Daily vitamin C: no shield against colds, a little time saved
A Cochrane review found no consistent effect on how long colds lasted from taking high doses of vitamin C only after symptoms began.
Daily vitamin C supplements do not stop most people catching colds, though they may make colds slightly shorter, according to a Cochrane review of placebo-controlled trials.
The review, by Hemilä and Chalker, was published on 31 January 2013. It covered trials that gave at least 0.2g of vitamin C a day.
In trials of 10,708 people from the general community, regular vitamin C did not reduce the number of people who caught a cold.
Colds did run shorter. The review pooled 31 comparisons covering 9,745 colds. Regular supplements shortened them by 8% in adults and by 14% in children. For adults, the possible range was 3% to 12%.
On a cold that lasts a week, our own arithmetic puts that at roughly half a day for adults and a day for children.
One small group stood apart. In five trials of 598 marathon runners, skiers and soldiers on subarctic exercises, vitamin C halved the risk of catching a cold, and the authors wrote that it “may be useful” for people facing brief periods of severe physical exercise.
High doses taken once symptoms appear are a separate question. Seven comparisons, covering 3,249 colds, found no consistent effect on length or severity, though few such trials exist and none involved children.
The authors wrote that routine supplementation is “not justified” in the general population. Separately, they said it “may be worthwhile” for people with a cold to test on an individual basis whether taking it once symptoms start helps them, given the consistent effect on the duration and severity of colds in the regular supplementation studies and its low cost and safety.
The NHS’s common cold page says there is “little evidence” that supplements such as vitamin C, echinacea or garlic prevent colds or help people get better more quickly.
The NHS says on its vitamin C page that 40mg a day is what adults need. That is the figure for adults aged 19 to 64, and a varied, balanced diet should supply all of it. Good sources include citrus fruit, peppers, strawberries, blackcurrants, broccoli, Brussels sprouts and potatoes.
Taking more than 1,000mg a day can cause stomach pain, diarrhoea and flatulence, the same page says, and those symptoms should disappear once supplements are stopped. It also says anyone taking supplements should not take too much, as this could be harmful.
The review’s searches ended in November 2012, so later trials are not included, and the finding for runners, skiers and soldiers rests on five trials. The NHS vitamin C page was last reviewed on 3 August 2020, with its next review due in August 2023.
People should begin to feel better in about one to two weeks, the NHS says, and among its reasons to see a GP are a high temperature for more than 3 days, symptoms that get worse or do not get better after 10 days, and shortness of breath or chest pain.