The developmental and probabilistic nature of the functional consequences of iron-deficiency anemia in children.

It is often assumed that the psychometric tools currently available measure accurately the effects of iron-deficiency anemia (IDA) on cognition in young children and that such effects are rooted in cerebral changes. It is also assumed that snapshots of development within a clinical trial can document such effects. I challenge these assumptions on the basis of four considerations. The first is that there are multiple biological, physical and social-psychological factors that reorient the trajectory of different psychobiological domains in early life after intense and prolonged stress. Further, psychobiological development changes are not necessarily caused by brain changes; there are other mechanisms that also affect development (e.g., biomechanics). A second consideration focuses on intraindividual, interindividual and intergroup differences concerning the nature of the effect of IDA and the response to iron treatment. Individual and group factors can moderate the effects of IDA; for example, different stages of iron deficiency involve different systemic changes, which in turn affect different psychobiological domains. The third consideration is that differences in the time of measurement of an intervention within a randomized trial could lead to detecting effects in different domains or effects of different intensity within the same domain. Finally, developmental assessments with the traditional developmental scales during the first 18 mo of life yield equivocal findings. Snapshots of development will overlook the course of effects of a nutrition intervention over time. Repeated measures over time within the same domain are considered particularly useful to draw the course of development.