CRD Summary: This review assessed the prophylactic effects of zinc supplementation for the prevention of childhood diarrhoea and respiratory illness. The authors concluded that zinc supplementation showed modest decreases in occurrence, severity and duration of diarrhoea and frequency and severity of respiratory illness. The authors' conclusions and recommendations for further research are likely to be reliable.

[Zinc compounds included zinc acetate, zinc sulphate, zinc gluconate, methionine and riboflavin.]

CRD Commentary: This review had clearly stated inclusion criteria in terms of participants, study design, interventions and outcomes. The authors searched relevant databases and made efforts to identify additional studies by reviewing reference lists of relevant literature. It was unclear whether any attempt was made to identify unpublished studies and whether language restrictions were applied, therefore, relevant studies may have been missed and language bias could not be ruled out. Evidence of publication bias was found and investigated appropriately. Appropriate efforts were made to minimise reviewer bias and error in the study selection and data extraction processes. The study selection process included a formal validity assessment using strict preset minimum quality inclusion criteria. Considerable heterogeneity was evident between studies in the meta-analyses, which was investigated through appropriate sub-group analyses. The significantly high levels of heterogeneity should be borne in mind when interpreting results. The authors also recognised that effect size was dependent upon nutritional status and baseline serum zinc levels of participants, and that all included studies were conducted in the developing world. Therefore, the findings may not be generalisable to the developed world nor to certain subgroups of children. Despite the identified issues, the authors' conclusions and recommendations for further rigorous research together reflect the evidence presented and are likely to be reliable.