Vitamin C for preventing and treating the common cold

Vitamin C supplementation appears not to reduce the incidence of colds in the general population, yet vitamin C may be useful for people exposed to brief periods of severe physical exercise. There is a consistent effect of vitamin C on the duration and severity of colds in the regular supplementation studies, but this finding is not replicated in therapeutic trials.

Comment: The quality of evidence is downgraded by inconsistency (unexplained variability in results).

A Cochrane review included 63 trials, reported in 44 publications. All trials tested 0.2 g/day or more of vitamin C. The trials fall to following categories

1. Community regular supplementation trial arms (n=43) which evaluated the effects of regular daily supplementation with vitamin C (i.e. vitamin C each day over the study irrespective of the presence of colds) on reducing the incidence or duration or severity of naturally occurring colds.
2. Community therapeutic trial arms (n=10) that evaluated the therapeutic effects of high-dosage vitamin C after natural common cold symptoms had commenced.
3. Community trials (n=7), which did not report data suitable for meta-analysis.
4. Laboratory trials (n=3) in which volunteers were intentionally exposed to known viruses after vitamin C or placebo administration.

The primary end point was the risk ratio (RR) of developing at least one cold whilst taking vitamin C regularly over the study period, ranging from 2 weeks to 5 years. In the general community trials, where people had no heavy short-term physical stress, the pooled RR was 0.97 (95% CI 0.94 to 1.00; 24 trials, n=10708). Trials involving marathon runners, skiers and soldiers on subarctic exercises yielded a pooled RR of 0.48 (95% CI 0.35 to 0.64; 5 trials, n=598).

Thirty-one comparisons examined the effect of regular vitamin C on common cold duration (9745 episodes). In adults the duration of colds was reduced by 8% (4% to 12%) and in children by 14% (7% to 21%). In children, 1 to 2 g/day vitamin C shortened colds by 18%. The severity of colds was also reduced by regular vitamin C administration. 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.

Clinical comment: Routine vitamin C supplementation is not justified in the prevention of common cold, yet vitamin C may be useful for people exposed to brief periods of severe physical exercise.

Ədəbiyyat

1. Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev 2013;1():CD000980.