This study aimed to determine whether 1g/day vitamin C supplementation affects the rate, length or severity of upper respiratory infections in adolescent swimmers during winter.
The randomized, double-blind, placebo-controlled study found that vitamin C does not affect the rate of respiratory infections in competitive swimmers, but does decrease the duration and severity of the infections in male swimmers.
Constantini, N. W., Dubnov-Raz, G., Eyal, B., Berry, E. M., Cohen, A. H., & Hemila, H. (2011). The effect of vitamin C on upper respiratory infections in adolescent swimmers: A randomized trial. European Journal of Paediatrics, 170, 59-63. Doi: 10.1007/s00431-010-1270-z