Every day, around the world, several million children develop diarrhoea. A new Cochrane review brings together the evidence on one possible treatment, zinc supplementation, and provides a reliable estimate of the benefits of this intervention.

Zinc is an essential element for the human body, with a role in cellular growth, differentiation, and metabolism. Nearly one in three children in the world are zinc deficient. The problem is most severe in low and middle income countries where the main dietary sources of zinc, including red meat, fish and diary products might be in short supply or too expensive. Thus, although the review has particular relevance to these countries, where diarrhoea is the second most common cause of death in children under five, killing two million per year, it also nicely illustrates the strengths of pooling knowledge from multiple studies.

This new Cochrane review of zinc supplements for acute or chronic diarrhoea contains randomised trials in which zinc was compared with a placebo and given for at least the minimum daily allowance of 5 mg. In fact, in many of the trials, the dose was higher than this: 20 mg or more per day in most of the studies. Nearly 6200 children in 18 trials were included, with the bulk of the studies being done for acute diarrhoea. Five trials focused on chronic diarrhoea.

The trials differed somewhat in the outcome measures and time points they reported but the reviewers were able to combine the results in a couple of particularly powerful analyses. Seven of the trials reported on whether the diarrhoea had stopped by seven days after the start of treatment. This evidence, from approximately 4100 children, showed a reduction of around 30% in the relative risk for children given zinc rather than placebo. Another analysis of nine trials, including nearly 3000 children, showed that a typical duration of diarrhoea of 3 to 5 days was reduced by about 12 hours on average.

These benefits were found to be concentrated in the children between six months and five years of age. There were two large trials in younger children but the more than 1300 children studied in these trials revealed no evidence of a beneficial effect for zinc supplementation. Apart from this subgroup analysis, the authors did not find evidence for more or less effect in other groups of children. They found similar effects regardless of nutritional status, geographic region, background zinc deficiency, and type of zinc

This Cochrane review is available in full at www.mrw.interscience.wiley.com/cochrane/clsysrev/articles/CD005436/frame.html

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July 21 2008