Dr. David R. Murdoch of the University of Otago in Christchurch, New Zealand, and colleagues conducted a randomized trial to examine the effect of vitamin D supplementation on incidence and severity of upper respiratory tract infections in healthy adults.
The study, conducted from February 2010 to November 2011, included 322 healthy adults. Participants were randomly assigned to receive an initial dose of 200,000 International Units of oral vitamin D3, then 200,000 IU one month later, then 100,000 IU monthly, or placebo administered in an identical dosing regimen for a total of 18 months.
The study, published in the Journal of the American Medical Association, found there was no statistically significant differences in the number of colds per participant -- an average of 3.7 colds per person in the vitamin D group and 3.8 per person in the placebo group. The duration of symptoms per cold was an average of 12 days for each group.