Iron deficiency anemia (IDA) is the most common single nutrient disorder, affecting 20–25% of the children worldwide, with a higher proportion of having ID without anemia (Stoltzfus, 2001
). Recent epidemiological data also suggest that 12% of the Hispanic children and 6% of the white and 6% of the African–American children in the USA have ID (Brotanek, Gosz, Weitzman, & Flores, 2007
). Infants with ID or IDA generally test lower in mental and motor development and show affective differences (i.e. wariness, hesitance, less positive affect, and less social interaction). Most studies continue to show behavioral and affective differences after iron therapy (Grantham-McGregor & Ani, 2001
; Lozoff et al., 2006
Several studies have found that infants with ID, compared with those with good iron status, are more likely to show poorer mental and motor performance at preschool or school age (see Lozoff et al., 2006
; Thomas, Grant, & Aubuchon-Endsley, 2009
for reviews). Our longitudinal study in Costa Rica followed participants from infancy to the transition to adulthood (19 years) and documented adverse persisting effects of early, chronic, and severe ID on cognitive performance years later (Lozoff, Jimenez, & Smith, 2006
). However, less emphasis has been given to the long-term social–emotional effects of ID. This oversight is noteworthy because executive cognitive deficits are widely recognized as a major risk factor for adjustment problems (Moffitt & Caspi, 2001
). The few available studies showed that children with poor iron status at birth were significantly less alert and compliant with rules at 5 years of age (Tamura et al., 2002
). Children in the Costa Rica study described above, who had chronic, severe ID in infancy, displayed lower levels of physical activity, positive affect, and verbalization compared with children with good iron status in infancy during a structured task at school entry (Corapci, Radan, & Lozoff, 2006
). Differences in mother-interaction quality (i.e. poor mother–child reciprocity and maternal responsivity) beyond infancy were also observed (Corapci et al., 2006
). The 11- to 14-year assessment of the Costa Rica study—the only follow-up study into adolescence—found that children with early, chronic, and severe ID were at increased risk for externalizing (i.e. aggression, defiance) and internalizing (i.e. anxiety, depression) problems, as rated by both teachers and parents (Lozoff, Jimenez, Hagen, Mollen, & Wolf, 2000
). In several studies, the iron status effect remained statistically significant after control for child and family background factors.
Studying the developmental course of social–emotional adjustment, especially in countries where the prevalence of ID is high, may help understand how early ID restricts children from reaching their developmental potential (Walker et al., 2007
). The concept of “functional isolation” in the nutrition field has been used to help explain long-lasting effects of chronic ID. According to the functional isolation hypothesis (Levitsky, 1979
), nutritional deficiencies contribute to changes in infant affect and activity, which in turn compromise infants’ ability to seek and/or receive stimulating and responsive interactions from their caregivers, thereby contributing to their isolation from the environment and poorer developmental outcomes over time. Lozoff and colleagues (Lozoff et al., 1998
) have offered an integrated biological and environmental view of brain–behavior relations. They postulated that chronic, severe ID has a direct impact on neurobiological changes in the developing brain (e.g. myelination, neurotransmitter systems, and neural metabolism). Given that dopamine and serotonin metabolism and functioning are involved in emotion processing, attention, and behavioral activation/inhibition (Beard, 2003
), alterations in these neurotransmitter systems are of particular relevance to the social–emotional domain and may explain the observed pattern of wariness/hesitance, low activity, and low positive affect observed among ID infants. The integrated model also includes the contribution of limitations in caregiving (e.g. lack of responsivity and stimulation) to maintain or even accentuate these child characteristics, especially in a disadvantaged environment. Such dynamic, reciprocal influences among the neural, social, and ecological factors combine over time to undermine child self-regulatory competence, which refers to the ability to modulate affective, attentional, and motivational states and delineates pathways to psychopathology (Posner & Rothbart, 2000
The present study included social–emotional evaluations in infancy and four subsequent follow-ups (5, 11–14, 15–18, and 19 years) from an ongoing study in Costa Rica to address two novel research questions. First, rather than examining the effect of ID on social–emotional functioning at a given point at time, we investigated the pattern of change in externalizing and internalizing problems over time between participants with chronic, severe ID and those with good iron status in infancy. Second, we examined whether early infant characteristics (e.g. low positive affect and activity level) might operate as vulnerability factors in the context of chronic and severe ID. Finally, the present study also extended the current knowledge by investigating the role of early chronic ID on behavior problems beyond early adolescence.
Based on the functional isolation hypothesis and past research, we hypothesized that children with chronic ID in infancy would show persisting levels of externalizing and internalizing problems from early childhood to adolescence and beyond, compared with children with good iron status in infancy. We also predicted that early chronic ID status would interact with infant characteristics consistent with functional isolation (i.e. higher levels of wary/distressed emotionality, lower activity) to result in greater behavior problems over time.