Section
3
Key Factors Influencing Body Composition and Its Distribution
Body composition changes as children grow and mature. Many factors influence these changes, including hormonal, environmental, and disease processes.
Growth is associated with increases in fat-free mass and fat mass, and changes in the relative proportions of these body components, which have important implications for accurate measurement of body composition. The timing and distribution of changes in FM also have important implications for current and future health, including the risk of developing adult obesity and various metabolic complications such as insulin resistance, type 2 diabetes, high blood pressure, and abnormal blood lipids. These topics are presented in greater detail below.
Growth is associated with increases in fat-free mass and fat mass, and changes in the relative proportions of these body components, which have important implications for accurate measurement of body composition.
Changes in Total Body Fat from Birth to Adulthood
Total percent body fat is on average 11 to 15% at two weeks of age in a healthy full-term newborn,7,8 and this fat is primarily located in the subcutaneous layer.
Amounts of intra-abdominal or visceral adipose tissue are thought to be negligible at birth.9 Total percent body fat increases to about 30% by 6 months of age and begins to gradually decline during early childhood to about 19% in girls and 14% in boys at 10 years.10 Percent body fat then increases in girls between the ages of 9 and 20 years but decreases in boys after age 13 years, due to more rapid increases in FFM in boys than in girls. In both sexes, total body fat mass increases during adolescence as well as slowly with age during adulthood. However, boys have a greater increase in FFM relative to weight than girls, and girls have a greater increase in FM relative to weight than boys, resulting in the decrease in total percent body fat in boys and the increase in girls. Sex differences in percent body fat become more pronounced during early adolescence. Sex specific distributions of percent body fat by age in a nationally representative sample are also available.11,12
Critical Developmental Periods of Growth: Adiposity Rebound and Puberty
After a period of rapid weight gain in infancy, the velocity of weight gain slows during early childhood before the pubertal growth spurt.
BMI demonstrates an interesting growth pattern during this period. In general, BMI declines after infancy to its lowest individual value at about age 6 years and then increases until adulthood. Longitudinal data on weight collected from birth to 15 years in the Avon Longitudinal Study of Parents and Children (n=625) documented rapid weight gain in infancy and a second period of rapid weight gain that begins between ages 7 and 11 years. The corresponding BMI data demonstrated the occurrence of an adiposity nadir (lowest point) before age 7 years and a rebound in most children between ages 7 and 9 years.13,14
The increase in BMI following the nadir is known as the adiposity rebound. Children who experience adiposity rebound at younger ages are more likely to have overweight or obesity later in childhood, adolescence, and adulthood.15,16 Longitudinal data on BMI in a New Zealand cohort studied from birth until age 26 years demonstrated different patterns of growth and risk of obesity among those with early (<5.5 years for boys and <5 years for girls), average (between 5.5 and 7.5 years for boys and between 5 and 7 years for girls), and late adiposity rebound (≥7.5 years for boys and ≥7 years for girls).15 Those children with early rebound had a higher BMI beginning at about age 5 years that was maintained throughout adolescence and adulthood. The risks of having overweight and obesity at age 26 years were about threefold higher for the early rebound compared to the average rebound group. These findings suggest that early adiposity rebound correlates with higher risk of overweight and obesity later in life.
During puberty, both FFM and FM increase, and the relative changes and their timing during development differ by sex. Thus, body composition changes during the pubertal period must be evaluated in relation to stage of pubertal maturation for boys and girls.17
Girls
Progressive increases in FM and percentage of body fat during puberty in adolescent girls are documented.18 Evidence from several epidemiologic studies in the past 30 years indicates a relationship between earlier onset of puberty in girls and increased BMI. Most of these studies examined the age of menarche as the primary marker for the timing of onset of puberty, because it requires only a recollection of age at onset of menarche by the child and no physical examination. However, increased BMI is also correlated with earlier attainment of other markers of female puberty, including breast development and pubic hair. The self-assessment stage of sexual maturation using drawings and pictures may result in girls with obesity overestimating their Tanner breast stage due to the presence of greater breast fat tissue.19 The question of whether earlier puberty is the cause or the result of increased body fat has not been resolved. However, longitudinal studies suggest that increased body fat or a rapid increase in BMI predicts earlier onset of puberty. Thus, although obesity is not the only factor contributing to early puberty in girls, the downward shift in the United States during the past 30 years in the age of onset of puberty and the age of menarche in girls could be partially explained by the higher prevalence of obesity over this time period.
