Comment: The quality of evidence is downgraded by inconsistency (unexplained variability in results).
A Cochrane review [Abstract] 1 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
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=10 708). 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 (9 745 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 (3 249 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.
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