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Objectives, research design, and implementation of the INCAP longitudinal study

Jean-Pierre Habicht and Reynaldo Martorell



The INCAP longitudinal study (1969-1977) was carried out in four Guatemalan villages to assess the effects of intra-uterine and preschool malnutrition on growth and mental development. To achieve this, food supplements were provided to pregnant women and young children. Two villages were given a high-protein, high-energy drink and two a no-protein, low-energy drink. Both supplements contained vitamins and minerals. The key features of the final study design implemented in 1969 are noted, together with its strengths and weaknesses. The selection of the study villages is described in detail, and the nature of the randomized nutritional intervention and that of the medical care programme offered to all the villages are presented. The methods of data collection and quality-control procedures are also detailed. The information should be of interest to those wishing to use and understand the INCAP/Cornell data set.


The longitudinal study on the effects of early nutrition supplementation carried out by the Institute of Nutrition of Central America and Panama (INCAP) in Guatemala in 1969-1977 continues to be one of the richest sources of information about the importance of nutrition for growth and development in children from developing countries. Some details about the study, such as its design and implementation, have been published but are dispersed in the literature. Other important information is available only in project documents or not at all. This article reviews the objectives, design, and implementation of the study to make the information accessible in a single source to the scientific community. In 1988-1989 the children of the study were revisited in what has come to be known as the Guatemala follow-up study [1]. The information presented here is useful for understanding both the earlier and the later data sets and their related publications.


The design eventually implemented in the longitudinal study called for comparison of outcomes in pregnant and lactating women and in their children between two pairs of villages exposed to different supplements (one large and one small village for each treatment). The major motivation for the study was to assess the impact of intra-uterine and preschool malnutrition on behaviour [2]. Therefore, food supplements were provided and their consumption was measured in pregnant women and in young children. The measurement of supplement outcomes in children was extended to seven years of age, reflecting the concern that it is only after about the age of five that behavioural tests begin to have good predictive value for school achievement and test performance at older ages [3, 4].

Nutrition supplementation was to produce an unambiguous contrast between well-nourished and malnourished children. Because protein deficiency was perceived to be the major cause of malnutrition at the time the study was being planned [5], major emphasis was placed on improving protein malnutrition while assuring enough extra energy to allow for protein use. The feeding intervention took advantage of INCAP's extensive experience with Incaparina [6], a nutritious gruel with a high protein and moderate energy content that was widely accepted in Guatemala. Thus, the pregnant mothers' and childrens' diets were improved with an Incaparina-based drink, referred to as atole, the Guatemalan name for a hot maize gruel. This drink had to be prepared at the time of ingestion and therefore required a central kitchen and feeding hall.

The centrally located feeding stations precluded random allocation of treatment to individuals within a village; instead, allocation was by village. To encourage attendance, the villages had to be small and compact enough to permit all inhabitants to reach the feeding stations easily. Random allocation was done between villages paired to be as similar as possible; one pair of villages was relatively large (about 900 people each) and one was small (about 500 people each).

The comparison beverage was a sweet, cool, coloured, fruit-flavoured drink called fresco. Drinks of this type were much appreciated in the area. It was originally intended that the fresco would be devoid of nutritional value, in effect to be a placebo as a control for the social stimulus and other factors associated with supplementation. The use of cyclamates for sweetening was considered, but concern about carcinogenicity led to sugar being used instead, which of course introduced energy. Finally, other nutrients were introduced, as discussed below, in an attempt to narrow the contrast between the atole and fresco groups to differences in energy and, above all, in protein. Consequently, the fresco should not be viewed as a placebo control to the atole, because it contained some energy and important concentrations of micronutrients. Instead, both drinks are referred to as supplements in this and previous publications.

Originally, three pairs of supplementation villages were specified, but budgetary constraints reduced the number to two, as noted above. A less accessible pair of large villages was dropped early in the study, with dire consequences for statistical power. As implemented, the effects of improved protein nutrition were to be ascertained through comparisons of results before and after the intervention in the two atole and two fresco villages. The before-after comparison was, however, possible only for selected variables collected with adequate sample sizes in 1968, before the intervention began, or for variables collected in the first months of the study, which could not be immediately affected by supplementation (e.g., the height of seven-year-old children).

The effect due to the study activities per se on behaviour was to have been estimated by contrasting outcomes before and after the study in the villages not receiving protein (i.e., the fresco villages) to those in villages not visited in the interim, termed supercontrol villages in project documents. However, budgetary constraints curtailed baseline data collection in these supercontrol villages, nor were data collected in these communities in 1977, at the end of the study.

Thus the final design was a four-way table involving contrasts between atole (A) and fresco (F), ideally before (b) and after (a) the beginning of the study (table 1). Since fresco and atole were randomized within a pair of small and a pair of large villages, the mean effect across village size and within a treatment-time period has only two degrees of freedom. The mean effect in atole villages, Aa, is expected to be better (usually larger) than the mean in fresco villages, Fa. When data are available prior to 1969, the difference Aa - Ab is expected to be greater than the difference Fa - Fb.

TABLE 1. Design of the INCAP longitudinal study

Village size Period of data
Before 1969(b) 1969-1977 (a)
Villages receiving atole (A)
Large (L) ALb ALa
Small (S) ASb ASa
Mean Ab Aa
Villages receiving fresco (F)
Large (L) FLb FLa
Small (S) FSb FSa
Mean Fb Fa

Although the allocation of treatment across villages was random, ingestion of the supplements was voluntary and therefore subject to self-selection. The implication of this combination of random error and self-selection bias for data analysis and interpretation is discussed later.

Finally, it is important to consider that conducting the study involved intensive contact between data collectors and villagers. The data collection activities were designed and implemented to affect all villages equally and therefore cannot be a source of bias in the fresco versus atole contrasts. However, the study setting does affect the validity of extrapolations to other populations if its activities had an impact on outcomes synergistically with the supplements. These same concerns apply to the medical care that was provided to the study villages; it too may have potentiated or diminished the impact of the supplements.

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