NEW YORK (Reuters Health) - For the average person, popping vitamin C pills is unlikely to ward off the common cold or shorten its length or severity. However, for people exposed to short bouts of extreme physical exercise or cold temperatures, vitamin C may markedly reduce their risk of catching a cold.

Amber Sweet hybrid oranges are seen in an undated photo from the Department of Agriculture. For the average person, popping vitamin C pills is unlikely to ward off the common cold or shorten its length or severity. However, for people exposed to short bouts of extreme physical exercise or cold temperatures, vitamin C may markedly reduce their risk of catching a cold. REUTERS/USDA/Randall Smith/Handout

The findings stem from a review of 30 published studies involving 11,350 people who took at least 200 milligrams of vitamin C each day.

Based on pooled data, regular ingestion of vitamin C did nothing to lower the risk of the common cold in the ordinary population, report reviewers in the latest issue of The Cochrane Library, a publication of the Cochrane Collaboration, an international organization that evaluates medical research.

There was a slight reduction in the duration and severity of common cold symptoms with vitamin C, compared with placebo, but the magnitude of the effect was so small its clinical usefulness is doubtful, the experts report.

Therefore, it is senseless for most people to take vitamin C every day to reduce their risk of catching a cold, according to co-author Harri Hemila of the University of Helsinki, Finland and her colleagues.

An exception appears to be when individuals are exposed to short periods of extreme physical stress. In six trials involving a total of 642 marathon runners, skiers, and soldiers taking part in sub-arctic exercises, vitamin C supplements reduced the risk of the common cold by 50 percent.

Vitamin C, for the average person, also failed as a “therapeutic” for the common cold. Trials of high-dose vitamin C taken after the onset of cold symptoms showed “no consistent” effect on either the length of a cold or the severity of symptoms.

Hemila and colleagues caution that there were only a few therapeutic trials and their quality varied. One trial showed a borderline benefit from downing an 8-gram dose of vitamin C at the beginning of a cold, and two trials using 5-day supplementation reported benefits.

More trials testing vitamin C as a possible treatment for the common cold are needed, the reviewers conclude.

The value of vitamin C in preventing and treating the common cold has been the subject of controversy for six decades. Nonetheless, vitamin C is widely sold and used for these purposes.

The authors of the Cochrane report conclude, based on the data at hand, that the routine use of vitamin C to prevent the common cold is “not rationally justified for community use. It could be justified in people exposed to brief periods of severe physical exercise or cold environments.”

SOURCE: The Cochrane Library 2007.