The study, which appears in PLoS Medicine,argues that "the lack of effect of prophylactic vitamin C supplementation on the incidence of common cold in normal populations throws doubt on the utility of this wide practice."
Robert M Douglas of the Australian National University, Canberra, and Harri Hemilä of the University of Helsinki, Finland, reviewed the best quality studies on vitamin C and the common cold done over the last 65 years. All of these studies compared a daily dose of 200mg of vitamin C or more against a dummy pill (placebo).
In order to assess whether supplemental vitamin C can reduce the risk of picking up a cold, the authors focused on 23 studies done in the general population, using doses of up to 2g daily. They found that vitamin C did not reduce the risk.
In these prevention studies, those people who were given vitamin C and then caught a cold experienced a small reduction in the duration of the cold compared with those taking a placebo. The authors say that the clinical significance of this minor reduction "is questionable, although the consistency of these findings points to a genuine biological effect."
But the authors did find evidence that the vitamin could help prevent colds in people exposed to extreme physical exertion or cold weather. They found six studies in which the vitamin or a placebo was given to marathon runners, skiers and soldiers exposed to significant cold and/or physical stress.
Those taking the vitamin experienced, on average, a 50 per cent reduction in common cold incidence. The authors urge "great caution", though, in making generalisations from this finding in 6 studies that is mainly based on marathon runners.
The authors then tried to assess the efficacy of vitamin C as a possible treatment for an established cold. The authors found seven trials (all in adults) evaluating whether vitamin C taken when their symptoms started would shorten the cold.
When they looked at all seven studies together, they found no benefit from taking the vitamin. But in one of the seven trials, patients took a single very high dose of the vitamin (8 g) on the day their symptoms started and experienced a shorter illness compared with people who took a placebo pill.
Both authors say that the results in this single trial are "tantalising and deserve further assessment."
A great deal of research currently focuses on vitamin C, an ingredient that has many preconceptions attached to it. Another recent study questioned vitamin C's ability to help exercise performance.
Earlier research had showed that the vitamin reduced oxidative stress indicating a potential boost during exercise. But a team from the University of Colorado says that they found no improvements in real exercise capacity for young or old men and women, by either acute or long-term intake of ascorbic acid.