The longitudinal intervention study: Design and implementation
The study was planned as an intervention in which one group of
mother-infant dyads received food supplementation while a second group remained
untreated. Whereas the treated group would tell us if an adequate nutritional
status could be attained even under adverse socioeconomic conditions, the
untreated group would provide important information about the natural course and
consequences of chronic malnutrition (Madrigal and Avila, 1990).
This project was conceived as a study of cases and not as an
epidemiological survey. At that time there were data suggesting that the problem
of marginal malnutrition was related to breast-feeding (Martinez and Chávez,
1967). For this reason it was decided to devote a substantial amount of effort
to measuring the quantity of breast milk produced and ingested. A great deal of
effort also went into making behavioral observations of the children. This
component was included because the research team believed that suboptimal child
behavior was one of the main consequences of malnutrition. Because behavioral
studies can only be done by direct observation in the households, the sample
size could not be large. Based on a statistical procedure for taking into
account the possibility of dropouts from the study, it was estimated that 20
dyads would be enough to test the behavioral hypotheses.
The supplemented and unsupplemented groups did not enter the study
at the same time, because the people in the villages could have questioned the
provision of food to some but not to other children. For this reason, during the
first year of the study, only the nonsupplemented women and their newborns were
recruited into the study. During the second and third year of the study, all the
women who became pregnant received food supplementation. At the time of birth of
the children, dyads with similar physical and socioeconomic characteristics to
the unsupplemented group were recruited for continued supplementation.
The United States National Institutes of Health (NIH) initially
funded the study for four years and later extended this period to seven years.
The Mexican Council for Science and Technology (CONACYT) funded the project for
another seven years. The project has continued to be funded by smaller grants,
one of which was provided by the United Nations University (UNU).
The study was initiated in February 1968, and the first three
months were devoted to a general study of the community and to establishing a
close relationship with the families. Following this period, all the pregnant
women in the village were studied. By the end of the first year, in June 1969, a
group of 41 mother-infant dyads had been recruited. Twenty of these 41 women
were selected for the measurements of milk production and intake and behavioral
observations. The selection was based on socioeconomic status and maternal
health, age, and anthropometry. The growth of the children of the remaining 21
dyads was followed longitudinally into the adolescent period.
The women and their children born in 1968 and 1969 were not given
supplemental food and did not receive any type of intervention except in
emergency situations. These children grew up with the support of their families
under the usual conditions of the village. They were breast-fed for a prolonged
period of time, and weaning foods were usually introduced with hesitation and at
a very late age in an insanitary environment that constantly led to constant
These children were born with low birth weight. They grew well in
the first three months but then their growth velocity declined and therefore
they began to suffer malnutrition. Of the 20 children who were selected for the
full study, two were treated and dropped from the study because they developed
severe malnutrition, one with edema and the other with marasmus. One of them was
replaced by another child. Another child died of an infection under very
difficult circumstances and one child emigrated. Therefore this group had a
final sample size for the full study of 17 children. The total number of
newborns of that year was later further reduced from 41 to 36 due to two
additional migrations. This group has been analyzed and included in several
reports dealing with preschoolers and teenagers.
The following year a second group of pregnant women was recruited
and supplemented twice per day with a nutritious drink immediately following the
first report of amenorrhea. The drink was made by mixing milk with fruits and
was designed to provide 400 kcal per day and appropriate amounts of iron,
niacin, riboflavin, and vitamins A and C. The intake of the food supplement was
monitored, and it was shown that it provided 325 kcal per day. Supplemented
women had similar socioeconomic and physical characteristics as the women in the
nonsupplemented group that was recruited the previous year.
Supplemented subjects were matched at birth with their
counterparts in the unsupplemented group, according to the physical and social
characteristics of the mother-infant dyed. In addition to the 20 supplemented
women who were included in the full study, another 20 women were also given food
supplements and studied in some aspects of their development. The fact that the
experimental and the control groups entered the study at different times was
necessary, not only for interactions with the community but for logistic
The children in the experimental group began to receive
supplementary food as soon as they showed the first signs of growth faltering,
at about 12 weeks of age. First, the children were offered a bottle with milk
during the night. When the children started to request to be breast-fed more
often, even though they were being offered the bottle with milk, they were also
given fruits and vegetables. Afterwards the infants were fed ad libitum with
milk and a variety of strained foods. The research team always advised the women
to continue breast-feeding during this time.
When the children were four years old, they were supplemented
twice per day with a sandwich and a glass of milk. When the children began
attending school, they sometimes missed one of the daily episodes of
supplementation because they preferred to remain playing at school during recess
instead of going out to receive their supplement. However, the children always
received the supplementation after school hours. The supplementation
intervention ended when the children were 10 years of age
Throughout the study, special care was taken to ensure that the
only between-group difference was the nutritional supplementation. Measures were
taken to balance the amount of contact with research workers and any other
procedure that could have been considered a nonnutritional intervention.
Throughout the 24 years of existence of the Centro Rural de
Tezonteopan, a variety of nutrition and child development parameters have been
studied. The unsupplemented children are, as this report is being written, 22
years old and therefore have become young adults. The children in the
supplemented group are now between 17 and 20 years of age. The range of ages in
the latter group is explained by the fact that the large amount of effort needed
for planning and implementing the project caused a slowdown in the rate of
recruitment of subjects during the last years of the intervention phase.
The different types of studies are presented in the Results. The
special methodologies that were employed are presented and discussed below.
a. All the women were included in longitudinal
follow-ups of anthropometry and in a special study on fertility and
b. The studies on food intake during early infancy included
measurements of milk volume using a 72-hour test-weighing procedure at 2, 8, 16,
24, 36, 56, and 78 weeks of age.
c. The behavioral follow-up looking at mother-child interactions
was the most significant component of the study. These observations were made
between breast-feeding episodes while the observer was seated in a corner of the
house pretending to read a book. Every 40 seconds the investigator looked over
the book to make a "visual photograph" of the mother-child interaction (e.g.,
holding, feeding, kissing, verbalizing, degree of physical activity, etc.).
Seventeen parameters were captured and written down every time a "visual
photograph" was created. This procedure was carried out for 1.5 hours during the
morning after the child woke up and for another 1.5 hours in the afternoon. This
methodology is derived from that used for ethological observations of primates
and is the one that captured the most important differences between the
supplemented and unsupplemented children.
d. A similar methodology was used during the school period. The
researchers made a hole in the wall of the classroom so that they could make
behavioral observations of the children (e.g., standing, sleeping, attention
span) while attending class. Each observation period lasted 1.5 hours and
yielded information that discriminated between supplemented and unsupplemented
children. Several national and international knowledge and problem-solving tests
were also administered during the school period.
e. The physical activity of the infants was evaluated by observing
the number of contacts that the heel made with the bed. Afterwards physical
activity was measured as the number of steps taken in a specified period of time
(10 minutes per hour for 10 consecutive hours).
f. The longitudinal assessments involving neurological,
psychological, and cognitive measurements were done following traditional
methods (A Chávez and Martínez, 1982).
g. Morbidity was recorded daily and the study also included a
microbiological assessment of fecal contamination of household objects and
The experimental design was selected to test the hypothesis that
nutrition during early childhood is an environmental factor that has a strong
negative impact on long-term human development and function. The design of the
study also contributed to a better understanding of the development of children
in a deprived environment. It was also possible to study the life cycle in the
families, since it included the follow-up of subjects from the time that they
were in their mother's womb until they became pregnant.
The final objective of the project was to identify the critical
point at which interventions that will achieve an optimal development in
socioeconomically disadvantaged populations are most