Fifteen studies (4,149 patients, range 44 to 1,382) were included; three were cluster-randomised trials. All studies were rated as being at high risk of bias (most studies gave little or no information about randomisation and allocation concealment methods). One reference appeared to combine data from two separate trials.
Children who received iron supplementation had significantly more haemoglobin (MD 6.97g/L, 95% CI 4.21 to 9.72; nine studies, 12 treatment arms; Ι²=82%) and ferritin (MD 11.64µg/L, 95% CI 6.02 to 17.25; five studies, six treatment arms; Ι²=48%) than controls. Iron supplementation also significantly improved cognitive development (SMD 0.25, 95% CI 0.06 to 0.45; two studies) but not height or weight (seven studies each).
No trials reported the effects of iron supplementation on iron-deficiency or iron deficiency anaemia. One trial reported anaemia as an outcome, with no significant difference found between groups. There were no significant differences between groups for other haematologic parameters.
The effect on haemoglobin and ferritin was highly significant in children who were iron-deficient at baseline but was not significant in children who were iron-replete at baseline; this appeared to explain the statistical heterogeneity. Further subgroup analyses were reported.