It is surprising how little is known about childhood obesity and overweight in preterm infants. In this sample of prematurely born 11-year-old adolescents with prenatal polydrug and postnatal environmental risk factors, we did not find higher rates of obesity compared with national estimates. The 24% prevalence estimate for obesity in this sample is similar to national prevalence estimates of obesity for African-American youth (22%) from the National Health and Nutrition Examination Survey 2003 to 2004.45
However, we did find that obese children had higher BMIs compared with national estimates than did the overweight children, suggesting that obese children in this study are toward the upper end of the obesity curve, and therefore likely at greater risk for disease. The 16.7% prevalence estimate for overweight appeared to be lower than national estimates (40%) for African-Americans. National estimates are based on representative samples and include term and preterm infants.6,43,45
Thus, it is possible that the distribution of weight is different in term and preterm infants. The results of regression analyses where we adjusted for covariates showed that typical factors relevant in the obesity literature among mostly term infant samples also predicted obesity and overweight in our preterm sample, although our SGA findings are unique.
SGA significantly predicted overweight in our study. Yet, SGA in preterm infants is rarely accounted for in the “fetal origins” literature. For example, a study showing increased risk of cardiovascular disease–related death given low birth weight found the highest standardized mortality ratios for preterm adults; SGA was not included.46
We were unable to locate obesity studies of gestational age, birth weight, and SGA in preterm children.15,16
Available studies often exclude relevant early growth variables, such as growth velocity.16,47,48
Inconsistencies in the literature may be related to these methodological issues. For example, preterm birth, low birth weight, and SGA have been associated with increased risk for adolescent overweight15
but not associated with BMI among 8-year-old children.47
In nonobese populations, SGA preterm children were the lightest in weight,16,47
and extremely low-birth-weight preterm infants were lighter compared with term infants.48
Our study suggests that SGA is associated with increased risk for overweight at 11 years independent of both birth weight and postnatal growth velocity for children born preterm.
Contrary to our predictions, we found higher birth weight and not low birth weight was related to overweight and obesity at 11 years among preterm children. However, similar findings have been reported in term children at ages 3 to 613,49
and 7 years,7
but not in preterm adolescents. The majority of evidence examines low-birth-weight and not other birth-weight categories, suggesting that low birth weight is associated with increased risk of childhood overweight and obesity.50
In fact, among term children, some have found a stronger tracking effect for later adiposity among low-birth-weight versus normal or high-birth-weight children.13
The landmark studies of men and women born during famine in England around the 1900s provided early evidence for the association between low birth weight and chronic disease,51
but did not distinguish preterm from term births.52,53
Our findings suggest that the mechanism that increases risk for obesity as birth weight increases in term children is similar among preterm children, and this risk extends into adolescents. Higher birth weight in this case is not synonymous with LGA, which has also been associated with overweight among term children.49,54
Further, there were too few LGA children in this preterm sample overall and who later became overweight/obese to warrant further examination. Interestingly, the fact that there is increased obesity risk for normal-weight preterm children as birth weight increases suggests an interaction between the intrauterine and postnatal environment that warrants further investigation.
In adults, there is mixed evidence suggesting that both decelerated52
and accelerated growth velocity50
among low-birth-weight infants is associated with the highest risk of cardiovascular disease and overweight/obesity onset. In children, our findings are consistent with literature, suggesting that across different definitions of growth velocity (e.g., 12 to 24 months of infancy) and adiposity indicators (e.g., BMI and waist circumference), rapid weight gain during early life increases risk for adiposity.55,56
Although the large majority of research is not limited to SGA infants, it has been established that low-birth-weight infants are more likely to show rapid infancy growth than normal and higher-birth-weight infants.57
We did not find a significant SGA by growth velocity interaction. This finding is consistent with evidence suggesting that outcomes related to rapid growth velocity are similar for SGA and normal-birth-weight populations.56
We found an association between inadequate exercise and adolescent overweight and obesity. This has been reported in some studies44
but not others.58
However, exercise has not been previously studied as a predictor for obesity in preterm infants. Our findings have implications for intervention, as physical exercise is increasingly being used to combat obesity in children.40,59–61
We did not find an association between excessive television viewing and childhood adiposity, which is inconsistent with the term literature,1,41
nor did we find an effect for regular health food consumption. Although evidence from the USDA suggests a modest effect for fruit and vegetable intake protecting against childhood adiposity,62
other studies found no association between these two variables.58
The two studies of childhood overweight and obesity in preterm populations have used different methodological approaches to examine effects of gender.7,63
Both studies were in term children; neither examined SGA status. One study found that female gender was associated with obesity at 7 years after controlling for birth weight, growth velocity, and other confounders.63
The other study, also at age 7, stratified by growth velocity and birth weight found that accelerated growth velocity was more strongly associated with higher BMI among boys but not girls with lower birth weight.7
Differences in methodological approaches could explain the discrepancy in findings. Findings from our study suggest that female gender is associated with 11-year overweight and obesity in preterm children.
Our finding that higher reported maternal prepregnancy BMI is associated with increased risk for childhood adiposity is consistent with previous literature.8,64
Our study finding is unique from previous studies, which typically did not control for birth weight, SGA, and other relevant early growth variables among preterm infants.
The fact that MLS was not initially designed to examine overweight and obesity is a limitation. However, MLS provides a rare opportunity to longitudinally study childhood obesity and overweight in a sample of high-risk children. Although our findings are limited to preterm infants, this population is understudied in the field, and we were able to determine a unique effect for SGA. Although we found no main effect for in utero tobacco or cocaine exposures, we were unable to test SGA interaction effects due to small sample numbers. It may be that the association between SGA and adolescent obesity in this preterm sample is due to an interaction between SGA and prenatal cocaine or tobacco exposures. In utero tobacco exposure is associated with SGA,65,66
and maternal smoking is associated with childhood obesity.8,67
Evidence for SGA and prenatal cocaine exposure is limited and mixed.20,68
The effect of prenatal cocaine exposure on childhood obesity is still unknown. In light of our unique findings for SGA and childhood obesity among preterm infants, the combined effect of SGA and tobacco and cocaine exposure on obesity warrant further exploration.
The use of BMI is this study is both a limitation and strength. The limitation of BMI is that it is an indirect measure for adiposity that may reflect lean body mass rather than body fatness.69
As such, BMI as a measure of overweight and obesity tends to have high specificity for identifying adolescents with a high percentage of body fat, but low sensitivity.70
The utility of BMI is that it correlates with more direct measures of body fatness, is the most widely used adiposity measure,71
and has been used with youth born prematurely.7,72
Our findings extend to preterm children evidence that SGA, accelerated growth velocity, and higher birth weight7,13,50
are independently associated with increased risk of childhood adiposity. This suggests that multiple growth-related processes are involved in the risk of childhood adiposity for preterm infants, one of which may be fetal programming as indicated by the SGA effect. The findings also indicate that both prenatal and postnatal processes (inadequate exercise) are involved, providing opportunities for prenatal and postnatal behavioral interventions.