Zinc intake, status and indices of cognitive function in adults and children: a systematic review and meta-analysis
Warthon-Medina M, Moran VH, Stammers AL, Dillon S, Qualter P, Nissensohn M, Serra-Majem L, Lowe NM · 2015
grade Cmeta-analysisindependentunreplicated
Sample
18 studies met inclusion (12 RCTs - 11 in children, 1 in adults - and 6 observational); the child meta-analysis pools 6 RCTs
Population
Children and adults; the RCTs are mostly from developing countries where zinc deficiency is considered a major constraint on child development.
Design
Systematic search of EMBASE, MEDLINE and Cochrane to March 2014, including both supplementation trials and dietary-intake studies. Only six child RCTs could be pooled, which the authors name as the main limitation. Graded C for the small number of pooled trials and heterogeneous designs.
Key findings
Nine of eighteen studies reported a positive association between zinc intake or status and at least one cognitive measure - the standard pattern of a literature that looks positive in narrative summary. The meta-analysis of the six child RCTs found NO significant effect on any cognitive index: intelligence SMD <0.001 (95% CI -0.12 to 0.13, P = 0.95), executive function SMD 0.08 (95% CI -0.06 to 0.22, P = 0.26), motor skills SMD 0.11 (95% CI -0.17 to 0.39, P = 0.43). The gap between the narrative count of "positive" studies and the pooled randomized estimate is the whole lesson.
Genetic confound
None internally for the RCT pool. The observational studies in the review carry the usual diet-quality confounding and are not the basis of the pooled estimate.
DOI / URL
Effects
| Outcome | Metric | Value | Measure | Timing | Vs | Horizon | Class |
|---|---|---|---|---|---|---|---|
| Intelligence, children (near g) | pooled SMD, 6 RCTs | <0.001 (95% CI -0.12 to 0.13), P = 0.95 - null | standardized | end of supplementation | business-as-usual | end-of-treatment | g |
| Executive function, children (near transfer) | pooled SMD, 6 RCTs | 0.08 (95% CI -0.06 to 0.22), P = 0.26 - null | mixed | end of supplementation | business-as-usual | end-of-treatment | near-transfer |
| Motor skills, children | pooled SMD, 6 RCTs | 0.11 (95% CI -0.17 to 0.39), P = 0.43 - null | mixed | end of supplementation | business-as-usual | end-of-treatment | near-transfer |
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- Breakfast, school meals, and micronutrients — what feeding children actually buysmixedconf: mediumgc: low