A meta-analysis of 72 trials examining the benefits of regular vitamin C supplementation concludes that although vitamin C may have a modest yet consistent effect on the duration of colds, it has no effect on the incidence of colds, except in people exposed to short periods of extreme physical stress.
The review was published online January 31 in Cochrane Database of Systemic Reviews.
"Vitamin C (ascorbic acid) for preventing and treating the common cold has been a subject of controversy for 70 years," write Harri Hemilá, MD, PhD, from the Department of Biochemistry, University of Helsinki, Finland, and a colleague.
Vitamin C has been touted for treating respiratory infections since it was first isolated during the 1930s, the authors point out. The number of people taking vitamin C supplements rose dramatically in the 1970s, after Nobel laureate Linus Pauling, citing earlier placebo-controlled trials, concluded that vitamin C could help prevent colds as well as reduce cold symptoms after onset.
Still, much controversy continues about the benefits of vitamin C supplementation.
In an effort to shed more light on that issue, these authors used several databases (CENTRAL, MEDLINE, EMBASE, CINAHL, LILACS, and Web of Science) to identify and review results from 72 placebo-controlled trials carried out since 1966. They excluded trials that used less than 0.2 g per day of vitamin C.
Twenty-nine of the trials, involving 11,306 participants, were aimed at determining the risk of developing a cold while taking vitamin C during varying periods of study. The authors of this review say that in studies focusing on community populations as a whole (involving 10,708 people), the pooled risk ratio (RR) of developing a cold while taking vitamin C was 0.97 (95% confidence interval [CI], 0.94 - 1.00), representing a nonsignificant 3% reduction in risk.
However, in 5 trials that assessed the effects of supplementation on people involved in extreme physical activities (marathon runners, skiers, and soldiers on subarctic exercises), the pooled RR was 0.48 (95% CI, 0.35 - 0.64), representing a statistically significant 52% reduction in risk.
As to the supplement's effects on duration of symptoms, the reviewers evaluated data from 31 studies (9745 cold episodes) and report that vitamin C supplements were associated with about an 8% reduction in duration of symptoms in adults and a 14% reduction in children.
"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," the authors write.
Seven of the assessed studies examined the effect of therapeutic vitamin C (up to 8 g/day begun after onset of symptoms) on a total of 3249 episodes of colds. The authors say the pooled data showed no consistent effects of vitamin C on either severity or duration of colds.
"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," the authors write.
They emphasize that more randomized controlled trials are needed to assess all the effects of vitamin C on the common cold.
"Nevertheless," they conclude, "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."
This study was supported in part by the Australian National University, Canberra, Australian Capital Territory and the Commonwealth Department of Health and Ageing. The authors have disclosed no relevant financial relationships.
Cochrane Database Syst Rev. Published online January 31, 2013. Abstract
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Cite this: Vitamin C for Colds Shows Limited Benefit: Meta-Analysis - Medscape - Feb 08, 2013.