CRD Summary: This review assessed the effect of supplementary oral zinc in children with acute and persistent diarrhea. The authors concluded that the duration and severity was reduced through zinc supplementation. The reliability of this conclusion was unclear given uncertainty over clinical significance of some outcomes and between-study variability.

[The included studies assessed zinc salt of any formulation (including sulphate, sulphate/copper sulphate, gluconate or acetate) at doses of 5 mg/day or over for any duration.]

CRD Commentary: The review addressed a clear question and was supported by clear inclusion criteria. The search strategy was extensive, but search terms were not reported. The authors searched for unpublished material to reduce the possibility of publication bias, but they did not formally test for its presence. It was unclear whether there were any language restrictions in place, thus there was the possibility of language bias. The authors did not state how the papers were selected for review or how many reviewers performed the selection, which may mean that the selection process was subject to bias. It was unclear whether the outcomes were pre-specified. Mortality was reported in very few trials, but was initially specified as a primary outcome, therefore, it was uncertain whether the primary outcomes were those that were originally listed or those that were most frequently reported. WMD were reported in days and in these results might be statistically significant due to the large numbers of patients, but they might not have represented findings that were clinically significant. A fixed-effects meta-analysis was used, but given the presence of significant heterogeneity between studies this seemed questionable. The influence of one large study with a short duration of diarrhoea was apparent through the standardised gravity results. In light of the methodological shortcomings and the significant heterogeneity between studies, the authors’ conclusions should be interpreted with caution.