Although the role of
vitamin C in
common cold incidence had been studied extensively, the level of
vitamin C intake has not been unequivocally shown to affect the incidence of colds. In the present study the six largest
vitamin C supplementation (> or = 1 g/d) studies, including over 5000 episodes in all, have been analysed, and it is shown that
common cold incidence is not reduced in the
vitamin C-supplemented groups compared with the placebo groups (pooled rate ratio (RR) 0.99; 95% CI 0.93, 1.04). Consequently these six major studies give no evidence that high-dose
vitamin C supplementation decreases
common cold incidence in ordinary people. Nevertheless, the analysis was continued with the hypothesis that
vitamin C intake may affect
common cold susceptibility in specific groups of people. It was assumed that the potential effect of supplementation might be most conspicuous in subjects with low dietary
vitamin C intake. The average
vitamin C intake has been rather low in the UK and plasma
vitamin C concentrations are in general lower in males than in females. In four studies with British females
vitamin C supplementation had no marked effect on
common cold incidence (pooled RR 0.95; 95% CI 0.86, 1.04). However, in four studies with British male schoolchildren and students a statistically highly significant reduction in
common cold incidence was found in groups supplemented with
vitamin C (pooled RR 0.70; 95% CI 0.60, 0.81). Thus, these studies with British males indicate that
vitamin C intake has physiological effects on susceptibility to
common cold infections, although the effect seems quantitatively meaningful only in limited groups of people and is not very large.