Vitamin C and Colds: 11,306 People, One Clear Answer

Not in ordinary people. The Cochrane review reports that in general community trials involving 10,708 participants, the pooled risk ratio was 0.97 with a 95 per cent confidence interval of 0.94 to 1.00. A risk ratio of 1.00 would mean no effect at all, and the interval reaches it. Whatever effect exists in the general population is too small to matter, and this is one of the most thoroughly tested questions in nutrition.

Because of the second finding, which is genuine and gets quoted without its context. Five trials involving 598 marathon runners, skiers and soldiers on subarctic exercises yielded a pooled risk ratio of 0.48 with a confidence interval of 0.35 to 0.64. That is roughly a halving. The population is the entire point: people undergoing short periods of severe physical stress in cold conditions. A finding in soldiers on subarctic exercises does not transfer to an office worker in November.

Slightly, and the honest version is less impressive than the percentage sounds. Duration was reduced by 8 per cent in adults, with a confidence interval of 3 to 12 per cent, and by 14 per cent in children, 7 to 21 per cent. On a seven-day cold, 8 per cent is about half a day. That is a real, statistically significant effect and it is not the difference between being ill and being fine.

The review reports 14 per cent in children against 8 per cent in adults, and the confidence intervals are wide enough that the two overlap at the edges. It is a consistent direction rather than a settled explanation. What the review establishes is that both effects are real and modest; why children's colds respond more is not answered by the pooled numbers themselves.

A great deal, and it is why this is one of the few supplement questions that can be called settled for the general population. The incidence analysis drew on 29 trial comparisons and 11,306 participants, with 10,708 in the community trials specifically. When a question has been asked at that scale and the answer is a risk ratio of 0.97, further small trials are unlikely to overturn it.

The review's therapeutic evidence is about regular supplementation begun before illness rather than a dose started once symptoms appear, and those are different questions. Vitamin C is inexpensive and, at ordinary doses, well tolerated, so the calculation is not really about risk. It is about expectation: half a day off a cold is what the pooled evidence supports, and anyone promising more is going beyond it.

There is a defensible one if you are genuinely in the stressed-athlete category, where the risk ratio was 0.48. For everyone else, this particular reason does not hold up. That says nothing about vitamin C's other roles in the body or about correcting a genuine deficiency, which are separate questions with separate evidence. It says that colds are not a good reason.

Vitamin C is water soluble and excess is largely excreted, which is why very high doses are sold freely, but high intakes are commonly associated with gastrointestinal effects such as diarrhoea and cramping. The trials in this review used a range of doses, and nothing in the finding suggests that pushing the dose higher converts a 0.97 risk ratio into a useful one. This article is not medical advice.