I cannot believe that we are still talking about whether vitamin C prevents colds. No such luck.
What triggered this is a recent and quite detailed critique of a 2018 meta-analysis of studies of this question: “Extra Dose of Vitamin C Based on a Daily Supplementation Shortens the Common Cold: A Meta-Analysis of 9 Randomized Controlled Trials.”
Its conclusion: “The combination of supplemental and therapeutic doses of vitamin C is capable of relieving chest pain, fever, and chills, as well as shortening the time of confinement indoors and mean duration.”
You can read the detailed critique of this study for yourself, but I thought this was settled years ago by one of my all-time favorite nutrition studies: Karlowski TR, Chalmers TC, Frenkel LD, Kapikian AZ, Lewis TL, Lynch JM. Ascorbic acid for the common cold. A prophylactic and therapeutic trial. JAMA 1975; 231: 1038–1042.
This fabulous study was done at NIH using NIH employees. As the abstract puts it,
Three hundred eleven employees of the National Institutes of Health volunteered to take 1 gm of ascorbic acid or lactose placebo in capsules three times a day for nine months. At the onset of a cold, the volunteers were given an additional 3 gm daily of either a placebo or ascorbic acid.
The initial analysis of the data showed a highly significant effect of vitamin C in preventing colds or reducing symptoms. But the trial had one major flaw: it had an usually big dropout rate.
One hundred ninety volunteers completed the study. Dropouts were defined as those who missed at least one month of drug ingestion. They represented 44% of the placebo group and 34% of those taking ascorbic acid.
These were good investigators. They asked the dropouts why they had dropped out. The reason: the study subjects knew (well, they thought they knew) whether they were taking vitamin C or the placebo.
The investigators reanalyzed the data according to what the study subjects thought they were taking. Those who thought they were taking vitamin C had fewer colds and reduced symptoms—regardless of whether they were taking vitamin C or the placebo. And those who thought they were taking the placebo had more colds and worse symptoms regardless of whether they really were taking the placebo or were actually taking vitamin C.
The authors’ cautious conclusion:
Analysis of these data showed that ascorbic acid had at best only a minor influence on the duration and severity of colds, and that the effects demonstrated might be explained equally well by a break in the double blind.
My conclusion: Vitamin C is one terrific placebo.
Nothing wrong with that, but that’s why I can’t believe investigators are still arguing about it.