In this issue of the Archives, Lozoff et al1 report the findings of a long-term follow-up of children at 10 years of age who were participants in a double-masked, randomized controlled trial conducted from 1991 through 1994 in Santiago, Chile, testing iron-fortified (12.7 mg/L) vs low-iron (2.3 mg/L) infant formula use from 6 to 12 months of age.2 All infants enrolled in the original trial were screened to have no iron deficiency anemia at the outset. Among 57% of the original sample of children who were reached, an intent-to-treat analysis showed that the mean scores on tests of spatial memory and visual-motor integration at 10 years of age were lower in the iron-fortified group compared with the control.1 The mean effect sizes for both these outcomes were small (−0.21), whereas effect sizes for other measures, such as overall IQ, arithmetic, visual perception, and motor coordination, showing a similar negative trend were even smaller, ranging from −0.08 to −0.16, and not significant. Stratified analyses revealed large negative effect sizes (−0.85 to −1.36) of the iron formula among children who had high hemoglobin (Hb) levels (Hb level >12.7-13.0 g/dL, 5.0%-5.5% of the sample) at the start of the trial, whereas those with low Hb level (<10.4-10.7 g/dL, 9%-24% of the sample) benefitted somewhat with the intervention. For a large proportion of children with Hb concentrations in the middle of the distribution (10.8-12.7 g/dL, 70%-90% of the sample) the iron intervention did not have any impact on the measured outcomes. (To convert Hb to grams per liter, multiply by 10.0.)
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