REVIEW ERS ' CONCLUSIONS The failure of vitamin C supplementation to reduce the incidence of colds in the normal population indicates that routine mega-dose prophylaxis is not rationally justified for community use .
But evidence shows that it could be justified in persons exposed to brief periods of severe physical exercise and /or cold environments .
Also , the consistent and statistically significant small benefits on duration and severity for those using regular vitamin C prophylaxis indicates that vitamin C plays some role in respiratory defence mechanisms .
BACKGROUND The role of oral vitamin C ( ascorbic acid ) in the prevention and treatment of the  common cold  has been a subject of controversy for at least sixty years .
Public interest in the topic continues to be high and vitamin C continues to be widely sold and used as a preventive and therapeutic agent for this common ailment .
OBJECTIVES To discover whether oral vitamin C in doses of 200 mg or more daily , reduces the incidence , duration or severity of the common cold when used either as continuous prophylaxis or after the onset of cold symptoms .
Vitamin C pharmacokinetics in healthy volunteers: evidence for a recommended dietary allowance.
Determinants of the recommended dietary allowance ( RDA ) for vitamin C include the relationship between vitamin C dose and steady-state plasma concentration , bioavailability , urinary excretion , cell concentration , and potential adverse effects . Because current data are inadequate , an in-hospital depletion-repletion study was conducted . Seven healthy volunteers were hospitalized for 4 - 6 months and consumed a  diet  containing  < 5 mg of vitamin C daily .  Steady-state plasma and tissue concentrations were determined at seven daily doses of vitamin C from 30 to 2500 mg .  Vitamin C steady-state plasma concentrations as a function of dose displayed sigmoid kinetics . The steep portion of the curve occurred between the 30- and 100-mg daily dose , the current RDA of 60 mg daily was on the lower third of the curve , the first dose beyond the sigmoid portion of the curve was 200 mg daily , and complete plasma saturation occurred at 1000 mg daily . Neutrophils , monocytes , and lymphocytes saturated at 100 mg daily and contained concentrations at least 14-fold higher than plasma . Bioavailability was complete for 200 mg of vitamin C as a single dose . No  vitamin C  was excreted in urine of six of seven volunteers until the 100-mg dose . At single doses of 500 mg and higher , bioavailability declined and the absorbed amount was excreted . Oxalate and urate excretion were elevated at 1000 mg of vitamin C daily compared to lower doses . Based on these data and Institute of Medicine criteria , the current RDA of 60 mg daily should be increased to 200 mg daily , which can be obtained from fruits and vegetables . Safe doses of vitamin C are less than 1000 mg daily , and vitamin C daily doses above 400 mg have no evident value
REVIEW ERS ' CONCLUSIONS The failure of vitamin C supplementation to reduce the incidence of colds in the normal population indicates that routine mega-dose prophylaxis is not rationally justified for community use .
But evidence shows that it could be justified in persons exposed to brief periods of severe physical exercise and /or cold environments .
Also , the consistent and statistically significant small benefits on duration and severity for those using regular vitamin C prophylaxis indicates that vitamin C plays some role in respiratory defence mechanisms .
BACKGROUND The role of oral vitamin C ( ascorbic acid ) in the prevention and treatment of the  common cold  has been a subject of controversy for at least sixty years .
Public interest in the topic continues to be high and vitamin C continues to be widely sold and used as a preventive and therapeutic agent for this common ailment .
OBJECTIVES To discover whether oral vitamin C in doses of 200 mg or more daily , reduces the incidence , duration or severity of the common cold when used either as continuous prophylaxis or after the onset of cold symptoms .