Plain language summary
The upper limit of a nutrient’s intake has been defined as the maximum intake from food, water, and supplements that is unlikely to pose risk of adverse health effects to most individuals in the general population. The aim of this study was to determine the levels of zinc intake at which adverse effects are observed in children aged 0–3 years. This study is a systematic review and meta-analysis of fifty-eight articles from fifty-five studies. Almost all studies were randomised controlled studies (n=52) and the rest were quasi-experimental studies. Results show: - that zinc supplementation had a significant adverse effect on serum ferritin, plasma/serum copper concentration, serum transferrin receptor, haemoglobin, haematocrit, and the odds of anaemia in ≥1 of the subgroups of pooled data. - a significant reduction of the lactulose:mannitol ratio. - that there weren’t significant effects of zinc supplementation on c-reactive protein, erythrocyte superoxide dismutase [antioxidant enzyme], zinc protoporphyrin [chemical compound], blood cholesterol, or iron deficiency anaemia Authors conclude that the recommended maximum zinc doses might need to be adjusted for children at risk or recovering from iron or copper deficiency. Additionally, the study’s findings may be used to undertake dose–response modelling to estimate tolerable upper intake levels of zinc in children aged 0–3 years.
Abstract
Zinc supplementation reduces morbidity, but evidence suggests that excessive intakes can have negative health consequences. Current guidelines of upper limits (ULs) of zinc intake for young children are extrapolated from adult data. This systematic review (PROSPERO; registration no. CRD42020215187) aimed to determine the levels of zinc intake at which adverse effects are observed in young children. Studies reporting potential adverse effects of zinc intake in children aged 0-3 y were identified (from inception to August 2020) in MEDLINE, Embase, and the Cochrane Library, with no limits on study design. Adverse clinical and physical effects of zinc intake were synthesized narratively, and meta-analyses of biochemical outcomes were conducted. Random effects models were used to generate forest plots to examine the evidence by age category, dose, dose duration, chemical formula of zinc, and zinc compared with placebo. The Joanna Briggs Institute Critical Appraisal Checklist, Cochrane Risk of Bias 2, and Grading of Recommendations Assessment, Development, and Evaluation (GRADE) guideline were employed to assess risk of bias and to appraise the certainty of evidence. Fifty-eight studies assessed possible adverse effects of zinc doses ranging from 3 to 70 mg/d. Data from 39 studies contributed to meta-analyses. Zinc supplementation had an adverse effect on serum ferritin, plasma/serum copper concentration, serum transferrin receptor, hemoglobin, hematocrit, and the odds of anemia in ≥1 of the subgroups investigated. Lactulose:mannitol ratio was improved with zinc supplementation, and no significant effect was observed on C-reactive protein, erythrocyte superoxide dismutase, zinc protoporphyrin, blood cholesterol, and iron deficiency anemia. The certainty of the evidence, as assessed using GRADE, was very low to moderate. Although possible adverse effects of zinc supplementation were observed in some subgroups, it is unclear whether these findings are clinically important. The synthesized data can be used to undertake a dose-response analysis to update current guidelines of ULs of zinc intake for young children.
Methodological quality
Jadad score
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Not applicable
Allocation concealment
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