The idea that the vitamin can prevent and treat a cold became popular following the 1970 publication of Nobel laureate Linus Pauling's book, Vitamin C and the Common Cold. "Pauling's book was very influential. But I'm confident that the general population doesn't stand to gain any great benefit in terms of colds from taking regular doses," says Robert Douglas of the Australian National University in Canberra, who co-authored the review.

Douglas and Harri Hemilä of the University of Helsinki, Finland, considered 55 studies dating from 1940 to 2004. These studies compared the effect of at least 200 milligrams daily versus a placebo.

From 23 studies investigating prevention in the general population, the pair concluded that regular doses of vitamin C do not reduce the risk of a cold. But they found the duration of any cold that did develop was shortened, though "only by about half a day in adults, and slightly more in children", Douglas notes.

Extreme physical stress
However, people suffering extreme physical stress through exertion or freezing temperatures did benefit substantially from taking vitamin C. Studies of skiers, soldiers and marathon runners suggest that regular vitamin C can slash their incidence of colds in half. Pauling was heavily influenced by a ski school study, in particular.

"I don't understand why - but it does seem there is a small subset of people who do seem to have substantial prevention benefit from taking vitamin C, when for the general population it's zilch," Douglas says.

As well as considering the effect of regular daily does, the pair also looked at whether starting to take vitamin C as soon as a symptoms appeared could shorten a cold. They found no evidence that it could, except for in one study which involved a huge 8 gram dose on the first day of symptoms.

"For all except this 8 gram group, the evidence is quite unimpressive that taking largish doses makes any difference at all once a cold has started," says Douglas.

Journal reference: PLoS Medicine (Vol 2(6), p e68)