Does vitamin C cut colds in people who exercise?

Short answer

Not in everyday life: in 10,708 people, cold risk differed by only 3%. But in 598 people doing short bouts of extreme exercise, like marathons or training in severe cold, colds fell by more than half.

Does vitamin C cut colds in people who exercise?
'Vitamin C → stronger immunity → fewer colds' — does this formula hold?
Many people reach for vitamin C at the first sign of a cold. It is also common to hear that hard training makes you catch colds more easily. We checked whether vitamin C really cuts colds.
For most people, colds did not drop
A 3% difference in trials with over 10,000 people
An analysis that pooled trials of daily vitamin C found that in 10,708 people from the general population, for every 100 who caught a cold on a dummy pill, 97 did on vitamin C. The number of colds stayed practically the same.
But people doing extreme exercise were different
Cold risk fell to 0.48 times
In 5 trials with 598 people doing short bouts of extreme exercise, such as marathons, a ski camp or training in severe cold, those taking vitamin C had 0.48 times the cold risk. Colds fell by more than half.
A key trial: 2 weeks after an ultramarathon
Cold symptoms fell from 68% to 33%
People racing an ultramarathon longer than 42 km took either 600 mg of vitamin C or a dummy pill daily. In the 2 weeks after the race, 68% of the dummy pill group had cold symptoms, versus 33% of the vitamin C group.
What about taking it after you catch a cold?
Starting after symptoms showed no difference
In 7 comparisons (3,249 colds) where vitamin C was started after symptoms began, cold length and severity did not drop consistently. The benefit appeared only with regular daily use beforehand.
Daily use makes colds a little milder
8% shorter and 15% less severe
Across 31 comparisons (9,745 colds) of daily use, colds in adults were 8% shorter. In 10 trials using 1 g or more a day, cold severity fell by 15%. For a week-long cold, that is about half a day shorter.
Bottom line: it matters before extreme exercise
In everyday life, it did not cut the number of colds
Vitamin C did not cut the number of colds in the general population. Colds were halved in people doing short bouts of extreme exercise like marathons, who took 0.6-1 g daily. Starting it after symptoms began made no difference.