To investigate whether vitamin C intake affects common cold susceptibility in specific groups of people.

The author used previous literature reviews and two reference lists. The search was extended by searching MEDLINE and SciSearch for additional studies published since 1988.

Study designs of evaluations included in the review

For the large-scale studies, the authors searched for placebo-controlled double-blind studies in which at least 1 g/day vitamin C was regularly administered, and in which there were 200 or more common cold episodes.

In the two UK study groups, an a priori choice of study design was not stated. The subgroup of 5 British studies was selected as a surrogate definition for low vitamin C intake.

Specific interventions included in the review

The vitamin C doses ranged from 1 to 3 g/day in the large-scale randomised controlled trials (RCTs), from 0.05 to 1 g/day in the British studies, and from 0.08 to 4 g/day in the studies of British males. The control groups received placebo.

Participants included in the review

Male and female adults, students and schoolchildren were included.

Outcomes assessed in the review

A decrease in susceptibility to infection by the common cold was measured by the common cold incidence rate, or a reduction in the recurrence of common cold infections.

How were decisions on the relevance of primary studies made?

The author does not state how the papers were selected for the review, or how many of the reviewers performed the selection.

The author does not state that they assessed quality.

The author does not state how the data were extracted for the review, or how many of the reviewers performed the data extraction.

The common cold incidence rate in the study groups was calculated from the published results for the large-scale studies and the group of British studies. For the group of studies of British males, the proportion of participants falling sick during the study was calculated from the published results.

In certain studies where only the mean number of episodes per person was given, the total number of episodes was calculated from the published results.

For the large-scale study and the group of British studies, the rate ratio or relative risk (RR) was calculated directly from the numbers of participants and episodes. For the group of studies of British males, the RR was calculated as the ratio of the proportions.

95% Confidence intervals (CIs) were calculated for the RRs in the three groups of studies. Where published results were available, the p-values were recalculated using the chi-squared test.

How were the studies combined?

The pooled RRs were calculated with 95% CIs by averaging the individual logarithm values, using the inverses of the variances as the weights.

How were differences between studies investigated?

The author does not state how differences between the studies were investigated.

The review analysed three sets of studies.

Six RCTs were included in the review of large-scale studies on common cold incidence and vitamin C. There were 1,747 participants in the vitamin C group and 1,766 participants in the placebo group.

Five studies were included in the review of British studies on common cold incidence and vitamin C. There were 1,100 participants (581 males and 519 females) in the vitamin C group and 1,674 participants (1,238 males and 436 females) in the placebo group. Three of the studies were RCTs, one was double-blind but not randomised, and one was neither double-blind nor randomised. One of the RCTs used an active placebo of low-dose vitamin C.

Four studies were included in the review of vitamin C intake in British males. There were 597 participants in the vitamin C group and 530 participants in the placebo group. Two of the studies were RCTs, one was double-blind but not randomised, and one was neither double-blind nor randomised.

In the 6 large-scale studies there was no evidence that high-dose vitamin C supplementation decreases common cold incidence in ordinary people (RR 0.99, 95% CI: 0.93, 1.04). In the group of British studies, the 4 studies of vitamin C supplementation for females showed no marked effect on common cold incidence (RR 0.95, 95% CI: 0.86, 1.04). The 4 studies of vitamin C supplementation for male schoolchildren and students showed a statistically-significant reduction in common cold incidence (RR 0.70, 95% CI: 0.60, 0.81).

In the third group of studies on British males, there was a statistically-significant reduction in the number of males with recurrent common cold infections in the grouup receiving vitamin C supplementation (RR 0.54, 95% CI: 0.40, 0.74; p=0.00005).

The studies with the British males indicated that vitamin C intake has physiological effects on susceptibility to common cold infections, although the effect seems quantitatively meaningful only in limited groups of people and is not very large.

The author has relied on previous reviews as the basis for the literature search, which were then supplemented by searches of two databases. It is unclear whether additional relevant studies may have been missed, since there is no mention of unpublished data, abstracts and non-English language studies in the searches. [A:Additional information received from the author shows that the search strategies were very extensive.]

The selection methods for the studies, and the validity and quality assessment measures of the included studies, were not included. In addition, the methods used to extract the data were not reported.

The author summarised the data in tabular format, and pooled the results from the individual studies using rate ratios and relative risk statistics. Heterogeneity between the studies was neither tested nor evaluated.

The author's conclusions should be viewed with caution because the quality of the included studies was not assessed, differences between the studies were not evaluated, and the only statistical significance was found in a very specific subset of the study populations and these results may be attributable to other causes.

Practice: The author did not state any implications for practice.

Research: The author states that further studies are required for a more accurate estimate of the practical significance of this effect, and that dietary requirements of doses of vitamin C may be higher than those officially recommended.

Juho Vainio Foundation; Academy of Finland.

This additional published commentary may also be of interest. Nutritional discussion forum. Br J Nutr 1997;78:857-66.

This is a critical abstract of a systematic review that meets the criteria for inclusion on DARE. Each critical abstract contains a brief summary of the review methods, results and conclusions followed by a detailed critical assessment on the reliability of the review and the conclusions drawn.