In addition , numerous trials have shown no difference regarding the most commonly studied endpoint : frequency of symptoms .
In 5 trials with participants under heavy acute physical stress ( 3 with marathon runners )  , vitamin C  reduced the incidence of colds by 52 % ( 95 % CI : 36 % to 65 % ) , but there is no evidence that vitamin C prevents colds in the general population .
Proponents of evidence -based medicine ( EBM ) emphasize that conclusions about medical interventions should be based on controlled trials with clinical ly relevant outcomes , so that the greatest weight should be given to the systematic review s of the trials .
So far , therapeutic supplementation has not been shown to be effective .
The effect of vitamin C on upper respiratory infections in adolescent swimmers: a randomized trial
The risk of upper respiratory infections ( URIs ) is increased in people who are under heavy physical stress , including recreational and competitive swimmers . Additional treatment options are needed , especially in the younger age group . The aim of this study was to determine whether 1 g/day vitamin C supplementation affects the rate , length , or severity of URIs in adolescent swimmers . We carried out a r and omized , double-blind , placebo-controlled trial during three winter months , among 39 competitive young swimmers ( mean age 13.8 ± 1.6 years ) in Jerusalem , Israel .  Vitamin C had no effect on the incidence of URIs ( rate ratio = 1.01 ; 95 % confidence interval ( CI ) = 0.70–1.46 ) . The duration of respiratory infections was 22 % shorter in vitamin C group , but the difference was not statistically significant . However , we found a significant interaction between vitamin C effect and sex , so that vitamin C shortened the duration of infections in male swimmers by 47 % ( 95 % CI : −80 % to −14 % ) , but had no effect on female swimmers ( difference in duration : + 17 % ; 95 % CI : −38 % to + 71 % ) . The effect of vitamin C on the severity of URIs was also different between male and female swimmers , so that vitamin C was beneficial for males , but not for females . Our study indicates that vitamin C does not affect the rate of respiratory infections in competitive swimmers . Nevertheless , we found that vitamin C  decreased the duration and severity of respiratory infections in male swimmers , but not in females . This finding warrants further research
In addition , numerous trials have shown no difference regarding the most commonly studied endpoint : frequency of symptoms .
In 5 trials with participants under heavy acute physical stress ( 3 with marathon runners )  , vitamin C  reduced the incidence of colds by 52 % ( 95 % CI : 36 % to 65 % ) , but there is no evidence that vitamin C prevents colds in the general population .
Proponents of evidence -based medicine ( EBM ) emphasize that conclusions about medical interventions should be based on controlled trials with clinical ly relevant outcomes , so that the greatest weight should be given to the systematic review s of the trials .
So far , therapeutic supplementation has not been shown to be effective .