|Effects of Improved Nutrition in Early Childhood : The institute of Nutrition of Central America and Panama (INCAP) Follow-up Study; Proceedings of an IDECG workshop, July 1990, Bellagio, Italy, Supplement of The Journal of Nutrition (International Dietary Energy Consultative Group - IDECG, 1994, 198 pages)|
|Nutritional supplementation during early childhood and bone mineralization during adolescence(¹,²)|
The design and methods of the original INCAP longitudinal study have been described in detail elsewhere (Martorell et al.1995a); thus, only details of the study pertinent to the analyses presented here are described. Two supplements were provided on demand in four rural communities in Guatemala; two of the villages received Atole, a high-calorie, high-protein drink, whereas the other two villages received Fresco, a low-calorie, no-protein drink. Also, Atole contained calcium and phosphorus whereas Fresco did not contain these minerals. Both supplements contained identical concentrations of iron, fluoride and some vitamins. Villagers were allowed to consume as much or as little supplements as they desired, and consumption was measured to the nearest 10 mL on a daily basis for all pregnant and/or lactating mothers and children 0-7 y of age. The growth and development of children during the first 7 y of age were studied in all subjects who were <7 y of age in 1969, when the study began, and in all children born into the study from 1969 to 1977, when the study ended.
The follow-up study, which began in 1988, was a cross-sectional follow-up of participants from the four study villages (Martorell et al. 1995a). Bone mineralization was measured on a subsample of 372 healthy adolescents and young adults stratified by age, gender and village, who had been exposed to nutritional supplementation during early childhood and who agreed to participate in the assessments of physical performance (for details, see Haas et al. 1995). At the time of examination, the subjects were 11-27 y of age. Measurements of bone mineral concentration (BMC), radial bone width (BW) and bone density (BD = BMC/BW), were made at the one-third distal site using a Norland Model 2780 Digital Bone Densitometer (Fort Atkinson, WI). The bone measures were obtained using single-photon absorptiometry from a 125 I source (Sorenson and Camerson 1967). Such measures are highly correlated with body calcium and with skeletal weight and are an appropriate method for assessing bone mineralization in the appendicular skeleton (Mazess 1983). Anthropometric dimensions, including stature and weight, were taken by trained observers using recommended methods (Lehman et al. 1988). The exact age of each subject was known from the birth records of the longitudinal study.
Of those 372 subjects examined, the files of 11 subjects were found to have missing data for either height, weight or bone mineralization measures and the files of 5 subjects could not be linked with those from the original INCAP longitudinal study and, therefore, no information was available on supplement intakes during early childhood. The analyses presented here relate to those 356 subjects for whom complete data on all variables of interest were available.
The effects of early nutritional supplementation on adolescent
bone mineralization, weight and stature were analyzed relative to the total
cumulative amount of energy consumed from the supplements by the subjects during
the first 7 y of their lives. Cumulative intakes were determined in part by age;
for example, those born in 1962 could only have been supplemented in their
seventh year of life whereas those born in 1969 could have received supplement
from birth to 7 y. Cumulative intakes were also determined by individual
patterns of attendance and consumption (Schroeder et al. 1992). Because of the
upwardly skewed nature of the distribution of values of cumulative energy intake
of supplement, a square root transformation was used for all analyses.
Preliminary analyses demonstrated that such transformation reduced skewness and
kurtosis to acceptable levels. The five outcome measures were transformed to
gender specific scores (Standard deviation scores) to allow for comparisons of
the magnitude of the response to energy supplementation among outcomes. The
results are presented in standard deviation units for each outcome measure per
100 square root of kcal, (SD/100
) which can be thought of as the effects on status during adolescence of consuming ~10,000 kcal/41,840 kJ of supplemental energy during childhood.
The effects of early supplemental energy intake on adolescent bone mineralization, weight and stature were examined using the multiple linear regression techniques of Statistical Analysis Systems (SAS Institute 1992a) to control for covariates and potentially confounding variables. Random effects models (SAS Institute 1992b) were developed to estimate the unadjusted effects of energy supplementation on each of the three bone measures, weight and stature and the effects after adjusting for age, gender (males = 1, females = 0) and supplement type (Atole = 1, Fresco = 0). Use of these models allowed us to treat supplement type as a random variable. Finally, weight and/or stature were included in the bone mineralization models to address the secondary objective of testing for the presence of a bone-specific response to supplementation. Effects were considered statistically significant at P<0.05.
Various functional forms of all variables were considered. For example, we compared the fit of genderspecific simple linear and curvilinear models relating each subject's age and the five outcome measures. Although in general the curvilinear relationships provided better fits to the data, the addition of either weight or height to the bone mineralization models removed the effects of the higher-order age terms and interactions, suggesting that the higher-ordered terms for age were colinear with height and weight and thus, their inclusion would "over-control" for the effects of age on bone mineralization status in the models. Therefore, gender-specific linear terms for age (i.e., age + gender + gender*age) were fit in the models for bone mineralization, and the higher order terms, age: and age2*gender were added to the models to describe age-related variations in weight and stature. Interactions between amount and type of supplement consumed also were evaluated for inclusion in the models. Because we were interested in evaluating the potential importance of these interactions, they were considered statistically significant at P<0.10.