“Zinc supplements can reduce the severity and length of cold symptoms,” reported the Daily Express. It said that taking zinc syrup, lozenges or tablets within a day of developing symptoms helps people fight off colds quicker.
The news story is based on a review of 15 trials in more than 1,300 people. Zinc was found to reduce the duration of colds by about a day on average if taken within 24 hours of symptoms starting. People taking zinc were also half as likely to have symptoms after a week and had less severe symptoms than people taking a placebo. Taking zinc for at least five months also appeared to protect people against catching colds, while children who took zinc were less likely to be absent from school or to be prescribed antibiotics.
This is an update of a Cochrane review, capturing new research that has been published since the first version was written in 1999. The Cochrane Library is generally considered to be one of the most respected and reliable sources of evidence. Importantly, the researchers say they cannot recommend taking zinc for colds without further research into what dosage, formulation and duration of treatment will produce benefits with the least risk of side effects (which include bad taste and nausea for zinc lozenges).
For many people, the limited benefit seen here may not seem worth the expense and possible side effects of taking zinc. This is an individual decision. It is possible that zinc may benefit some groups of people more than others, such as those with lowered immune systems. However, it is not clear whether this is the case as the review did not find any studies in these populations.
Where did the story come from?
The study was carried out by researchers from The Cochrane Collaboration, an international, non-profit organisation which publishes high-quality systematic reviews of health care research. The study was published in the Cochrane database of systematic reviews. No external funding was supplied.
The study was reported accurately by The Daily Telegraph, the BBC and the Daily Express, which all stressed that no recommendations had been made about dose and duration of treatment. The BBC also included comments from an independent expert, who remained doubtful about zinc's benefits as a cold treatment in current formulations and said that zinc’s toxicity would be a “potential concern” if taken over long periods.
What kind of research was this?
This is an update of a systematic review of randomised controlled trials (RCTs), which investigated whether zinc supplements can reduce the incidence, severity and duration of common cold symptoms. The researchers were interested in zinc’s ability to both prevent and treat cold symptoms. A systematic review of RCTs is considered the “gold standard” in research methodology. It aims to identify, appraise, select and combine all available high-quality research evidence relevant to a specific question. The previous version of the review (published in 1999) included eight studies and concluded that there was limited evidence on which to base conclusions about the effectiveness of zinc. More research has since been published and the review has been updated to capture this.
The researchers say that there is currently no proven method of prevention or treatment for the common cold and that even medication that is only partly effective could still reduce the amount of sickness, absenteeism and economic loss caused by it. The researchers say that laboratory studies have found that zinc has anti-viral properties. They suggest that zinc might attach itself to certain receptor sites in the nose, preventing the viruses from entering the body.
What did the research involve?
The researchers searched three different research databases to identify relevant studies for their review. Their search criteria were for randomised, double-blind, placebo-controlled trials in which zinc supplementation was used for at least five consecutive days to treat cold symptoms, and trials that looked at zinc use for at least five months throughout the cold season to prevent a cold.
Supplementation with zinc was either with syrup, lozenges or tablets and the specific dose and duration varied across the trials.
The researchers were mainly interested in whether people taking zinc had reduced duration of illness and severity of symptoms, and had fewer colds. They also looked at other outcomes, including the proportion of people who still had symptoms after three, five or seven days of treatment, people who took days off school, antibiotic use and side effects.
The search was carried out using standardised methods, including using the bibliographies of published papers to look for unpublished trials. Two review authors assessed the studies to ensure that only appropriate trials were included and independently reviewed the results for inclusion in the analysis. All studies were assessed for risk of bias, using an established Cochrane tool, and for heterogeneity (the degree to which studies vary in their populations, methods and outcomes, which can affect the results of a systematic review).
The authors combined the results of the trials and synthesised the data using standard statistical methods.
Crossover and cluster-randomised trials were excluded from this review.
What were the basic results?
The researchers included 15 trials involving 1,360 people in their analysis. Of these, 13 trials with 966 participants assessed treatment of colds with zinc. Two trials, with 394 participants, assessed prevention. All 15 trials were conducted in high-income countries among healthy people, and the participants’ ages ranged from one year to 65 years old. Zinc was provided in the form of syrup, lozenges or tablets.
Not all studies were useful for all of the analyses, but the researchers grouped the relevant ones together to assess their main outcomes. Overall:
- A pooling of results from six studies showed that the intake of zinc was associated with a significant reduction in the duration of colds (standardised mean difference [SMD] -0.97, 95% confidence interval [CI] -1.56 to -0.38).
- Pooling five studies found that the severity of symptoms was also significantly reduced with zinc intake (SMD -0.39, 95% CI -0.77 to -0.02).
- People taking zinc were less likely to have cold symptoms beyond seven days of treatment (odds ratio [OR] 0.45, 95% CI 0.2 to 1.00) when the five relevant studies were pooled.
- Taking zinc for at least five months reduced the incidence of colds (incident rate ratio 0.64, 95% CI 0.47 to 0.88) in the two studies that measured this outcome.
- Two studies combined showed that school absence and antibiotic prescriptions were lower in people taking zinc for at least five months.
- Overall, there were more adverse events in people taking zinc (OR 1.59, 95% CI 0.97 to 2.58). Those taking zinc were over twice as likely as those taking a placebo to get nausea (OR 2.15, 95% CI 1.44 to 3.23) and bad taste (OR 2.64, 95% CI 1.91 to 3.64).
How did the researchers interpret the results?
The researchers say the results indicate that taking zinc within 24 hours of the start of cold symptoms reduces the duration and severity of symptoms in healthy people. When taken for at least five months, it reduces cold incidence, school absences and antibiotic prescriptions.
However, they say that zinc supplementation can potentially lead to side effects. They say that because of this “and the differences in study populations, dosages, formulations and duration of treatment, it is difficult to make firm recommendations about the dose, formulation and duration that should be used”.
A systematic review and meta-analysis of RCTs is the best way to arrive at estimates of effectiveness. However, it is not always appropriate to pool study results and one guide of this is to measure "heterogeneity" between studies. In this review, there was a high level of heterogeneity between the studies that were pooled to determine the effect of zinc on the duration of cold symptoms. This may suggest that it was inappropriate to pool them in the first place. It certainly makes this particular finding less conclusive. The researchers note that the differences are likely due to the different zinc preparations used in the studies, the study populations that were combined (adults and children) and how long there had been symptoms of a cold before supplementation began.
More research will undoubtedly be published on this topic. Overtime this can be added into systematic reviews such as this and as the evidence base grows it will permit subgroup analyses that can answer outstanding questions about dose and who will benefit most. Until then it seems that the body of evidence for zinc supplementation seems to be swinging in favour of its benefit, and the choice of whether to take supplements should be an individual one. Many healthy people may not see the potential benefits of taking zinc as being worth the expense of supplements or the possible side effects that can include nausea. Some of the preparations don’t taste very good either. People should stick to recommended daily allowances that are specified as maximum doses on supplement preparations.