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 suggests that routine vitamin C supplementation does not prevent the common cold but modestly reduces its duration and severity in the general population, and halves the risk of catching a cold in people exposed to extreme physical stress, while the few therapeutic trials of vitamin C treatment of the common cold have been inconsistent.
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 may decrease the duration of cold symptoms, but does 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. Past influential reviews of the subject have concluded that there are no beneficial effects beyond a placebo. A 2013 Cochrane systematic review of placebo-controlled trials using vitamin C doses of at least 200 mg/day found that routine vitamin C supplementation did not prevent colds in the general population, but halved the common cold risk in people exposed to short periods of extreme physical stress, shortened colds by 8 percent in adults and up to 18 percent in children, and reduced severity of symptoms. Only seven clinical trials tested vitamin C as a treatment after the onset of cold symptoms, and three of those trials showed a benefit, but considering the benefits of routine vitamin C supplementation, the review concludes, "it may be worthwhile for common cold patients to test on an individual basis whether therapeutic vitamin C is beneficial for them."
A systematic review of vitamin C for cold-induced asthma found three relevant studies, all of which found benefits from the administration of vitamin C, either against asthma attacks or against bronchial hypersensitivity that is a characteristic of asthma.
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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