Vitamin C
According to the Cochrane review on vitamin C and the common cold, 1 g/day or more of vitamin C does not influence common cold incidence in the general community. However, in five randomized double-blind placebo-controlled trials with participants who were under heavy short-term physical stress (three of the trials were with marathon runners), vitamin C halved the incidence of colds. In the dose of 1 g/day or more, vitamin C shortened the duration of colds in adults by 8% and in children by 18%. Vitamin C also decreased the severity of colds.