That's the word from a brand new review of some 60 years of clinical research by Robert M. Douglas, MD, emeritus professor at Australian National University, Canberra, and colleagues.
Vitamin C affects resistance to viruses in lab animals. That's led to decades of speculation that vitamin C supplements could be used to treat or to prevent the common cold.
It's also led to a large number of clinical trials, which tested a wide range of vitamin C doses in a wide range of people. In a prestigious Cochrane Review, Douglas and colleagues used widely accepted techniques for analyzing all these studies.
- When taken after a cold starts, vitamin C supplements do not make a cold shorter or less severe.
- When taken as a daily preventive medicine, vitamin C very slightly shortens cold duration -- by 8% in adults and by 13.6% in children.
- When given as a preventive medicine to highly fit people in conditions of extreme cold -- data based mostly on marathon runners -- vitamin C cuts the risk of getting a cold in half.
The average adult who suffers with colds for 12 days a year would still suffer for 11 days a year if that person took a high dose of vitamin C every day.
"It would not seem reasonable to ingest vitamin C regularly in the mega-dose range throughout the year if the only anticipated benefit is to rather slightly shorten the duration of colds, which occur for adults only two or three times a year," Douglas and colleagues suggest.
For the average child who suffers about 28 days of cold illness a year, taking daily high-dose vitamin C would still mean 24 days of cold illness.
"Such a benefit is not trivial but is it worth the cost of long-term prophylaxis?" Douglas and colleagues ask.
The only strong effect of vitamin C was in preventing colds among people engaged in extreme physical exercise in extremely cold conditions.
"Caution should be exercised in generalizing this finding that is mainly based on marathon runners," Douglas and colleagues caution.
Douglas and colleagues report their findings in The Cochrane Collaboration.