Thirty-two studies (7,089 children) were included in the review. Only four studies were considered to be at low overall risk of bias. The duration of the interventions ranged from four weeks to one year.
Cognition: Compared to control, iron supplementation significantly improved global cognitive scores (SMD 0.50, 95% CI 0.11 to 0.90; nine RCTs; Ι²=93%). Subgroup analysis also reported a significant beneficial effect among children who were anaemic at baseline (SMD 0.29, 95% CI 0.07 to 0.51; six studies; Ι²=22%), but there were no significant differences among children without anaemia (four studies; Ι²=0%).
There were no significant differences between iron supplementation and control for global intelligence quotient scores (five RCTs; Ι²=97%). Subgroup analysis found that children with anaemia at baseline had significant improvements in their intelligence quotient after supplementation (MD 4.55, 95% CI 0.16 to 8.94; three studies; Ι²=28%), but results were not significant for non-anaemic children (two RCTs; Ι²=0%).
Compared to control, iron supplementation produced a superior performance on the maze test (MD 1.30, 95% CI 0.90 to 1.70; four RCTs; Ι²=0%) and clerical task scores (SMD 0.44, 95% CI 0.14 to 0.75; four RCTs; Ι²=30%). There were no other significant differences reported for the remaining cognitive tests.
Growth: There were no significant differences between control and iron supplementation for absolute height (five RCTs) but there was a difference for height for age (z score) for children who received iron supplementation compared to those who received the control (MD 0.09, 95% CI 0.01 to 0.17; six RCTs; Ι²=10%). Results were similar for the sensitivity analysis including studies with a low risk of bias.
Weight: There were no significant differences in absolute weight between iron supplementation and control groups (five RCTs), although one study reported children with iron deficiency anaemia had a significant improvement in weight for age score compared to control but this effect was not reported for children without anaemia.
Haematologic and iron indices: Compared to control, iron supplementation significantly reduced the prevalence of iron deficiency (RR 0.21, 95% CI 0.07 to 0.63; four RCTs; Ι²=85%) and decreased the risk of anaemia (RR 0.50, 95% CI 0.39 to 0.64; seven RCTs; Ι²=85%)
Safety: There were no significant differences between groups for malaria infection (four RCTs), or for gastrointestinal adverse events (four RCTs).
Other results were reported in the review