Vitamin C and sex differences in respiratory tract infections.

Vitamin C and sex differences in respiratory tract infections In their systematic review of sex differences in respiratory tract infections (RTIs), Falagas et al. concluded that males develop RTIs more frequently than females, in particular lower RTIs, and the course of the infection is often more severe in males than in females. 1 In 1997, I reported a meta-analysis of British trials on vitamin C and the common cold which gives a complementary viewpoint on sex differences in RTIs. 2 In four trials with males, vitamin C supplementation reduced common cold incidence by 30% (95% CI: À40% to À19%), but had no effect in four trials with females (estimate À5%; 95% CI: À14% to +4%). The divergence in the confidence intervals suggests different effects on males and females. Three studies reported data for both males and females and the largest of these, by Baird et al., 3 found highly significant interaction between sex and vitamin C effect on common cold incidence (Table 1). The two smaller trials had wide confidence intervals that overlapped between males and females. 2 Furthermore, in four trials with British males, vitamin C reduced recurrent colds during the study period by 46% (À60% to À26%), but had no effect on females. 2 In particular, Tyrrell et al. 4 found that therapeutic vitamin C during the first cold episode reduced subsequent colds in males by 40% (À63% to À3%), 2 but not in females (À7%; À45% to +54%). The Baird et al. 3 and Tyrrell et al. 4 studies were randomised placebo-controlled double-blind trials and their findings cannot be dismissed on methodological grounds. Because large-scale trials give no evidence that high-dose vitamin C supplementation (X1 g/day) decreases common cold incidence, 2 the findings with British males call for special explanations. Several surveys had reported low dietary vitamin C intake in the UK and thus the benefit of supplementation may be explained by treating marginal deficiency. 2 This explanation is consistent with the estimated low daily vitamin C intake in Baird's study, 50 mg/day, and the particularly low dosage of vitamin C supplementa-tion, 80 mg/day. 3 Usually plasma and leucocyte vitamin C concentrations are lower in males than in females although it is not clear to what extent this is due to dietary and physiological differences between the sexes. 2 Concluding from the British studies, 2–4 it seems that sex differences in RTIs may be generated by variations …