Vitamin C Supplementation and Respiratory Infections: a Systematic Review

Military Medicine,  Nov 2004  by Hemilä, Harri

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Furthermore, Kimbarowski and Mokrow18 observed a reduced risk of pneumonia in influenza patients and Dahlberg et al.17 reported 50% lower incidence of respiratory infections more severe than the common cold in their vitamin C group, but the difference was not statistically significant.

Amount of Vitamin C in Diet and in Supplements

One particular problem in the interpretation of vitamin C trials is the great variation in the dietary intake levels and supplementation dosages. A different value of outcome between vitamin C and control groups may result from a particularly low dietary intake in the control group ("marginal deficiency") or from the high-dose supplementation in the vitamin C group. In the former case, a small dosage of supplement might produce a similar effect, whereas the latter case requires the particularly high dosage. Previously, low dietary intake of vitamin C, rather than high-dose supplementation, was proposed better to explain the decrease in common cold incidence found in trials with British men.13

The trials in Tables I and II vary widely in dietary vitamin C intake. Sabiston and Radomski19 estimated that the food rations contained at most 40 mg/day of vitamin C, which is considerably lower than the current U.S. recommendation for men (90 mg/day).34 Pitt and Costrini16 failed to estimate dietary vitamin C intake, but in their subjects, the mean vitamin C level in plasma was rather high initially, 57 µmol/L, corresponding to a dietary intake of 100 mg/day or more.35 Glazebrook and Thomson23 estimated that their control students obtained only 15 mg/day of vitamin C in foods. On the other hand, the control group of Peters et al.26,27 received some 500 mg/day of vitamin C from foods and self-supplementation. Thus, vitamin C intakes among the control groups vary up to 30-fold. The dosage of supplements also varied dramatically, from 0.05 to 2 g/day (Tables 1 and II). Accordingly, the supplement dosages vary up to 40-fold.

Such dramatic variations in dietary vitamin C intakes and in supplement dosages preclude any simple comparisons of the trials in Tables I and II and any straight generalizations to other population groups, although all of the trials test whether vitamin C supplementation affects respiratory infections.

Conclusions

Several trials with military personnel and with participants under conditions comparable to those of military recruits have found that vitamin C substantially reduced the incidence or severity of respiratory infections. Although there is great variation in the technical quality of the analyzed trials, eight of the trials were double blind and placebo controlled. Furthermore, the technical shortcomings in two less satisfactory trials18,23 fail to discount their findings.

The estimates of benefit calculated in Tables I and II require cautious interpretation. If vitamin C does affect respiratory infections, it seems evident that no single estimate applies to all population groups because various factors, such as the dietary intake level of vitamin C, the dosage of supplements, the definition of outcome, and the level of exertion, most likely modify the effect. Consequently, the estimates presented in the tables in all probability do not directly apply to other population groups. Nevertheless, the large number of statistically significant benefits observed in these trials seems to warrant further examination of the role of vitamin C in respiratory infections, particularly in military recruits.