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 .
Winter illness and vitamin C: the effect of relatively low doses.
After their r and om -llocation to one of three treatment aroups , 622 volunteers received either  vitamin C or placebo in a maintenance dose of 500 mg once weekly and a therapeutic dose of 1500 mg daily on the 1st day and 1000 mg on the next 4 days of any illness . Two forms of vitamin C were employed : a sustained-release capsule containing ascorbic acid and a regular tabet containing a mixture of sodium and calcium ascorbate . In the 448 subjects who completed an average of 15 weeks in the study of total of 635 episodes of illness were recroded . Respiratory symptoms were recorded on at least 1 day in 92 per cent of these episodes . There were no consistent or significant differences in the sickness experience of the subjects receiving the sustained-release vitamin capsules compared to those receiving the vitamin tablets , but subjects in both vitamin groups experienced less severe illness than subjects in the placebo group , with approximately 25 per cent fewer days spent indoors because of the illness ( P smaller than 0.05 ) . These results are compatible with the belief that supplementary vitamin C can reduce the burden of winter illness , but the intake need not be as high as has sometimes been cl aim ed
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 .