Vitamin C for preventing and treating the common cold ( Review )

Seven comparisons examined the effect of therapeutic vitamin C (3249 episodes). No consistent effect of vitamin C was seen on the duration or severity of colds in the therapeutic trials. The majority of included trials were randomised, double-blind trials. The exclusion of trials that were either not randomised or not double-blind had no effect on the conclusions. Authors’ conclusions 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. Regular supplementation trials have shown that vitamin C reduces the duration of colds, but this was not replicated in the few therapeutic trials that have been carried out. Nevertheless, given the consistent effect of vitamin C on the duration and severity of colds in the regular supplementation studies, and the low cost and safety, it may be worthwhile for common cold patients to test on an individual basis whether therapeutic vitamin C is beneficial for them. Further therapeutic RCTs are warranted. P L A I N L A N G U A G E S U M M A R Y Vitamin C for preventing and treating the common cold The common cold is a major cause of visits to a doctor in high-income countries and of absenteeism from work and school. There are over 200 viruses which can cause the common cold symptoms including runny nose, congestion, sneezing, sore throat, cough, and sometimes headache, fever and red eyes. Symptoms vary from person to person and cold to cold. Since the common cold is usually caused by one of the respiratory viruses, antibiotics are useless and therefore other potential treatment options are of substantial public health interest. Vitamin C has been proposed for treating respiratory infections since it was isolated in the 1930s. It became particularly popular in the 1970s when Nobel laureate Linus Pauling concluded from earlier placebo-controlled trials that vitamin C would prevent and alleviate the common cold. Over two dozen new trials were undertaken thereafter. Vitamin C has been widely sold and used as a preventive and therapeutic agent. This review is restricted to placebo-controlled trials testing 0.2 g/day or more of vitamin C. Regular ingestion of vitamin C had no effect on common cold incidence in the ordinary population, based on 29 trial comparisons involving 11,306 participants. However, regular supplementation had a modest but consistent effect in reducing the duration of common cold symptoms, which is based on 31 study comparisons with 9745 common cold episodes. In five trials with 598 participants exposed to short periods of extreme physical stress (including marathon runners and skiers) vitamin C halved the common cold risk. The published trials have not reported adverse effects of vitamin C. Trials of high doses of vitamin C administered therapeutically, starting after the onset of symptoms, showed no consistent effect on the duration or severity of common cold symptoms. However, only a few therapeutic trials have been carried out and none have examined children, although the effect of prophylactic vitamin C has been greater in children. One large trial with adults reported benefit from an 8 g therapeutic dose at the onset of symptoms, and two therapeutic trials using five-day supplementation reported benefit. More trials are necessary to settle the possible role of therapeutic vitamin C, meaning administration immediately after the onset of symptoms.

T he role of vitamin C in the prevention and treatment of the common cold has been a subject of controversy for at least 60 years. Public interest in the subject, stimulated originally by the vigorous advocacy of Nobel laureate Linus Pauling during the 1970s [1], continues to be high. We have recently updated the Cochrane Review [2] on this topic (Text S1), incorporating 55 comparative studies that have been carried out over a period of 65 years.

The Updated Review
We sought to discover whether vitamin C in doses of 200 mg or more daily (Figure 1) reduces the incidence, duration, or severity of the common cold when used either as continuous prophylaxis or after the onset of cold symptoms. Criteria for inclusion were placebo-controlled trials to prevent or treat the common cold using oral doses of vitamin C of 200 mg/day or more. Literature from 1940 to 2004 was methodically screened.
An overview of the results of the three meta-analyses is shown in Table  1. Incidence was not altered in the subgroup of 23 community studies where prophylactic doses as high as 2 g daily were used. But a subgroup of six studies of marathon runners, skiers, and soldiers exposed to signifi cant cold and/or physical stress experienced, on average, 50% reduction in common cold incidence.
Duration of cold episodes that occurred during prophylaxis was signifi cantly reduced in both children and adults. For children this represented an average reduction of 14% in symptom days, while in adults the reduction was 8%.
For the seven trials that evaluated the therapeutic impact of vitamin C used at the onset of symptoms (all in adults), benefi ts were not observed for duration of episodes, although one of the large trials recorded a statistically signifi cant reduction in the duration of colds among participants administered a single vitamin C dose of 8 g on the day of symptom onset [3].

Implications of the Review
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. The clinical signifi cance of the minor reduction in duration of common cold episodes experienced during prophylaxis is questionable, although the consistency of these fi ndings points to a genuine biological effect.
In special circumstances, where people used prophylaxis prior to extreme physical exertion and/or exposure to signifi cant cold stress, the collective evidence indicates that vitamin C supplementation may have a considerable benefi cial effect; it was the results of one of these six trials, with schoolchildren in a skiing school [4], that particularly impressed Pauling [1]. However, great caution should be exercised in generalizing from this fi nding, which is based mainly on marathon runners.
No benefi ts have been observed from therapeutic use of doses totalling 10 g that was divided for the fi rst three days of illness. The equivocal fi ndings of the large study, which used 8 g only on the day of onset of respiratory symptoms [3], are tantalising and deserve further assessment.
None of the therapeutic trials carried out so far has examined the effect of vitamin C on children, even though the prophylaxis trials have shown substantially greater effect on episode duration in children.
Study quality for the trials included in these three meta-analyses was

Open access, freely available online
June 2005 | Volume 2 | Issue 6 | e168 The Best Practice section summarizes the current evidence on an important health intervention.  Vitamin C was identifi ed in the 1930s by Albert Szent-Györgyi, who received his Nobel Prize partly for this work. He found that paprika is a particularly rich source of the vitamin, which made it possible to produce kilograms of it for research purposes ([1963] Annu Rev Biochem 32: 1-14). Nowadays, the most convenient way to increase vitamin C intake is by way of 500-mg tablets, but further research is needed to explore the conditions in which supplementation may be benefi cial.

Vitamin C for Preventing and Treating the Common Cold
variable, but sensitivity analysis, where we excluded studies from the analysis that were less adequately blinded or randomized, did not change the general conclusions of the Cochrane Review.
Future work on this topic should explore the value of high dose therapy-in particular, in childrenand the mechanisms underlying the observed prophylaxis benefi ts in those exposed to substantial physical and/or cold stress.