|Social Policy Report, Volume X, Number 5, 1996 (Society for Research in Child Development, 1996, 32 pages)|
Research efforts are gradually revealing a finer-grained picture of the processes linking nutrition and observable behavior. Various mechanisms involving both biological and behavioral aspects of development are implicated and bear investigating.
Biological. Earlier research linking PEM and behavior suggested that undernutrition interfered with the development of the central nervous system. Undernutrition reduced brain weight and the number of brain cells, which in turn were seen as the cause of irreversible detriments in cognitive and motor performance (NAS, 1973). This emphasis on brain growth focused attention almost exclusively on the period of maximal brain growth, seen as the period of greatest vulnerability.
New findings indicate, however, that periods before and after that of maximal brain growth may be equally important. It is now understood that critical aspects of central nervous system development - for example, gliogenesis, macroneurogenesis, and early glial and neuronal migrations - precede the period of maximal brain growth. Other Later processes, such as synaptogenesis and myelinization, may also be sensitive to insult and remediation (Levitsky & Strupp, 1995; Strupp & Levitsky, 1995).
Research during the last decade has shown that the effects of undernutrition late in gestation are similar to those occurring early. In the case of previously undernourished animals, for instance, the period of brain growth can be extended, and remarkable recovery has been observed. Such evidence is leading researchers to consider a broader range of possible biological mediators, including brain differentiation and changes in neuroreceptor sensitivity. For example, perturbations at the sub-cellular level, as suggested by alterations in sensitivity to pharmacological challenges, persist after periods of early undernutrition and nutritional rehabilitation (Levitsky & Strupp, 1995).
Behavioral. In the mid 1970s the concept of functional isolation, referring to restricted interaction with the environment, was proposed to explain the long-term behavioral effects of early undernutrition (Levitsky, 1979; Levitsky & Barnes, 1972). It was hypothesized that it is the differential experience of the organism rather than disrupted brain growth that mediates the effects of early undernutrition over time. The child who is undernourished attempts to maintain energy balance by reducing energy expenditure and withdrawing from environment stimulation. Such withdrawal limits the child's capacity to take in environmental information and thereby acquire the skills and knowledge necessary for normal behavioral development (Levitsky & Strupp, 1984). This concept arose from evidence suggesting that the behaviors affected by early undernutrition were similar to those produced by early environmental isolation.
Although the functional isolation hypothesis was initially developed as an alternative to a purely biological explanation of nutrition mediated behavioral deficits, the two explanations may in fact be compatible rather than conflicting. Functional isolation may actually influence both central nervous system and behavioral development. While not all aspects of the CNS may be sensitive to environmental influences, and the extent of effects may be relatively small (Bedi & Bhide, 1988), evidence from behavioral neuroscience studies illustrates how restricting experience may adversely influence development (Diamond, 1988; Greenough & Black, 1992) and efficiency (Stone, 1987) of specific brain structures and processes. In addition to the influence of functional isolation on both brain and behavior, subsequent neural changes may further accentuate the effects of functional isolation on ultimate development.
Recently, the functional isolation proposition has been elaborated to explain in greater detail some of the mechanisms that may contribute to long-term adverse effects of undernutrition on cognitive development (Pollitt et al., 1993). This revised proposal hinges on the well-documented effects of undernutrition on body size, neuromotor development, and physical activity. If the child is small and physically underdeveloped and inactive, he or she may
(1) induce behaviors and social responses from caretakers that would generally be reserved for younger children;
(2) undertake less exploration of the environment; and
(3) consequently lag in acquiring the motor skills, cognitive abilities, and social behaviors that typify normal development.
These patterns can operate independently and interactively, with cumulative effects, such that the child ultimately falls behind in competencies attained by well-nourished children. Investigating how nutrition promotes individual differences in children's motor skills, exploration, and play behavior would provide a welcome test of this hypothesis.