Tuesday May 8 2012
Evidence says zinc may ease colds, but causes side effects too
Scientists have found that taking zinc tablets “may shorten the duration of a cold”, The Independent reported today.
For many years, laboratory research has suggested that zinc can stop cold viruses from multiplying, but it has not been clear whether this means taking zinc can prevent or ease a cold. To examine the issue, scientists pulled together all relevant studies of zinc as a cold treatment and conducted a thorough range of analyses of their overall findings. They found that zinc lozenges may shorten the length of a cold by one or two days more than taking a dummy placebo treatment, but that taking zinc was also associated with side effects.
Notwithstanding the newspaper’s excitement, the conclusions of this study are not new. A previous systematic review published in 2011 also found that zinc reduced the duration and severity of cold symptoms, although the trials it included varied considerably in methods, study populations and dosage timing. Overall, this variation made the results less reliable.
The common cold is generally a mild illness. Given that zinc carries the potential for side effects such as nausea and an unpleasant taste, zinc supplements are probably not suitable as a treatment for most people. Furthermore, large high-quality trials to assess the effectiveness and safety of zinc for the common cold would be needed before any recommendations could be made. It should be possible for a person to get all the daily zinc they need from a normal balanced diet.
Where did the story come from?
The study was carried out by researchers from the Hospital for Sick Children, Toronto, and MacMaster University, Ontario. There was no external funding for this study. The study was published in the peer-reviewed Canadian Medical Association Journal.
The research was accurately reported in The Independent, although the paper may have implied that this was the first time a study on zinc and the common cold had been undertaken. The issue was in fact examined in a recent review conducted by the prestigious Cochrane organisation, which found a modest reduction in the length of cold symptoms among people who had been using zinc supplements over a number of months. Cochrane reviews are generally considered to be among the highest levels of evidence, and draw upon all relevant evidence on a particular topic, rather than just selected studies supporting a particular view.
What kind of research was this?
This was a systematic review and meta-analysis of randomised controlled trials evaluating the effectiveness and safety of zinc as a treatment for the common cold. A randomised controlled trial, in which a treatment is compared to a placebo or another intervention, is the best direct way to assess the effectiveness of a treatment as it tests two or more treatments among two or more groups of randomly selected individuals. In a systematic review, researchers search through all relevant literature sources to identify all trials that have assessed the question of interest, and then use rigorous criteria to assess the quality of this evidence. Systematic reviews can also include a meta-analysis, a statistical method that combines the results of all identified studies into a single, larger set of data and results.
The researchers point out that colds are very common: on average, adults get colds two to four times and children eight to ten times a year. Although not serious for most people, colds lead to substantial amounts of time off work and school.
Colds can be caused by several viruses, of which rhinoviruses are the most common. Lab research has shown that the mineral zinc is known to help block replication of rhinoviruses and other viruses which affect the respiratory system, so might be a potential treatment for colds. The authors of this review also suggest that zinc may reduce the severity of cold symptoms by reducing activity in one of the main nerves in the face.
The authors point out that the recent Cochrane review concluded that zinc was effective at reducing the duration and severity of cold symptoms, but that the studies covered in the review varied so much in their design and nature that the results may be unreliable. On this basis, they say, the effectiveness of zinc remains uncertain.
In their own systematic review, the researchers say they tried to improve on the previous review by including more trials and obtaining additional data from the study authors.
What did the research involve?
The researchers conducted a search of several electronic databases and other sources for any studies on zinc as a treatment for the common cold, published up to September 2011. They included studies from all years and in any language, and included all randomised controlled trials comparing zinc taken orally with a placebo or with no treatment for common colds. All potentially relevant articles were screened and assessed to decide whether they were eligible for inclusion in the review. Study authors were also contacted for further information when required.
The researchers assessed the studies for any risk of bias using a validated method. Risk of bias occurs if the results of a study are affected by factors such as whether participants know which treatment they are using, which can affect their impressions of how effective a treatment is. They pooled all the relevant data from the studies included and carried out a statistical analysis using validated methods.
The researchers also carried out “subgroup” analyses to assess whether factors such as age, formulation, dosage and timing of treatment, and funding source (whether industry or independent) had any effects on the results.
What were the basic results?
The researchers included 17 studies in their review, involving 2,121 participants. The results of 14 of these studies were combined in the meta-analysis. They found that people taking zinc had cold symptoms for an average of one to two days less than patients given a placebo treatment (mean difference -1.65 days, 95% confidence interval [CI] -2.50 to -0.81). However, they pointed out that differences between trials in terms of design, dosages, timing and age of participants make this result less reliable. This problem persisted even after subgroup analyses.
The other findings are summarised below:
- Zinc shortened the duration of cold symptoms in adults by between two and three days (mean difference -2.63, 95% CI -3.69 to -1.58).
- Zinc had no significant effect on duration of cold symptoms in children (mean difference -0.26, 95% CI -0.78 to 0.25).
- Adverse events were more common in the zinc group than in the placebo group. These included unpleasant taste and nausea. The results show that 477 per 1,000 people taking zinc got side effects, compared to 385 per 1,000 in the control groups.
- Higher doses of zinc had greater effects on reducing symptom duration than low doses.
How did the researchers interpret the results?
The researchers say they found “moderate quality” evidence to suggest that zinc taken orally reduces the duration of cold symptoms, but that large high-quality trials are needed to properly assess the potential benefits and side effects of taking zinc for a cold. “The questionable benefits must be balanced against the potential adverse effects,” they argue.
For many years, it has been suggested that zinc supplements could relieve the symptoms of the common cold, but there was little clarity until a 2011 evidence review. This recent review suggested a modest reduction in the length of symptoms, although the results were not entirely conclusive. In a bid to help clarify the matter, researchers have conducted another review of evidence on this issue.
It suggests that zinc may help shorten the common cold. However, as with the previous reviews, the results are undermined by the high variability between trials in terms of their methods, study population and treatment aspects (such as zinc dosage and when zinc was first taken). In addition, while zinc may help shorten a cold, it also seems to have side effects such as nausea and an unpleasant taste. As the authors point out, large-scale randomised trials would be needed to assess whether the potential benefits of zinc outweigh the side effects, as well as what dose might be optimal. However, colds are a minor illness for most people, so whether such a trial is warranted is debatable.
From a nutritional perspective, the recommended daily intake of zinc is 5.5–9.5mg a day for men, and 4–7mg a day for women. It should be possible to obtain this amount through a normal balanced diet. If supplementary zinc is taken, the Food Standards Agency and Department of Health recommend that no more than 25mg a day is taken, as too much may cause anaemia and weakening of the bones.