Vitamin C Supplementation and Respiratory Infections: a Systematic Review

Military Medicine, Nov 2004 by Hemilä, Harri

Dahlberg et al.17 performed a large-scale, double-blind, placebo-controlled trial with infantry recruits in Northern Sweden in the wintertime. Allocation to the study groups was by odd and even identity numbers. Low doses of vitamin C had no effect on the number of participants who caught the common cold. However, the number of participants in the vitamin C group with more severe respiratory infections was only one-half that of the placebo group level, although the difference was not statistically significant.

Kimbarowski and Mokrow18 carried out a therapeutic trial in the former Soviet Union with military personnel hospitalized for influenza A infections. The method of allocation was not described and placebo was not used. Nevertheless, the distribution of influenza severity in the vitamin C and control groups was similar, indicating that the groups were comparable in this respect. The authors focused primarily on the usefulness of a laboratory test in evaluating the severity of uncomplicated influenza and excluded from further studies those participants who caught pneumonia during vitamin C supplementation. The number of pneumonia cases was reported and it was significantly lower in the vitamin C group. The authors were uninterested in pneumonia per se, and therefore the placebo effect and information bias28 seem unlikely explanations for the reported difference.

Kimbarowski and Mokrow18 also noted that most vitamin C patients were released on the 9th day of illness, whereas the control patients were released 2 to 3 days later, mostly on the 12th day of illness. However, the authors failed to publish the actual mean and SD, thus p cannot be calculated for the stay in hospital.

Sabiston and Radomski19 carried out a small randomized, double-blind, placebo-controlled trial with troops on a winter exercise in Northern Canada. The vitamin C group saw not only significantly fewer common cold episodes, but also a significantly shorter duration of constitutional symptoms as well. Moreover, it is worth noting the uneven distribution of colds in the 14 tents. There was at least one cold in nine tents: six tents had colds only in the placebo subjects, and three tents had colds in the placebo and vitamin C subjects, but no tents had colds only in the vitamin C subjects.19 Accordingly, vitamin C appeared to increase resistance against colds so that vitamin C subjects fell ill only if exposed to infected placebo subjects within the same tents but not if exposed to infected subjects outdoors.19,30

Elliott20 carried out a small randomized, double-blind, placebo-controlled trial in a Polaris submarine. Vitamin C had no effect on common cold incidence, but Elliott failed to report explicit data. Nevertheless, the vitamin C group saw fewer days of morbidity for productive and nonproductive cough, hoarseness, and sore throat. The effect on productive cough and sore throat was statistically significant.

Liljefors21 performed a small randomized, placebo-controlled, double-blind cross-over trial in Sweden in the autumn. The subjects participated in military repetition exercise and were considerably older than ordinary recruits (mean age 36 years in the Liljefors trial). Vitamin C had no effect on common cold incidence in this small and short trial.