Micronutrient deficiencies

As noted earlier, some micronutrient deficiencies that coexist with protein and energy deficiency have adverse effects on behavior in laboratory animals, on mental and motor development of infants and toddlers, and on the cognitive functioning of older children. It is important that we not overlook the role of micronutrient deficiencies in studies of the effects of undernourishment on child develop meet. In some investigations such deficiencies are a confound; in others they can be conceptualized as an effect modifier. In the first instance, the dietary intake of children in populations previously considered at risk of PEM were likely to have been deficient in vitamins and minerals, not in energy and protein, thus confounding results (Allen, 1993; Beaton, Calloway, & Murphy, 1992). In the second, nutritional factors that cause PEM could also be causing micronutrient deficiencies (e.g., in iron and zinc) that are known to affect, in turn, mental and motor development in children (Golub et al., 1995; Pollitt, 1995); thus, the outcomes may vary, depending on the presence or absence and severity of deficiencies.

Iron. Infants and toddlers who are iron-deficient anemic consistently perform less well on tests of mental and motor development than their peers whose body iron stores are replete (Lozoff, 1990; Walter, 1989). Yet supplementary iron has not generally reversed the developmental delay in this age group, except in a randomized trial in West Java, Indonesia (Idjradinata & Pollitt, 1993). In other studies, the developmental reversal was restricted to those cases where the iron supplementation resulted in normalizing the child's hemoglobin level. A preventative trial with the same age group yielded equivocal findings. The motor, but not mental, development of infants fed iron-fortified formulas was accelerated, compared to controls, up to 12 months of age; but this advantage was lost at 15 months (Moffatt, Longstaffe, Besant, & Dureski, 1994).

Evidence on the effects of iron deficiency on preschoolers and older school-age children is clearer. Compared to controls, children with iron deficiency scored lower on cognitive tests and performed less well on school tests (Pollitt, Hathirat, Kotchabharkdi, Missell, & Valyasevi, 1989; Seshadri & Gopaldas, 1989). Iron supplementation led to significantly improved performance on measures of overall intelligence and on tests of specific cognitive processes among iron-deficient children (Seshadri & Gopaldas, 1989; Soemantri, Pollitt, & Kim, 1985; Soewondo, Husaini, & Pollitt, 1989).

Research has yet to determine the role of iron in the brain in the cognitive and emotional detriments observed in iron-deficient children. It has been proposed that such effects are mediated by a deficiency in the functional activity of dopamine receptors (Yehuda & Youdim, 1989), but this hypothesis has yet to be fully tested in humans (Dallman, 1990). Alternatively, the impact of iron on cognitive performance may be mediated by changes in motivation or emotion that interfere with attentional processes which, in turn, interfere with cognitive performance. This question - how changes in iron status translate into changes in cognitive and noncognitive performance - remains an important area for future research.

Iodine. Maternal iodine deficiency in early pregnancy and associated thyroxine deficiency impair the development of the fetal central nervous system and can result in frank, irreversible cretinism in the child. Studies in Ecuador (Fierro-Benitez et al., 1989, Trowbridge, 1972) showed that correction of the maternal iodine deficiency before conception or in early pregnancy can improve the mental performance of offspring. Comparisons of primary school children in China, in areas with iodine deficiency versus areas with normal iodine intake (Ma, Wang, Wang, Chen, & Chi, 1989) and of goitrous vs. non-goitrous children in Chile (Muzzo, Levia, & Carrasco, 1987) showed better mental and psychomotor performance in the latter groups.

Two double-blind' intervention studies of primary school children yielded contradictory results. An intervention with goitrous Bolivian primary school children reduced the goiter rate but had no effect on physical or mental performance (Bautista, Barker, Dunn, Sanchez, & Kaiser, 1982), whereas iodized oil given to iodine-deficient children of similar age in Malawi did have a positive effect on mental and certain psychomotor test performance (Shrestha, 1994).

Zinc. Severe developmental zinc deficiency in laboratory rats disrupted brain growth and morphology and led to long-term behavioral changes that were qualitatively similar in many respects to those produced by general undernutrition (Golub et al., 1995). However, since severe induced zinc deficiency produces anorexia, it is difficult to discriminate between the effects of low zinc intake and an overall decrease in nutrient intake. Studies of marginal and moderate zinc deficiency in young monkeys have demonstrated effects on activity level, exploration, and performance on some cognitive tasks (Golub, Gershwin, Hurley, & Saito, 1985). In stunted school-age children, however, no differences were found between groups varying in zinc status, or within groups in response to zinc supplements, in scores on standardized tests of attention (Gibson et al., 1989).

At present, no experimental studies have discriminated among the effects of deficiencies of zinc, iron, protein, and energy. Thus, how different deficiencies may interact is unknown.