Fourteen RCTs (3,645 participants) were included. Ten studies (3,344 participants) were on children and four (301 participants) on premenopausal women. Nine RCTs ( 2,274 participants) provided data for meta-analyses. Results from five studies (1,070 participants) were discussed in narrative. Study duration ranged from four to 29 weeks.
The method of randomisation was described in five trials, partially described in three and not described in six. Allocation concealment was reported in one trial and was unclear in others. Full details of all quality assessment was reported.
Iron supplementation was associated with a benefit in attention or concentration (SMD 0.59, 95% CI 0.29 to 0.90, I2=0%; 179 participants). This could be interpreted as a moderate effect. In subgroup analysis the improvement remained apparent for children (132 participants), but showed no effect for premenopausal women (47 participants). The effect was weakened but remained statistically significant when one study that did not clearly report an improvement in iron status was removed.
Five trials met the inclusion criteria, but were not able to be included in the meta-analysis. One trial on children reported a beneficial effect on attention or concentration and four (one in premenopausal women and three in children) reported no statistically significant effect.
The overall pooling showed no evidence of an effect on intelligence, although there was evidence of heterogeneity (I2=81%). Subgroup analyses showed a benefit for those who were anaemic at baseline (SMD 0.54, 95% CI 0.26 to 0.81, I2=0%; 209 participants). Age group results suggested no effect in children (2,289 participants) and a positive effect in premenopausal women (72 participants). Three trials not included in the meta-analyses found no statistical difference in intelligence between iron and placebo groups.
Pooled results showed there was no evidence of any effect on memory (255 participants) or psychomotor function (255 participants) either in overall analysis or in subgroups by age. One of four studies not included in the meta-analysis reported a beneficial effect on memory; one trial reported on psychomotor function and found no effect.
Results for scholastic achievement were heterogeneous. Two studies found improvement and two found impairment; overall pooling showed no effect (heterogeneity 12=96%, 1,824 participants). Subgroup analyses appeared to show that iron supplementation was deleterious in those who were iron deficient. In the one study where participants were anaemic at baseline and the effect on iron status was clear, results suggested a harmful effect of iron (100 participants). One of three studies not included in the meta-analysis found an improvement with iron and two found no difference.
Adverse effects were not well reported; where reported there was no statistical difference in risk of dropping out with iron compared to placebo.
A funnel plot for the outcome of intelligence (the most commonly reported outcome), showed some indication of publication bias.