Vitamin C and the common cold
The common cold is caused by several groups of viruses including rhinoviruses, coronaviruses and respiratory syncytial virus. Colds are the leading cause of doctor's visits and the number one reason for absences from work and school. While human and animal studies have shown that vitamin C does significantly improve immune function, over 40 years of research suggest that vitamin C does not prevent the common cold or decrease the severity of symptoms in non-athletic people. It may be helpful for highly active people training in stressful conditions.
In the 1970s, Linus Pauling argued that vitamin C could significantly decrease the incidence of the common cold, which spurred a widespread belief that consuming more vitamin C will reduce the risk of catching a cold and reduce its severity. Spurred by Pauling's claims, NIH conducted one of the first double-blind, placebo-controlled studies of vitamin C. The results showed a moderate decrease in duration and severity of colds, however, a post-trial analysis by the authors suggested to them that the results may have been attributable to a failure of the double-blind protocol. A subsequent meta-analysis found only very minor reduction in duration, and the authors concluded that vitamin C had no value in treating the common cold.
In the past 40 years, numerous epidemiological and placebo-controlled trials have examined the effect of vitamin C supplementation on the prevention and treatment of colds. More than 30 clinical trials with over 10,000 participants have examined the effects of taking daily vitamin C in doses up to 2 g/day. The majority of studies of non-athletic people, when looked at collectively, led researchers to conclude that vitamin C does not prevent or treat the common cold, but highly physically active people training in stressful conditions (e.g. soldiers training in the Arctic) may benefit from supplementation. A recent literature review concluded that vitamin C consumption decreased the duration of cold symptoms, but did not prevent a cold or affect symptom severity.
Claims of vitamin C's efficacy in treating the common cold have been criticized by many researchers. The most-cited reviews of the subject have concluded that there are no beneficial effects beyond a placebo. A review of 72 studies, published in January 2013, found no significant effect of consuming vitamin C supplementation on the incidence of colds.
Sources of vitamin C
Generally, nutrients have more bioavailability in naturally occurring food sources. However, ascorbic acid’s bioavailability is equal in both food and supplemental sources. Therefore, while it is better to consume natural foods such as red peppers, oranges, strawberries and grapefruit, because they offer other nutrients that are likely to have a higher bioavailability, it is acceptable to consume vitamin C from a supplement and still maintain adequate intake levels.
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