This article summarizes some of the evidence available showing that vitamin D, especially at the 2,000 IU per day dosage, substantially reduces the incidence of colds or flu. The protective effect is comparable to or superior to that suggested by the best numbers available for the protective effect of flu vaccines (which of course aren’t likely to prevent colds). In addition to having no adverse side effects at that dosage, vitamin D has many other benefits, such as preventing asthma attacks, helping to maintain bone density, and possibly preventing multiple sclerosis. Perhaps that’s why Seattle has such a high incidence of MS: people don’t get enough vitamin D.

Vitamin D can be harmful at high doses – 100,000 IU or more – so one should be careful and know the exact amount contained in whatever supplement you choose to take.