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Results

Psychoeducational tests

Vocabulary. Table 1 presents the results of the hierarchical regression analyses) for the vocabulary test. In general, the results are in the predicted direction with older adolescents, subjects from higher SES families and those who entered school earlier and stayed in longer performing better on the measure of vocabulary. After controlling for all of these variables, the treatment contributed an additional 5% of the variance (Ff = 22.35, P<0.001) in performance with Atole subjects performing significantly better (b = 3.93) than Fresco subjects.

TABLE 1 Results of hierarchical regression analyses for vocabulary ¹

¹ Adapted from Pollitt et al. l 1993).
* P<0.05.
** P<0.01.
*** P<0.001.

When entered into the model, both interactive terms were significant and accounted for an additional 4% of the variance. In the case of the SES-by-treatment interaction, the slope for Atole subjects was nonsignificant whereas that for Fresco subjects was positive and significant (b = 1.373, P<0.001). Although there was no relation between SES and performance in Atole villages, performance improved in Fresco villages with increasing SES level (Fig. 1). At lower ends of the SES distribution, subjects who received Atole supplements performed significantly better than those who received Fresco supplements; whereas at higher SES levels there were no differences between them.

The grade-by-treatment interaction showed a different pattern. The slope was positive and significant for Atole (b = 3.861, P<0.001) but not for Fresco subjects. Differences between treatment groups increased with grade attained, such that children from Atole villages scored significantly higher than Fresco children at the upper ends of the grade distribution (Fig. 2). For those at the lower end of grade attainment, there were no differences between Atole and Fresco subjects.

Results of the hierarchical analyses for other outcome variables were similar (Table 2). After controlling for potentially confounding variables, there were significant effects of Atole on performance on tests of numeracy, knowledge, vocabulary and reading achievement. The percent of variance accounted for by inclusion of the treatment variable was generally small, yet statistically significant, ranging between 1 and 5%. Examination of significant interactive terms permits the identification of subgroups in whom effects were greatest. In almost all instances, effects of Atole were evident in children from families at the lowest levels of SES. In several cases (e.g., reading, vocabulary and reading achievement), effects were observed in children with the highest levels of education.

TABLE 2 Summary of results of hierarchical regression analyses for psychoeducational tests¹

¹ Adapted from Pollitt et al. (1993).

† After controlling for age at testing, gender, attendance, SES, age at school entry and maximum grade attained. SES = socioeconomic status.

*P<0.05.
**P<0.01.
*** P<0.001.

Information processing

Results of the regression analyses on reaction time of the memory task are presented in Table 3. In contrast to the results on the psychoeducational tests, none of the predictor variables were associated with performance, with the exception of grade attainment. After controlling for all potential confounders, treatment was associated significantly with performance, with Atole subjects having significantly faster reaction times (b = - 0.321, P<0.01) than Fresco subjects. Neither of the interactive terms was significant.

TABLE 3 Results of hierarchical regression analyses for memory reaction time¹

¹ Adapted from Pollitt et al. (1993).
* P<0.05.
** P<0.01.
*** P<0.001.

Results of hierarchical analyses for other information processing outcome variables were similar(Table 4). In general, the percent of variance accounted for by the models was small (between 3 and 10%), with males, higher SES, earlier school entry and higher grade attainment associated with enhanced performance. In seven analyses, there were three significant main effects of treatment. Atole subjects responded faster and more efficiently than Fresco subjects on the memory task and reached criterion faster on the paired associates task. None of the interactive terms was significant.

TABLE 4 Summary of results of hierarchical regression analyses for information processing tests 1

¹ Adapted from Pollitt et al. (1993).

† After controlling for age at testing, gender, attendance, socioeconomic status, age at school entry and maximum grade attained.

*P<0.05.
** P<0.01.

Discussion

The results of the INCAP follow-up study show that, after controlling for socioeconomic factors and formal educational experience, subjects exposed to Atole during the pre- and early postnatal period obtained significantly higher scores on measures of general intellectual abilities than subjects exposed to Fresco. Furthermore, better information processing abilities were observed in subjects exposed to Atole. There was great consistency to the results when examined by specific test; Atole subjects performed significantly better than Fresco subjects on tests of numeracy, general knowledge, reading and vocabulary achievement (Pollitt et al. 1993). Furthermore, significant interactions with SES were found for tests of numeracy, knowledge, reading achievement, vocabulary and the Raven's Progressive Matrices. Finally, significant interactions with grade attainment were reported on the three reading-related tests.

