Recent anthropometric and correlational studies involving specific nutrients have led to the following general conclusions:
1. Chronic, mild postnatal malnutrition is associated with a variety of cognitive and behavioral deficits in infants, school-age children and adults. The role of prenatal malnutrition is less clear; if relations are operative, they may well be restricted to the first two trimesters.2. Chronic mild malnutrition appears to be a necessary but not always a sufficient condition for producing behavioral deficits.
3. Data from studies correlating specific nutrients with cognition and behavior suggest the need to move beyond protein-calorie deficits when conceptualizing chronic mild malnutrition. Specifically, these results indicate the potential salience for cognition and behavior of animal source foods, iron, zinc and B vitamins.
4. Chronic mild malnutrition is embedded in a host of other biological and psychosocial contextual risk factors.
5. The probability of cognitive or behavioral deficits being associated with chronic mild malnutrition is increased under conditions when psychosocial-contextual risk factors are also present and may be increased under conditions where interactions among multiple low-level nutrient deficits are occurring.
6. There are suggestions that adequate nutrition may serve to buffer children against other risk factors.
Given these conclusions, what can correlational studies contribute to future research on nutrition and development?
Role of correlational studies. Experimental intervention studies are generally believed to be essential for proving that nutritional deficits cause behavioral deficits (Smart 1993). However, while controlling for nonnutrient influences, these studies rarely mirror the real world of the chronic, mildly malnourished individual. Mildly malnourished individuals rarely suffer from deficits of only a single nutrient, and chronic mild nutritional deficits rarely occur in isolation (Golden 1991). Chronic mild malnutrition is best viewed in terms of cumulative deficits (Saco-Pollitt et al. 1985), operating across risk factors and over time.
Correlational studies, while not proving causality, do have the advantage of enabling researchers to study large numbers of variables from multiple domains. As such, these studies may be more likely to model the actual processes encountered by mildly malnourished individuals. For example, correlational studies enable researchers to simultaneously study the effects on developmental outcomes of multiple nutrients, anthropometry, biomedical risk factors and both proximal (e.g., caregiver-child interaction) and distal (e.g., socioeconomic status) psychosocial conditions. These characteristics make correlational studies particularly useful for future research on a number of specific question,, some of which have been noted earlier, whereas others emerge from ambiguities or gaps in existing research. The domains suggested for future research are discussed below.
Nutrient-nutrient interactions. Given the possibility that the impact of low-level deficits in a specific nutrient may be amplified when other low-level nutrient deficits are present, correlational studies may be particularly useful in identifying which nutrient combinations should be the focus of subsequent experimental or intervention studies.
Nutrition and context. Three areas of focus may be particularly relevant in terms of studying the interrelationship of nutrition and context. First, as noted earlier, stronger predictions occur when nutritional and nonnutritional risk factors are combined. Similarly, some evidence suggests that adequate nutrition can act as a buffer against the detrimental impact of environmental or biomedical risk influences. Much more evidence is needed on the specific processes involved, e.g., what combination of nutritional and nonnutritional factors yields the strongest prediction? Is this prediction found when combining nutritional and nonnutritional risk factors best described by an additive or by an interactional nonlinear model, Under what conditions and to what degree can nutrition act as a buffer? Initial answers to these questions, involving data from multiple domains, may be best approached using multivariate. correlational model testing strategies (e.g., soft modeling techniques: Falk and Miller 1992), to determine which models offer the best fit for the data. The strongest models can then be subjected to more rigorous testing under experimental conditions.
Second, we have generally assumed that the better the nutrition, the more desirable the outcome. Although a logical assumption, the evidence is not totally consistent on this point. In a number of studies, better nutrition was found to be associated with less desirable outcomes. For example, more adequately nourished Egyptian female school-age children were rated as less involved in classroom activities by their teachers because more adequately nourished female children also tended to be more attention seeking, and attention seeking in females is not viewed as a desirable trait in. Middle Eastern countries (Wachs et al. 1995). More adequately nourished male children were shown to be less attentive in classroom activities in both Egypt (Wachs et al. 1995) and Jamaica (Grantham-McGregor, personal communication). Other studies provided evidence that more adequate nutrition may be associated with lower neonatal attention and orienting (Niestroj 1991), lower caregiver responsivity or involvement Graves 1978, Wachs et al. 1992) and more aggressively hostile behavior in school-age children (Barrett and Radke-Yarrow 1985). It is also worth noting that, according to several reports, adverse biomedical outcomes are associated with more adequate nutrition (Bruce-Chwatt 1985, Rush 1984). Whether adequate nutritional status has adaptive or maladaptive consequences appears to be a complex function of the biological costs of accommodating for inadequate nutrition, culturally based criteria of normality and contextual demands on the individual (Scrimshaw and Young 1989, Super 1990). Measurement of relevant cultural expectations and demands can be integrated into correlational designs, and specific predictions can be tested regarding contextual conditions that would lead to adaptive or maladaptive consequences of better nutrition for the individual.
Finally, as noted initially, inadequate nutrition is not solely a function of the availability of food. A variety of individual and contextual characteristics determines whether a child receives a greater or lesser share of available nutrients. Given the potential multiplicity of factors involved, correlational studies may be a necessary first step in identifying the most salient individual or contextual factors in determining who is more likely to receive a fair share of the ambient diet.
Sex differences. A number of studies suggested the possibility that males and females may be exhibiting different patterns of relations between nutritional adequacy and developmental outcomes (Neumann et al. 1992, Paine et al. 1992, Sigman et al. 1989b, Young-Se 1991). Given that not all studies report sex differences, it would be important to identify whether these findings are a random phenomenon or whether sex differences in children's reaction to mild malnutrition are associated with specific nutritional (e.g., differential adequacy of intake) or cultural characteristics. Correlational studies would seem an appropriate starting point for identifying specific parameters that are systematically associated with sex differences in reaction to nutritional intake.
Increasing the range of outcome variables and populations studied. The bulk of correlational research on nutrition-behavior relations has thus far involved young children's cognitive competence and adult work output. More emphasis needs to be placed on expanding the range of outcome parameters studied to include measures of social-emotional development, temperament, and mental health. Similarly, although there is a substantial body of research on nutritional influences in infancy and early childhood, less is known about influences on older children and adolescents or adults (with the exception of work output). More research is needed on these populations. Of particular interest would be studies encompassing all populations and outcomes relating nutrition to the caregiving activities of older siblings or the elderly because these are often the primary caregivers in less developed countries. Again, correlational studies relating adequacy of intake to specific caregiver activities would seem to be an appropriate starting point.
Long-term impact of early malnutrition. A number of recent studies demonstrated potential long-term consequences of early malnutrition (Pollitt et al. 1993, Sigman et al. 1991). It is not clear is whether they represent the direct long-term effects of early malnutrition or are a function of young malnourished children's increased sensitivity to later malnutrition. Although nutritional intervention studies are one way of approaching this topic, combining quasi-correlational (anthropometric) and correlational studies may be an alternative approach.
Using anthropometric measures such as height as an index of stunting, it would be possible to test whether concur rent correlations between nutritional intake and out comes are different in children with and without a history of early malnutrition.
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