Vitamin D supplements don't prevent colds or ease their severity in adults without deficiency, a randomized trial showed.
The number of upper respiratory infections wasn't any lower with large monthly doses of vitamin D than with placebo, at a mean of 3.7 versus 3.8 per person, for a nonsignificant risk reduction of just 3 percent, Dr. David R. Murdoch of the University of Otago in Christchurch, New Zealand, and colleagues found.
Missed workdays and duration of symptoms likewise were similar between groups, they reported in the Oct. 3 issue of the Journal of the American Medical Association.
Sufficient levels appear to be enough, Murdoch's group concluded from its VIDARIS trial.
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Healthy adults in developed countries who are already keeping their vitamin D in the range recommended for bone health don't need an extra boost, Dr. Jeffrey A. Linder of Brigham and Women's Hospital in Boston agreed in an accompanying editorial.
"The VIDARIS trial has rigorously addressed this question," he wrote. "Results suggest that vitamin D should join the therapies listed in the Cochrane reviews as being ineffective for preventing or treating upper respiratory tract infections in healthy adults."
That list with no or questionable benefit or significant side effects includes vitamin C, garlic, Echinacea, zinc, saline nasal irrigation, steam inhalation, increased fluid intake, and other popular strategies, Linder pointed out.
"It's very important, however, to keep in mind that this group had relatively normal levels to start with, and so this might not apply to people with lower levels, with actually a vitamin D deficiency," study co-author Dr. Carlos A. Camargo Jr. of Massachusetts General Hospital in Boston cautioned in an interview.
For example, in vitamin D-deficient children in Mongolia, a daily dose of the vitamin has been shown to have a dramatic impact, halving the incidence of upper respiratory infections.
Another trial in adults with chronic obstructive pulmonary disease showed a benefit of supplementation in reducing exacerbations only in vitamin D-deficient individuals with levels under 10 ng/mL.
Vitamin D is appealing as a preventive treatment because of its role in immunity and inducing antimicrobial compounds in the body.
The VIDARIS trial randomized 322 healthy adults in New Zealand to double-blind treatment with 200,000 IU oral vitamin D3 initially and 1 month later followed by monthly doses of 100,000 IU monthly or placebo on the same schedule for 18 months, covering two winter seasons.
The extra vitamin D did boost levels from a baseline of 29 ng/mL to over 48 ng/mL throughout the study period. A level above 20 ng/mL is generally considered adequate for bone and overall healthy in adults.
Monthly office visits for dose administration and completion of questionnaires about "sudden onset of runny nose, nasal stuffiness, sore throat, or cough not attributed to allergies" were supplemented with a home visit for nasopharyngeal swab sampling any time a participant contacted study staff to report symptoms.
However, the primary endpoint of those upper respiratory tract infections didn't differ between the groups over the course of the study, nor were there any secondary benefits.