmed in clinical trials before vitamin D can be recommended to prevent colds and flu.

"We are planning clinical trials to test the effectiveness of vitamin D to boost immunity and fight respiratory infection, with a focus on individuals with asthma and COPD, as well as children and older adults -- groups that are at higher risk for more severe illness," Ginde said.

"While it's too early to make any definitive recommendations, many Americans also need more vitamin D for its bone and general health benefits. Clinicians and lay people should stay tuned as this exciting area of research continues to expand," he said.

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Editor:Yang Jie