RESUMEN
In children under 5, a hemoglobin (Hb) cutoff of 11 g/dL is recommended by the World Health Organization to define anemia, yet few studies have examined whether this cut point accurately coincides with negative functional consequences. This systematic review and meta-analysis of iron intervention and observational studies aimed to clarify the consequences of low Hb concentration in children under age 5 years on growth, development, and chronic disease (functional outcomes) across the full range of Hb values. A literature search returned 5049 studies; of these, 56 intervention and 20 observational studies fit the inclusion criteria. Among iron supplementation trials, a metaregression indicated significant associations between intervention effects on Hb and their effects on motor and mental development. For each 1 standard deviation (SD) increase in Hb, motor scores increased by 0.28 SD and mental scores increased by 0.24 SD. Iron supplementation trials among children with lower Hb concentrations at baseline showed stronger associations between their effects on Hb and their effects on mental development (P-interaction = 0.008). Heterogeneity among observational studies precluded calculation of pooled associations between Hb and functional outcomes. Available evidence was not able to establish an inflection point at which decreasing Hb begins to be associated with negative functional outcomes. Future research is needed to examine associations of Hb with growth and development in populations with varying levels of Hb, inflammation, and in different ages and settings.