|Community-Based Longitudinal Nutrition and Health Studies : Classical Examples from Guatemala, Haiti and Mexico (International Nutrition Foundation for Developing Countries - INFDC, 1995, 184 pages)|
|4. The INCAP longitudinal study (1969-1977) and its follow-up (1988-1989): An overview of results|
The follow-up study is unique (Martorell and Rivera, 1992). There have been some follow-up studies in developing countries, but these have focused on very specific groups, such as survivors of severe malnutrition; long-term follow-up studies of nutrition interventions have never been carried out, to our knowledge. Another distinguishing feature of the follow-up study is its comprehensive nature: no study in developing countries has included the range of measures of human function that were included in the follow-up.
The follow-up study permits one to ask whether the benefits found in early childhood persist into adulthood. A novel contribution of the study is that it allows examination of functional effects that can only be measured later in life, extending, in effect, the horizon for evaluating nutrition interventions. Its central hypothesis is as follows: Better nutrition during early childhood leads to adults with a greater potential for leading healthy productive lives.
However, most subjects were adolescents when the data were collected in 1988. The issue of when it was best to carry out the study was debated intensely among the research members, and though it was agreed that it would be better to measure the subjects as adults, it was felt that the opportunity to carry out the study might not present itself later. It was also recognized that much would be learned about adolescence per se by going ahead with the study. This was seen as a significant contribution, since much less is known from nutrition studies in developing countries about adolescents than about young children.
The use of the word "potential" in the hypothesis was deliberate. It was realized that productivity in an economic sense could not be adequately assessed in a young population. This could only come later when the subjects formed independent households and settled into their occupations. Similarly, other functions, such as parenting, could only be measured later when they formed families.
Overview of Key Results from the Follow-up Study
Data were collected on many functional domains (Rivera et al., 1992), but in this brief overview of results, only three aspects are emphasized: body size and composition, work capacity, and intellectual performance. All three areas are very important, and improvements in one or all would be seen as contributing to human capital formation.
Three aspects of the body size and composition results stand out (Rivera et al., 1995). First, adolescents who were exposed to the atole during the first three years of life were taller and had greater fat-free masses than those who received fresco. However, there was some attenuation of the effects observed at age three. In other words, there was a small degree of catching up in fresco villages with respect to atole villages. It is interesting that the anthropometric effects were greatest in females. The cutoff point of less than 149 cm, equivalent to a height of 4 ft 11 in, is often used as a criterion of obstetric risk in women (Krasovec, 1991). Of those females over 16 years of age who had been exposed to the supplements from birth to three years of age, 49% of the fresco subjects had very short stature compared to 34% of atole subjects (Figure 4). Little or no growth in height occurs after 16 years (Martorell et al., 1995). Similar findings are obtained when the analysis is restricted to women more than 18 years old (fresco, 12/27 or 44%; atole, 9/32 or 28%).
Differences in fat-free mass also stand out. Females from atole villages had 2.1 kg more fat-free mass than females from fresco villages. These differences are equivalent to an effect size of about 0.5, that is, equal to a positive shift of 0.5 SD. Cohen (1977), author of a popular book on statistical power, labels an effect of this magnitude as medium.
Work capacity was significantly improved in subjects exposed to the supplements in their first three years of life, but only in males (Haas et al., 1995). This effect was large. Atole males had maximal oxygen consumptions (VO2max) that were 0.3l/min greater than those of fresco males. The difference is equivalent to about 0.7 SD, approaching what Cohen (1977) would call a large effect size. Another interesting finding is that the larger working capacity of atole males could not be explained by differences in fat-free mass (i.e., VO2max/kg fat-free mass was still greater in atole villages). The basis for these qualitative tissue differences between atole and fresco subjects is unclear.
A feature of all analyses carried out to date with respect to measures of intellectual performance is that they control for schooling variables (Pollitt et al., 1993) because the villages had differed in patterns of school attendance since before the beginning of the study (Habicht and Martorell, 1992; Bergeron, 1992). One of the fascinating discoveries is that intellectual performance was more affected in adolescence among subjects exposed to the supplements during pregnancy and the first two years of life than among those exposed to a supplement later in life. The behavioral analyses have focused on a different sample than the biological analyses (exposure during pregnancy and the first two years of life vs. exposure during the first three years of life, respectively). This is due to different perceptions about vulnerability in each area and to the fact that effects of atole on growth were found only in children less than three years of age (Martorell and Klein, 1980). The atole-fresco differences found in children were less than 0.2 SD, compared to differences of around 0.6 SD found in adolescence using a summary variable of intellectual performance (i.e., a factor score that combines literacy, numeracy, general knowledge, Raven's Progressive Matrices, reading, and vocabulary). To use Cohen's labels, the effects found in children can be described as small, while those found in adolescents can be called medium to large. There are also strong indications that the effects in adolescence are found only in those cohorts exposed to supplementation during pregnancy and the first two years of life. Examination of the subcomponents making up the summary variable shows that effects were found in four of six tests. Effects were found in both males and females.
Potential Significance of the Results of the Follow-up Study
First, the greater body size and increased fat-free mass found in females would be expected to impact positively on reproductive fitness (Krasovec and Anderson, 1991). Short stature is a risk factor for cephalopelvic disproportion, delivery complications, and maternal obstetric mortality. Also, greater fat-free mass has been found to lead to higher birth weights.
The improved working capacity in males might result in increased productivity in men engaged in hard physical labor. The literature clearly supports this expectation (Spurr, 1983).
Finally, sharp minds are valued by all societies and by parents everywhere in recognition that improvements in intellectual performance are bound to improve the capacity of individuals to function in a variety of settings. Consider two suggestions. One is that such improvements might lead to better employment opportunities and greater earnings. Another is that intellectually better-endowed adults will be better parents because they will be better providers as well as being able to meet the developmental needs of their children.