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If you are popping vitamin C tablets daily in hopes of avoiding the common cold, you are wasting your money.

A scientific review of 30 studies involving more than 11,000 volunteers found that the popular supplement offers little protection against coughs, sniffles and sneezes.

At best, vitamin C may shorten the illness - but not by much. It trimmed the duration of colds by 8 per cent for adults and 13 per cent for children, according to the findings published in the Cochrane Library, a non-profit publication that systematically reviews other medical literature.

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However, there is one group that seems to profit from a daily dose - people who are exposed to short periods of extreme physical stress such as marathon runners. Vitamin C reduced this group's risks of a cold by 50 per cent, noted one of the authors of the report, Harri Hemila of the University of Helsinki.

Why should this group see a benefit, but not others?

Dr. Hemila explained that short bursts of extreme exercise produce lots of free radicals, highly unstable atoms that may put the body at risk for infections. Vitamin C, an antioxidant, apparently neutralizes these free radicals.

Regular exercise, on the other hand, doesn't produce the same level of unchecked free radicals.

So, unless you happen to be a marathon runner, the gains of supplementation are so marginal that "it doesn't make sense to take vitamin C 365 days a year to lessen the chance of catching a cold," Dr. Hemila said.

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