MUSCULARITY AND FATNESS OF INFANTS AND YOUNG-CHILDREN BORN SMALL-FOR-GESTATIONAL-AGE OR LARGE-FOR-GESTATIONAL-AGE

Objective. There is growing interest in the extent to which body compo sition, both short- and long-term, differs in infants and children bor n at the extremes of birth weight. This is because a growing number of studies have linked low birth weight and fetal growth restriction to the chronic diseases in adulthood that often are obesity-related, and there is also evidence to suggest that heavy infants may be at increas ed risk for obesity in later life, again with the attendant obesity-re lated chronic diseases. Our objective was to compare anthropometric in dices of body composition of infants and young children born small-for -gestational-age (SGA, <10th percentile) or large-for-gestational age (LGA, greater than or equal to 90th percentile) with those of normal b irth weight status (appropriate-for-gestational-age, AGA) in a US samp le. Design. National sample of US-born non-Hispanic white, non-Hispani c black, and Mexican-American infants and young children, 2 to 47 mont hs of age, examined in the third National Health and Nutrition Examina tion Survey (NHANES III, 1988-1994), for whom birth certificates were obtained. The primary outcomes were normalized anthropometric indices (z scores or standard deviation units [SDU]) of nutritional status and body composition (mid-upper arm circumference, triceps and subscapula r skinfolds, mid-upper arm muscle and mid-upper arm fat areas (UFA), a nd the arm fat index). The outcomes thus were scaled to permit compari son across chronologic ages. Results. The prevalence of SGA was 8.6%, appropriate-for-gestational-age 80.9%, and LGA 10.5%. From ages 2 to 4 7 months, for infants and young children born SGA, there was a persist ent overall deficit in muscularity (mid-upper arm circumference and mi d-upper arm muscle area) of approximately -0.50 SDU, but less of a def icit in fatness, particularly at the youngest ages. For infants and yo ung children born LGA, there was a surfeit in muscularity of similar t o 0.45 SDU, with less of a surfeit in fatness, particularly at the you ngest ages. Across all ages, the mean UFA showed a statistically signi ficant deficit for SGA children (-0.27 +/- 0.10 SDU) and surfeit for L GA children (0.24 +/- 0.08 SDU). At individual ages for UFA and at ind ividual and all ages combined for skinfold thicknesses, there were no significant differences in level of subcutaneous fatness in the three birth-weight-for-gestational-age groups. There was a tendency in the f irst year for the arm fat index (% arm fat) to be significantly higher for SGA infants, but the effect did not persist after the first year. Conclusion. SGA infants remain smaller and LGA infants larger in size through early childhood, but the discrepancies in weight are primaril y attributable to differences in lean body mass (muscularity). Fatness is less affected. Thus, based on the fatness indicators used, at any given weight for infants and children 2 to 47 months of age, percent b ody fat appears to be relatively higher for children who were SGA at b irth and lower in those who were LGA at birth. These differences in bo dy composition for SGA infants support the evidence documenting a link between disturbances in intrauterine growth and chronic disease assoc iated with subsequent adiposity in adulthood.