Although many people swear by vitamin D to bolster their immune system and stave off colds, researchers from the University of Otago in Christchurch, New Zealand, have found that vitamin D supplementation has no effect on upper respiratory tract infections. They arrived at their  findings, published in the Journal of the American Medical Association, after following 322 healthy adults who were given either 200,000 IU oral vitamin D supplements or a placebo once a month for 18 months. In the end, there were no statistically significant differences in either group. Both groups averaged just under four upper respiratory tract infections each, with symptoms lasting an average of 12 days in both groups.