Researchers pooled six randomized, double-blind trials of zinc in school-aged children with ADHD. The trials included 489 children aged 7 to 10, of both sexes. Of these, 245 got zinc and 244 were in a comparison group. The trials ran in Iran, Chile, the USA and Turkey between 2004 and 2020. In four trials, children took zinc alongside methylphenidate. In two, zinc was given alone. Daily doses in the trials ranged from 10 mg to 40 mg, for 6 to 12 weeks. Parents, teachers or both rated the symptoms.
Across all six trials, children given zinc had lower overall ADHD scores than those on placebo. The difference was moderate, but the trials disagreed a lot with each other. Zinc did not clearly change hyperactivity or inattention scores, and each of those came from only three trials. The study found no clear link between the dose or the length of treatment and the change in overall scores. When any one of four trials was left out, the overall result was no longer clear.
The authors rated the certainty of the evidence as moderate to very low. They said too few studies were included to reach a firm conclusion. Most trials were in Asian groups with a moderate to high risk of low zinc, so the results may not apply elsewhere. The authors suggest zinc may help ADHD symptoms in children, but say the results need caution and larger, well-designed trials are needed.
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The numbers Stats and what the terms mean
The statistics behind the plain-language claims above.
- Talebi, 2021: difference in overall ADHD scores, zinc versus placebo (6 trials), SMD −0.62 Hedges' g; 95% CI −1.24 to −0.002.
- Talebi, 2021: how much the trials on overall ADHD scores disagreed, I2 = 89.9%, p < 0.001.
- Talebi, 2021: difference in hyperactivity scores (3 trials), SMD −0.93 Hedges' g; 95% CI −3.31 to 1.45, p = 0.44 (I2 = 98.6%).
- Talebi, 2021: difference in inattention scores (3 trials, 305 children), SMD 0.21 Hedges' g; 95% CI −0.09 to 0.51, p = 0.17 (I2 = 34.4%).
- Talebi, 2021: overall ADHD scores in trials of zinc with methylphenidate, SMD −0.50 (95% CI −0.96 to −0.03).
- Talebi, 2021: overall ADHD scores in trials using a non-Conners' scale, SMD −1.53 (95% CI −1.85 to −1.20, p < 0.001).
- Talebi, 2021: range of results when one of four trials was left out, each 95% CI crossing zero (CI range −1.42, 0.20).
- Talebi, 2021: checks for missing unpublished studies on overall scores, Begg's p = 1.00, Egger's p = 0.23.
What the terms mean:
- SMD (Hedges' g) An effect size: a number that shows how big a difference is, separate from whether it was statistically significant. Here it compares zinc with placebo; roughly, 0.2 is small, 0.5 is moderate and 0.8 is large, and a minus sign shows the direction of the change, not its size.
- 95% CI A confidence interval: a range of values around the result that fit the data reasonably well. A narrow range means a more precise result, and a wide one means a less precise result.
- p A p-value: a number from a statistical test that shows how unusual a result would be if there were really no effect. It depends on the study's own assumptions being right.
- I2 A measure of how much the trials' results disagreed with each other. Higher percentages mean more disagreement.
- Begg's and Egger's tests Checks for signs that studies with unwelcome results went unpublished. Here they found no evidence of that.
Source: Talebi S, Miraghajani M, Ghavami A, et al.. The effect of zinc supplementation in children with attention deficit hyperactivity disorder: A systematic review and dose-response meta‑analysis of randomized clinical trials. Critical Reviews in Food Science and Nutrition (2021). doi:10.1080/10408398.2021.1940833