Just because companies make immunity claims more often about vitamin C than about any other vitamin doesn’t make what they say more likely to be true.
Take the 2,150 people who got either a placebo or 60 mg (the DV) to 80 mg a day of vitamin C as part of a multivitamin. After an average of 15 months, those who were taking the multi got no fewer colds or respiratory infections.
What about megadoses? “Large amounts of vitamin C do not prevent colds,” says the University of Florida’s Susan Percival, “but they can shorten the duration if they’re taken every day before getting one.”
In 24 trials on nearly 11,000 adults, those who took about 1,000 mg of vitamin C every day for an average of three months had no fewer colds than those who took a placebo. But when the C takers got a cold, it was 8 percent shorter—about 12 hours less for a week-long illness.
The catch: you have to be taking vitamin C before the first sniffle. In seven studies, taking roughly 3,000 mg a day at the first sign of a cold didn’t stop it, shorten it, or lessen its severity.
The one group that may get fewer colds from megadoses of C: people undergoing brief periods of severe physical exertion. In studies of South African ultra-marathoners, Canadian military recruits on sub-Arctic winter exercise, and children attending ski school in the Alps, those who took an average of 670 mg of vitamin C every day were half as likely to later catch a cold as those who got a placebo.
Sources: J. Am. Geriatr. Soc. 55: 35, 2007; BMJ 331: 324, 2005; JAMA 288: 715, 2002; Cochrane Database Syst. Rev. 1: CD000980, 2013.