Body Composition Applications During Growth

Skeletal muscle mass has a central role in intermediary metabolism, aerobic power, and strength. Its mass increases as a portion of body weight during growth, accounting for 21% at birth and 36% at adolescence. The essential role of skeletal muscle in many physiologic processes throughout the lifespan makes understanding of factors affecting it significant. The greater incidence of type 2 diabetes melli-tus in adolescents in the US (particularly in girls from minority populations) and in Japan makes evaluation of race and sex differences in pediatric skeletal muscle mass (and adipose tissue or fat mass) especially important. Identification and characterization of differences could form the basis for further investigation of the associated metabolic implications.

Race differences in SM are known to exist as early as prepuberty. African-Americans have greater limb lean tissue mass compared to Asian and Caucasian children, while Caucasian children have greater amounts than Asians throughout Tanner stages 1 to 5. Race differences in total body bone mineral content adjusted for total body bone area, age, height, and weight have been reported in prepuber-tal African-American, Asian, and Caucasian females and males. African-American children had greater total body bone mineral content than Asian and Caucasian children, while differences between Asian and Caucasian children are less clear. Collectively, these findings suggest that the proportions of specific FFM subcomponents may differ by race. Although mechanisms leading to bone and skeletal muscle differences between races are not well understood, endocrine factors may be involved.

Sex differences in FFM have been reported from birth throughout childhood with females having smaller amounts than males. Total body bone mineral content is less in Tanner 1 females compared to males in African-Americans, Asians, and Caucasians. The mechanism for this sex difference in unclear. Gonadal steroids are significant mediators of adult sexual dimorphism of body composition, including fat-free soft tissues. Prepubertal females have higher concentrations of circulating estradiol than prepubertal males, and gonadotropin and gona-dal steroids increase gradually in both males and females from the age of 5 years. Thus, prepuberty is a period with sex differences in circulating concentrations of sex steroids and of changes in these concentrations with advancing age. The earlier skeletal maturation of females, for example, has been attributed to the greater estradiol level in females compared to males. However, non-hormonal (possibly genetic) mechanisms may also play a role.

Fat or adipose tissue distribution is recognized as a risk factor for cardiovascular disease in both adults and children. An android or male fat pattern, with relatively greater fat in the upper body region, is associated with negative metabolic predictors whereas a gynoid or female fat pattern, with relatively greater fat in the hip and thigh areas, is associated with less metabolic risk. More and more studies are showing that the syndrome develops during childhood and is highly prevalent among overweight children and adolescents. While the concept of the metabolic syndrome referred initially to the presence of combined risk factors including VAT, dyslipidemia, hypertension, and insulin resistance in adults, it is now known to exist in children, especially where obesity and/or higher levels of VAT are present. Although sex-specific patterns of fat distribution had previously been thought to emerge during puberty, sex and race differences in fat distribution are now known to exist in prepuber-tal children. The implications are that a specific body composition pattern may differ by sex and race. An example is the relationship of blood pressure to central fat distribution in boys compared to girls where a significant positive relationship between trunk fat and blood pressure was reported in boys but not girls, and was independent of race, height, weight, and total body fat. Understanding the predictors of blood pressure in children is important since childhood blood pressure has been shown to track into adulthood in longitudinal studies. Children whose blood pressure levels were in the highest quintile, were two times more likely to be in the highest quintile 15 years later. Identification of clinically useful body composition measures would allow for the identification of children at increased risk for hypertension, who could benefit from monitoring.

Race differences in fat distribution among prepu-bertal Asians, African-Americans, and Caucasians also exist. Previous reports in adolescents have suggested significantly smaller hip circumferences in Asian females at all pubertal stages compared to Caucasians and Hispanics and greater trunk subcutaneous fat in Asian females compared to Caucasians. Differences in subcutaneous fat mass and fat distribution in Asian compared to Caucasian adults have also been described. Understanding the sex-and race-specific effects of puberty on regional body composition may help delineate the developmental timing of specific health risk associations.

Race difference in blood pressure has been reported in many studies of adults, where a higher prevalence of hypertension has been found among African-American women, placing this group at a higher risk for cardiovascular-related morbidities and mortality. Previous studies attempting to determine whether this race difference appears in childhood or early in adulthood have produced inconclusive findings.

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