At 6 months of age, compared to infants who were exclusively breastfed without iron supplementation, those who received oral iron supplementation showed an increase in hemoglobin (Hb) levels (MD 0.42, 95% CI 0.19-0.66, <i>p</i> < 0.001, <i>I</i> <sup>2</sup> = 76%) and a reduction in the incidence of iron deficiency (ID) (RR 0.38, 95% CI 0.15-1.00, <i>p</i> = 0.050, <i>I</i> <sup>2</sup> = 29%) and iron-deficiency anemia (IDA) (RR 0.58, 95% CI 0.40-0.84, <i>p</i> = 0.004, <i>I</i> <sup>2</sup> = 0). This evidence concerns the gene GSTM1 and anemia.