The power of a nutritional explanation

The assessment of effects with which we are concerned was framed within a quasi-experimental design. As such, the study faces most, if not all, of the well-known risks involved in this type of experimental approach that we listed in our earlier discussion (see Chap. IV). In this case, the villages, but not the individuals, were randomized into one of the two nutritional treatments. One might argue that utilizing this randomization would allow for a more powerful analytic strategy than electing to use the individual as the unit of analysis: theoretically, the four villages could have functioned as the units for analysis. However, in our view, the benefits of such a strategy would be illusory; the analysis would be legitimate only if the design had been that of a true experiment and included randomization as well as blinding of both subject and personnel. The intervention study failed to meet both these criteria. From our own analyses, we now know that randomization did not render the Atole and the Fresco villages comparable on all possible confounding variables, and neither subjects nor field personnel were blinded with regard to treatment.

Our analyses of the follow-up data established long-term developmental effects of nutritional supplementation, particularly among those at the lower end of the social and economic distribution in the villages. To our knowledge, no previous study has tested the effects of supplementary feeding during the first years of life on intellectual functioning assessed 10-15 years later. Our results are the first of this kind to be reported, and most readers of the nutrition-behavior literature will find them unexpected (Pollitt, 1988). However, it is necessary to review alternative explanations before an inference of a nutrition effect can be drawn.

In Chapter IV, we discussed the foremost threats to the validity of the conclusion that we propose; the most important among them were the nonequivalence between villages (both during the intervention and in the 10-year interval between the longitudinal and the follow-up study) with regard to factors capable of affecting the outcomes of concern, differences in the delivery and consumption of Atole and Fresco related to the differing nutritional properties of the two drinks, and the different patterns of attendance at the feeding stations where each of the supplements was administered. Although we lack the necessary information to reject them completely, our analyses of the available data converge to indicate that none of these alternatives can completely account for the differences in the cognitive functioning of subjects from Atole and from Fresco villages: the internal validity of the nutritional explanation is not compromised by these challenges.

In addition to ruling out alternative explanations, we have also established that a nutrition experiment did in fact take place and that the nutritional differences between the Atole and the Fresco villages had demonstrable developmental implications. The differences in the composition and the actual consumption of the two supplements suggest that the individuals who received the Atole were better nourished, particularly since Atole consumption truly supplemented the diet rather than merely substituting for other foods (see, e.g., Chap. IV).

Although the presence of a nutritional treatment is unquestionable, we cannot test the original protein hypothesis, nor can we specify which nutrient(s) determined the observed differences in outcomes. We do know that the two supplements were equivalent in micronutrients per unit of volume; however, the actual consumption of the Atole and Fresco supplements differed. Thus, when actual micronutrient consumption data were analyzed in terms of recommended dietary allowances (National Research Council, 1989), there was no equivalence between the two groups in that regard. Because the subjects in the Atole group consistently consumed more micronutrients than the subjects in the Fresco villages in the postnatal period, we cannot rule out the possibility that factors such as iron contributed to the test differences between groups (Pollitt, in press).

Finally, we must consider whether our nutritional explanation conforms with the current understanding of the nature and determinants of human development. We argued in Chapter III that both theory and data support the possibility of effects of early supplementary feeding on cognitive development in adolescence. Early supplementary feeding of nutritionally at-risk infants and preschool children results in a developmental advantage and is likely to have beneficial long-term effects, just as the provision of educational opportunities to young children living in poverty has been shown to have beneficial long-term effects on their social behavior.

In addition, longitudinal studies of children exposed to stress factors suggest that, while single events are generally not a sufficient condition to affect development, the probability of deviancy increases when multiple stress factors coexist or interact in synergistic fashion. We have found that those children who were at the lowest levels of the SES distribution and received Fresco performed less well on the battery of psychoeducational tests than any of the other groups of children.

In contrast to the strong evidence on the effects of Atole observed among the subjects in the cohort of maximum exposure, the range of supplement effects on the subjects with late exposure (after 2 years of age) was narrower. There were main effects of Atole on the knowledge and numeracy tests and a significant interaction between treatment and grade attained on reading achievement, similar to those found in the cohort of maximum exposure.

This difference in the range of effects observed among the two groups suggests that the behavioral development of children is more sensitive to nutritional factors during the first years of life, particularly during the period of rapid growth in the brain and body. However, the effects on this late exposure cohort show that, in a nutritionally at-risk population, dietary improvements after the second year of life, following the peak period of growth and change, will still have long-term developmental benefits.

In sum, the evaluations of competing explanations suggest that: the dietary experimental intervention is the most likely determinant of the observed differences in test performance.