Whether vitamin C prevents or treats the common cold has been argued over for seventy years, which is itself a clue that the effect is not large. A Cochrane review pooled the trials, excluding any using less than 0.2 grams a day. The interesting move was splitting the population: ordinary community trials on one side, and people under heavy physical stress on the other. Does the split matter?
In the general community, essentially nothing. Twenty-nine trial comparisons with 11,306 participants took vitamin C regularly. In the community trials, involving 10,708 people, the pooled for developing a cold was 0.97, with a 95 percent interval from 0.94 to 1.00. But five trials of 598 marathon runners, skiers and soldiers on gave a pooled risk ratio of 0.48.
Vitamin C for preventing and treating the common cold has been a subject of controversy for seventy years. A dispute that long-running is usually a sign that any real effect is small enough for study design to swing the answer, which makes a systematic review the right instrument.
The Cochrane review searched six databases through November 2012 along with two trial registers, and set two rules. Trials using less than 0.2 grams per day of vitamin C were excluded, and so were trials without a placebo comparison. during regular supplementation was assessed as the proportion of participants experiencing one or more colds during the study period.
Twenty-nine trial comparisons involving 11,306 participants contributed to the pooled analysis of developing a cold while taking vitamin C regularly. In the general community trials, involving 10,708 participants, the pooled risk ratio was 0.97, with a 95 percent interval from 0.94 to 1.00. That is a reduction of about three percent, with an upper bound sitting on no effect at all.
Then the exception. Five trials involving a total of 598 marathon runners, skiers and soldiers on subarctic exercises yielded a pooled risk ratio of 0.48, roughly halving the risk. Those are populations under sustained extreme physical stress, which is a plausible mechanism rather than an arbitrary slice. Weigh the two findings differently, though: one rests on more than ten thousand people and the other on 598.
wat. For ordinary people vitamin C barely moved the chance of catching a cold, and for marathon runners and soldiers it roughly halved it.
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It explains how a supplement can be both useless and effective depending on who takes it. The community result is the one that applies to almost everybody buying it.
How do we know?
Hemilä and Chalker searched six databases through late 2012 and two trial registers. They restricted the review to placebo-controlled trials using at least 0.2 grams of vitamin C daily, and pooled the incidence of colds during regular supplementation.
What is still unknown
The subgroup finding rests on five trials and 598 people under unusual physical stress, so it is far less robust than the community result it is quoted alongside. This covers incidence of colds, not how a cold feels.
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