|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)|
Our results suggest that the amount of energy consumed from supplement during early childhood had a significant and positive effect on the bone mineralization of these Guatemalan adolescents and young adults. Although studies have related short-term improvements in diet and nutritional status to enhanced bone growth, mineralization, and skeletal maturation of malnourished children (Guzmán et al. 1965, Himes et al. 1990, Martorell et al. 1979), we are unaware of other studies reporting positive effects of nutritional supplementation during early childhood on bone mineralization during adolescence and young adulthood.
The magnitude of the supplementation effects diminished, but remained significant after controlling for the three most important confounding factors: age, gender and supplement type. This is an important point, because these three factors together account for 33-65% of the variation in bone mineralization in these data. Further, because the correlations between age and body size are high (0.6-0.8) in this age group, the presence of age in the model also controls partially for variation in bone mineralization associated with body size. The persistence of the effects of energy supplementation after controlling for supplement type is important in light of the fact that only Atole contained protein, calcium and phosphorus - nutrients often cited for their important roles in bone development. Available evidence indicates no major deficiencies in calcium or phosphorus in this population (Flares 1971, INCAP 1969, Lechtig et al. 1972).
The effects of amount of supplemental energy consumed on bone mineralization became statistically nonsignificant after controlling for weight and stature at adolescence. The amount of supplemental energy consumed was associated positively with weight and with stature in these adolescents and young adults, after controlling for age and gender, but not supplement type. These results suggest that the effects of supplemental energy on bone mineralization were operating through the increases in weight and stature associated with supplementation.
The interpretation of these results also did not change in further analyses (n = 352) considering mat urational age (deviation in RUS age from chronological age) as well as chronological age in the models. Further, analyses by Pickett et al. (1995) and Khan et al. (1995) suggest no overall effects of supplementation on skeletal maturation or menarcheal status respectively. Thus, although maturation and menarcheal status influence bone mineralization during adolescence apart from chronological age (Himes and Huang 1993), these results suggest that the observed effects of supplementation on bone mineralization are not likely to be operating through changes in maturation related to supplementation. From this, we conclude that the observed effects of energy supplementation on bone mineralization were probably no greater than those associated with the overall somatic growth response in body size associated with supplementation and were probably mediated by this general somatic response to supplementation.
Two methodological limitations of the study, however, should be mentioned. These relate to the lack of randomization at the level of the individual in the design of the longitudinal study and the selection of subjects for the bone mineralization substudy.
In the longitudinal study, the subjects were not randomly assigned to differing levels of supplementation; that is, the children chose how much supplement they would consume (Schroeder et al.1992). Thus, the positive relationships between amount of supplement consumed during early childhood and bone mineralization during adolescence could result from selection bias if better-off children (i.e., larger children with bigger, more dense bones) chose to consume more supplement. Results from analyses of the longitudinal study have provided arguments against this possibility by demonstrating that rates of growth in weight and length were associated positively with level of sup plementation after controlling for important confounding factors such as initial body size, socioeconomic status, home diet and diarrhea! diseases (Schroeder et al. 1995). The limitations of this type of analysis are discussed in greater detail by Habicht et al. (1995).
Although the preceding arguments lend credence to the interpretation of the results as effects of energy supplementation on bone mineralization, it is important to note, that this selection bias issue could have been addressed directly if we had controlled for variation in bone mineralization during early childhood. This was not possible because such bone measures were not collected during the longitudinal study. Measures of the dimensions of the second metacarpal, however, were available for the majority of the subjects (n = 314; median age of measure = 3 mo), and from these measures, estimates of the bone cortical area can be made (Garn 1970). This area estimate has been shown to be closely related to bone mineral content (Harsman and Kirby 1972). When the regression models were fit again, controlling for this indicator of early childhood bone mineral content, the basic conclusions drawn from the analyses remained the same, although, in these analyses, there was some evidence of bonespecific effects of supplementation on both BMC and BD (P levels of 0.05-0.10).
During the follow-up study, bone mineralization measures were taken on only a subsample of the subjects. The subjects were selected based on their age, gender, village and willingness to participate in another substudy on physical work capacity (Haas et al. 1995). Given that this sample of adolescents and young adults may be different from the rest of the follow-up sample, sampleselection bias could exist. Therefore, analyses were performed comparing the weight, stature, and intake of supplement of subjects in the follow-up study who did, or did not, have bone measurements taken. Subjects who participated in the bone mineralization substudy were younger, lighter and shorter, but had consumed the same amount of supplemental energy during early childhood as compared with the rest of the subjects in the follow-up study. Further analyses demonstrated that the groups were not different in weight, but were shorter after controlling for age, gender and supplement type. The positive relationship between the amount of supplemental energy consumed during childhood and weight and stature during adolescence was not different between the groups for weight, but was for stature, with the bone group having a significantly more positive effect of supplementation. In summary, as suggested previously by Rivera et al. (1992), these analyses indicate that the subjects chosen for the subsample were not representative of all subjects participating in the follow-up study, and were, perhaps, those who responded more to supplementation. Thus, the magnitude of the sup plementation effects on bone mineralization presented here may be somewhat overstated.
Despite these limitations, the results provide important evidence that participation in nutritional supplementation programs can have demonstrable long-term nutritional benefits on bone mineralization for mild to moderately malnourished children. Supplementation trials of mild to moderately malnourished children using different designs and methodologies have been conducted in other populations in developing countries (Gopalan et al. 1973, Mora et al. 1981). Following up these participants could provide further evidence of the long-term impact of food supplementation programs and provide further support for the results presented here.