Elsewhere (Pollitt et al. 1993) we reported that mother's education, father's occupation and house quality, the three indicators of SES, correlated positively with scores from all the psychoeducational tests (including vocabulary) administered. As expected, subjects at the lowest end of the SES distribution were at the highest risk of poor cognitive test performance. Because of it, the finding that the interaction between treatment and SES accounted for significant portions of the vocabulary test scores and of other tests of the battery administered is particularly important for developmental theory and public health. The direction of the interaction shows that the strongest beneficial effects of the Atole on cognition were observed among those at the lowest end of the SES distribution. This selective effect was sufficient to cancel the expected test score differences between those at the lowest and highest end of the SES distribution in the group that received Atole. Conversely, the distribution of cognition test scores in the Fresco group follows what is generally a truism in developmental psychology: cognitive competence varies as a positive function of SES status. Accordingly, in this study, we view the effect of the Atole as a social equalizer.

Particular characteristics of the subjects also modified the effects of the treatment, as suggested by the significant interaction of treatment and maximum grade attained. Whereas maximum grade attained was independent of test performance among those that received Fresco, this was not the case among those subjects that received Atole. In this last group, the scores in the vocabulary test varied as a positive function of maximum grade. In our view, this effect indicates that Atole provided an impetus to take advantage of the formal educational experiences to which they were exposed.

Are the results due to the supplementation program? Answering this question requires critical examination of the internal validity of the data (Cook and Campbell 1979). Among the potential threats to internal validity are differences between Atole and Fresco in several areas: 1) pretreatment differences in social and economic characteristics; 2) differences in the provision of Atole and Fresco treatments; 3) differences in attrition and recruitment rates to the follow-up study; and 4) differential patterns of community development since the end of the longitudinal study in 1977 to the 1988 follow-up study. The possibility that differences in these areas provide an explanation for differences in test performance between Atole and Fresco subjects has been considered carefully elsewhere and rejected (Polite et al. 1993). Rather, the conclusion reached was that the nutritional differences between the Atole and Fresco supplements is the best explanation for the behavioral differences observed in the INCAP follow-up study.

A related issue regarding treatment effects and their programmatic implications is the nutritional status of the target population. The prevalence of growth retardation in the study population indicates high levels of malnutrition among infants and children. About 26% of the sample had severe stunting (³ 3SD below the reference median) and 43% had moderate stunting (2.9-2.0 SD below the median) at 3 y of age (Martorell et al. 1992).

Nutritional status is likely to interact with supplementary feeding to determine outcome. Thus, the external validity of the Guatemalan findings must be assessed in context and generalizations should be restricted to populations with nutritional status similar to that in the study population. However, it must be noted that. there is no theoretical or empirical reason to suspect; that the benefits of early supplementary feeding are observed exclusively below a certain level of nutritional risk.

On the basis of the results presented here and elsewhere (Pollitt et al. 1993), the internal validity of the effects of supplementation is high. However, although there is strong evidence for a nutritional effect, the specific nutrient or nutrients responsible for the changes observed cannot be identified from this study. Rather, the findings are informative of the potential of efforts to improve diets more generally. Thus, programs that are effective in improving diets in deficient areas, whether educational, food-based or other in nature, will achieve the same results.

The effects of public health programs, particularly as they refer to behavior, need to be evaluated in the context of a society's explicit and implicit social policy. In the context of rural Guatemala, the benefits and costs of nutrition programs to enhance development must be contrasted with those related to efforts to address other existing conditions that limit development. The school system, for example, is vastly inefficient and does not meet the basic educational needs of the population. Only about one half of the children enrolled in first grade ever finish primary school and a large percentage (> 20%) remain functionally illiterate.

Programmatic actions that focus on unmet nutritional needs and have beneficial effects on human cognitive development are potentially a step forward in social policy. However, in our view, such actions are deceptive if framed in the context of a social policy that disregards other basic human needs and does not attend to the overall quality of life. Unmet nutritional needs generally coexist with, among others, unmet needs in education, housing, sanitation and health care. Only by meeting all these needs in conjunction with nutritional needs will we truly have moved forward toward a fair humane society that sustains the rights of children and fosters cognitive, social and emotional development.

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