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 supplementation in normal subjects reduces constitutive ICAM-1 expression.
Regulation of monocyte adhesion molecule gene expression is via redox sensitive transcription factors . We have investigated whether dietary antioxidant supplementation with vitamin C ( 250 mg/day ) can modulate monocyte ICAM-1 expression in healthy male subjects with low plasma vitamin C at baseline . In a r and omised , double-blind , crossover study , monocyte ICAM-1 mRNA was analysed using quantitative reverse transcriptase PCR . Protein was determined by flow cytometry ( monocytes ) and ELISA ( plasma ) . Monocyte numbers were unaltered by supplementation . Subjects with low plasma vitamin C ( < 50 microM ) prior to supplementation expressed higher levels of monocyte ICAM-1mRNA , and showed a significant ( 50 % ) reduction in ICAM-1mRNA expression after 6 weeks of 250 mg/day vitamin C supplementation ( p<0.05 ) . This was paralleled by a reduction in sICAM-1 ( p<0.05 ) . For the first time , these results show that dietary vitamin C can modulate monocyte ICAM-1 gene expression in vivo , where regulation of gene expression represents a novel mechanism for benefit from dietary antioxidants
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 .