As for the rest of the population, the review finds no evidence that regular supplementation of vitamin C reduces the chance of coming down with a common cold, although it may reduce the duration and severity of illness.
The review appears in the 31 January issue of The Cochrane Library.
The Common ColdIn high-income countries, the common cold is a major reason for visiting the doctor, and taking time off work and school.
There are over 200 viruses that can cause common cold symptoms, which include runny nose, sneezing, sore throat, congestion, cough, and sometimes headache, red eyes and fever.
The symptoms vary from cold to cold and also from person to person. Because it is caused by a virus, the common cold can't be treated with antibiotics, so there is a lot of public health interest in finding treatments that can help.
Vitamin CVitamin C is widely sold and used as a preventive and therapeutic agent.
It has been proposed as a treatment for respiratory illnesses ever since it was first isolated in the 1930s.
It rose to popularity as a remedy in the 1970s when Nobel laureate Linus Pauling said placebo-controlled trials showed vitamin C was effective at preventing and alleviating the common cold.
For this latest review, the authors restricted their analysis to randomized placebo-controlled trials that tested the effectiveness of at least 0.2 g per day or more of vitamin C.
Modest Effect in General PopulationBased on 29 trials involving 11,306 participants that compared vitamin C with placebo, the Cochrane reviewers found regular use of vitamin C had no effect on common cold incidence in the general population.
However, their analysis of 31 studies covering a total of 9,745 common cold episodes, that compared vitamin C with placebo, shows it has a small but consistent effect in reducing the duration of symptoms.
Regular doses of 1 g or more per day have reduced the average duration of colds in adults by 8% and in children by 18%.
Most Benefit Seen in People Who Do Heavy Physical ActivityThe only trials that appeared to show that vitamin C reduced the risk of catching a common cold were those conducted in people who were under short-term physical stress.
An analysis of five randomized trials shows vitamin C halved the incidence of the common cold: three of the trials studied marathon runners, one was of Swiss schoolchildren at a skiing camp, and the other studied Canadian soldiers on a winter exercise.
A more recent randomized trial involving teenage competitive swimmers found regular use of vitamin C halved the duration of common colds in males but had no effect in females.
ConclusionThe authors conclude that:
"The failure of vitamin C supplementation to reduce the incidence of colds in the general population indicates that routine vitamin C supplementation is not justified, yet vitamin C may be useful for people exposed to brief periods of severe physical exercise."
And, although, trials looking at the effect of regular supplementation of vitamin C shows it reduces the duration of colds, "this was not replicated in the few therapeutic trials that have been carried out," they add.
But, they note that since the studies show a consistent effect of regular vitamin C use on reducing duration and severity of common colds, and bearing in mind that the supplement is not expensive and relatively safe, "it may be worthwhile for common cold patients to test on an individual basis whether therapeutic vitamin C is beneficial for them".
The authors call for further therapeutic randomized controlled trials, that is where the vitamin is taken as a treatment as soon as symptoms begin.
A study published earlier this month found that men who consume high levels of vitamin C are at twice the risk of kidney stones than men who do not.
Written by Catharine Paddock PhD