By Rita Rubin
WebMD Health News
Reviewed by Louise Chang, MD
Oct. 2, 2012 -- Looks like we may have to cross out another supposed way to prevent the common cold.
There were reasons to think otherwise. Various studies have suggested that vitamin D plays a protective role in the immune system. Vitamin D deficiency in children has been linked to higher rates of pneumonia and upper respiratory tract infections. And because the body makes vitamin D when in the sun, lower levels tend to occur in the winter, otherwise known as cold season.
Monthly Mega Doses
In the new study, New Zealand researchers assigned 322 healthy adults to take either vitamin D supplements or a placebo once a month for 18 months. For the first two months, those assigned to vitamin D took 200,000 IU (international units) once per month. For the rest of the study, they took 100,000 IU once per month.
The Institute of Medicine recommends that people 1-70 years old take in 600 IU of vitamin D daily, and that people over 70 years take in 800 IU daily.
This high-dosing regimen used in the study was chosen to achieve blood levels of vitamin D that had been linked to the lowest risk of disease in some previous studies.
Participants were asked to contact study staff whenever they came down with symptoms of a cold, such as a runny nose or sore throat. A staff member then visited to do a swab test and have the participant complete a survey about their symptoms.
Study Results Not Perfect
By the end of the study, both the vitamin D group and the placebo group had averaged just under four upper respiratory tract infections per person, according to the researchers' report in this week's Journal of the American Medical Association. With each episode, symptoms lasted an average of 12 days in both groups.
Only half of the swabs in each group tested positive for respiratory viruses, and two in each group were the flu.
"Our tests are good, but not perfect, and some viruses may have been cleared by the time the swab was collected," says researcher David Murdoch MD, head of pathology at the University of Otago in Christchurch, New Zealand. "There may also be viruses causing colds that were not part of the testing panel."
Murdoch says his team plans to retest the samples that had originally tested negative for respiratory viruses with more sophisticated methods.
Murdoch's trial "vigorously" addressed the question of whether vitamin D can prevent colds, Jeffrey Linder, MD, MPH, a doctor at the Brigham and Women's Hospital and Harvard Medical School in Boston, writes in an accompanying editorial.
What's still not clear is whether vitamin D supplements could protect against colds in people deficient in vitamin D or in children, neither of whom were in his study, Murdoch says. "A key factor in our study is that we had few participants with very low baseline levels of vitamin D."
Perhaps taking vitamin D supplements daily instead of monthly would lead to more consistently raised blood levels and, therefore, greater protection against colds, says Alexandra Yamshchikov, MD, an assistant professor in the infectious diseases unit at the University of Rochester Medical Center. She was not involved in Murdoch's study.
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SOURCES:David Murdoch, MD, head of pathology, University of Otago, Christchurch, New Zealand.Jeffrey Linder, MD, MPH, primary care doctor, Brigham and Women's Hospital and Harvard Medical School, Boston.Alexandra Yamshchikov, MD, assistant professor, infectious disease unit, University of Rochester Medical Center, Rochester, N.Y.
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