Vitamin C and acute respiratory infections

So far over 60 studies have examined the effects of vitamin C on the common cold. No effect on common cold incidence was observed in the six largest s tudies, indicating that vitamin C has no preventive effects in normally nou rished subjects in the Western countries. There are, however, smaller studi es reporting benefit. In three trials of subjects under heavy acute physica l stress, common cold incidence decreased by on average 50%, and in four tr ials of British males common cold incidence decreased by on average 30% in the vitamin C groups. The dietary vitamin C intake in the UK is low and con sequently the benefit may be due to the correction of marginal deficiency, rather than high vitamin doses. Regular vitamin C supplementation (greater than or equal to 1 g/day) has quite consistently reduced the duration of co lds, but the size of the benefit has varied greatly. In the four largest st udies the duration of colds was reduced only by 5%. In two of these studies , however, absence from school and work was reduced by 14-21% per episode, which may have practical importance. Three controlled studies recorded a re duction of at least 80% in the incidence of pneumonia in the vitamin C grou p, and one randomised trial reported substantial treatment benefit from vit amin C in elderly UK patients hospitalized with pneumonia or bronchitis. It seems that the preventive effects of supplementation are mainly limited to subjects with low dietary vitamin C intake, but therapeutic effects may oc cur in wider population groups. Further carefully designed trials are neede d to explore the effects of vitamin C.