In a recent meta-analysis published in BMC Public Health, researchers examined 15 comparisons from 10 trials on the effect of vitamin C supplementation on common cold severity and duration.
Their results indicate that the effect of vitamin C is particularly important in reducing the duration of severe symptoms.
Study: Vitamin C reduces the severity of common colds: a meta-analysis. Image Credit: Gargonia/Shutterstock.com
Background
The common cold is a major reason for acute illness across high-income countries, where it accounts for the largest share of visits to physicians and leads to missing school or work. The economic toll it exacts has been compared to that of hypertension and stroke.
Since colds are caused by viral infections, they cannot be effectively treated with antibiotics; regardless, every other common cold patient will receive antibiotics to treat their condition.
The common cold has been associated with temporarily lowered levels of vitamin C levels in the urine, plasma, and leucocytes of infected people.
Despite compelling evidence from randomized control trials and meta-analyses that vitamin C supplementation can reduce the duration and severity of colds, disproportionately influential publications (some of which were subsequently retracted) led to a persistent belief that it is not beneficial.
Further rigorous and high-quality evidence could be instrumental in persuading people of the value of vitamin C supplementation. Since the ultimate consequence of this non-life-threatening illness appears to be time lost from work and school, focusing on this outcome is an important avenue for investigation.
About the study
In a previous work, the authors investigated the effect of supplementing vitamin C on the severity of the common cold based on the standardized mean difference (SMD) scale.
In the current study, they built on this work by estimating the effect of vitamin C supplementation on the severity of the illness based on a more easily interpretable relative scale.
They also compared its effect on mild symptom duration versus severe symptom duration across trials that reported both effects. The two outcomes of focus were (1) common cold severity in terms of symptoms, duration of severe symptoms, and days spent indoors or absent from work and (2) how long the cold lasted overall.
Trials were included in the analysis if they were placebo-controlled, and a minimum of 1g of vitamin C per day was orally administered over the study period to people who were healthy at baseline.
These criteria allowed researchers to examine how regular supplementation would affect the colds that occurred during the study. The minimum dose was determined by previous findings that indicated a dose-response relationship in that range.
Following accepted meta-analysis protocols, the research team assessed the risk of different sources of bias.
To analyze the treatment effect, they used the relative symptom severity scale and calculated the ratio of means to estimate the relative effect.
Statistical analysis included pooling all included trials and using inverse variance. Forest plots were used for sensitivity analysis.
Findings
Ten trial reports and 15 comparisons were included in the meta-analysis. Most trial durations ranged between two and five months, except two studies that took place over one week and reported only severe symptoms. The vitamin C dose varied between 1g and 4g per day.
Regarding the risk of bias analysis, all the trials had placebos and were randomized and double-blinded. Most trials clearly showed that relevant variables were balanced and baseline; further, dropout rates were not significantly different between the different intervention groups.
Most trials reported the tablets for vitamin C and the placebo were similar. However, the researchers found evidence of contamination (where the placebo group received higher vitamin C doses than recommended) in six comparisons, which could lead to underestimating the treatment effect.
Results of the meta-analysis indicated that vitamin C supplementation reduced days absent from school (for students) and confined at home by 15%. The groups receiving the supplement also showed decreased common cold severity by 13%.
Across all 15 comparisons, the pooled effect of 1g or more of vitamin C was 15%, indicating a significant reduction in severity.
In terms of the duration of severe symptoms, the analysis found a reduction of 26% as compared to no significant effect of supplementation on mild symptoms. There were some indications that effects could be stronger for males compared to females.
Conclusions
In this study, the authors estimated a 15-26% decrease in severe symptoms caused by the common cold, which could be attributed to vitamin C supplementation.
Further, they showed that severe symptoms decreased, even though the overall duration did not.
The conclusions from the meta-analysis strengthen and extend existing evidence of the efficacy of vitamin C in reducing the impact of illness from the common cold.
Future studies can focus on the role of therapeutic vitamin C consumption after the onset of symptoms. However, vitamin C supplements have emerged as a safe, affordable, and effective safeguard against the common cold.
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