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Should you take Vitamin D supplements to prevent colds and shorten the misery?
Like other theories about the benefits of vitamin D, it seems like a reasonably good idea. After all, some lab studies suggest vitamin D might enhance immunity. And as everybody knows, people are more prone to respiratory infections during winter, when they cover up and get less vitamin D-generating sunlight.
The authoritative Institute of Medicine thought the notion was plausible enough to recommend more research on the possibility in its otherwise deflating report on vitamin D and health almost two years ago.
But a well-done study found zero difference in the incidence or duration of upper respiratory infections among 161 New Zealanders who took a hefty dose of vitamin D versus an equal number who took a fake pill. Participants were monitored for 18 months — longer than in previous studies.
The results appear in JAMA, the Journal of the American Medical Association.
Dr. Jeffrey Linder of Brigham and Women's Hospital in Boston says in an accompanying editorial that the findings suggests vitamin D should join the list of 13 other common-cold nostrums considered by the Cochrane Collaboration as ineffective, questionable or possibly harmful.
Those include Echinacea, zinc, steam inhalation, vitamin C, garlic, antihistamines, Chinese herbs, intranasal corticosteroids, intranasal ipratroprium (an asthma drug), extract of Pelargonium sidoides (a South African plant), nasal irrigation, extra fluid intake or antiviral drugs.
The vitamin D results are disappointing, Linder notes, since Americans suffer around a billion upper respiratory tract infections every year — three for every man, woman and child.
By the IOM's reckoning, the new study leaves a solid basis for vitamin D supplements only for bone health. The U.S. Preventive Services Task Force earlier this year wasn't even convinced of that.
The IOM Report recommends most children and adults consume 600 to 800 international units of vitamin D a day and get their blood levels of the active form, 25-hydroxyvitamin D or 25-OHD, to at least 20 nanograms per deciliter.
In the new study, volunteers who got the real vitamin D pills received 100,000 IUs per month, or about 3,300 units a day. That's more than five times the recommended daily allowance for adults.
The supplements increased volunteers' blood levels of 25-OHD from 29 (a near-normal level) to 43. Participants who achieved higher blood levels didn't do any better, in terms of frequency or severity of colds.