The review question and the inclusion criteria were clear. However, despite the criterion for enrolled children to be healthy, several trials had mean baseline levels of haemoglobin below the commonly accepted cut-off for diagnosis of anaemia. The authors searched several relevant databases and other sources without language restrictions, reducing the chances of language or publication biases or the omission of relevant trials. The authors reported using methods designed to reduce reviewer error and bias in the selection of studies, but not at other stages of the review process.
An acceptable tool was used to assess the validity of the included trials, but reporting of the result of this assessment was very limited. The decision to use meta-analysis appeared reasonable; steps were taken to assess and explore statistical and clinical heterogeneity between trials.
The authors' conclusions appear reasonable with regard to the safety of zinc supplementation in healthy children, as they were robust to the exclusion of trials containing anaemic children, but those relating to benefits in children with anaemia or zinc deficiency relate to work outside the systematic review and could not be assessed. Given concerns over the application of inclusion criteria and the poor reporting of aspects of the review, it is difficult to determine the reliability of the conclusions.