Boys
The nature and direction of the relationship between obesity and timing of pubertal onset in boys is inconsistent.20 The relationship between obesity and timing of pubertal onset has been studied in girls, but not as well in boys, possibly because of the lack of a convenient self-report marker for puberty in boys that can be obtained in an interview, such as age at onset of menarche for girls. Therefore, studies of boys must rely on physical examinations with accurate Tanner staging of pubic hair and genital development, and such studies are more difficult to perform on a large scale. It remains unclear whether boys with obesity and those with overweight differ with respect to their pubertal onset, as data are contradictory. Some studies report an association between increased BMI and earlier age at pubertal maturation, whereas other studies have found the reverse association.21 Lee et al. have speculated that greater estrogen production in boys with obesity compared to boys at a healthy weight could result in suppression and delay of the pubertal process.22 However, this might not be the case for boys who are overweight.
Like the timing of adiposity rebound, the timing of onset of puberty has important future health implications, as several adverse health outcomes have been associated with earlier onset of puberty, such as glucose dysregulation, higher blood pressure, increased risk of insulin resistance, type 2 diabetes, and cardiovascular disease in women and men. This risk also exists for post-menopausal breast cancer in women.23 A systematic review of data on the association of pubertal timing and adiposity and cardiometabolic risks in middle to late adulthood in both men and women noted that some studies suggest that earlier menarche may reflect greater childhood adiposity and early menarche itself has little impact on cardiometabolic risk, whereas other studies report that earlier maturation is an independent risk factor for adulthood obesity and other cardiometabolic risk factors.24 Strong evidence exists, however, for an association between earlier pubertal maturation and greater adult adiposity in women.24
Fat Mass Distribution
The adverse health effects of body fat are related to its location or distribution in the body as well as to the total amount of FM.
Available data suggest that metabolic disease risk occurs differentially by the location of different fat depots and that patterns of visceral adipose tissue (VAT), subcutaneous adipose tissue (SAT), and VAT/SAT ratio vary by age and sex. Higher amounts of VAT and higher proportions of VAT to SAT are major determinants of impaired glucose metabolism and hepatic fat accumulation over time. This altered fat partitioning often is not observed until adolescence and carries a higher risk of metabolic conditions, including insulin resistance and type 2 diabetes, independent of the overall body FM. This is because insulin resistance is related to a particular central and visceral fat distribution (greater VAT relative to SAT) and ectopic fat accumulation.25 These metabolic consequences are more pronounced in girls than in boys.26 Moreover, in adolescent girls with obesity, a low proportion of VAT relative to SAT, regardless of the amount of total body fat, has been reported to protect against fatty liver and glucose dysregulation.26 In a cohort of children and adolescents of Hispanic heritage followed over a 2-year period, accumulations of liver fat and VAT, and reduction in SAT each significantly and independently predicted diminished beta-cell function, an important risk factor for transition to type 2 diabetes.27 The impact of these changes in body fat distribution during puberty are important for understanding the pathophysiology leading to development of type 2 diabetes.
Sex Differences
Differences in body fat according to sex are evident as early as birth and extend throughout the life span.10
It is well established that beginning at birth, the percentage of total body fat differs by sex, such that females have a higher percent total fat than boys.8,28 These sex differences in body composition become more pronounced during the adolescent growth spurt and sexual maturation. The sex differences established during adolescence persist through adulthood, with both percentage subcutaneous fat and total body fat higher in females. The percentage differences between sexes in body composition reflect a larger FFM that consists of greater bone mineral content (especially of the limb skeleton) and greater skeletal muscle mass in males compared to females. Therefore, estimated absolute total FM is similar in male and female adolescents, but in terms of percentage of body weight, females have a greater percentage of weight as FM and males have a greater percentage of weight as FFM.
Race/Ethnicity
In the United States, striking race/ethnic differences in the prevalence of obesity and in body composition exist among children.29
Race/ethnicity differences have been reported in total body fat beginning at birth,28,30 at prepuberty,31,32 and during adolescence33 for studies involving children born in the United States. Among prepubertal children (aged 3 to 12 years) in New York City, children who were Asian had higher percent body fat compared with those who were African Americans and Caucasians.34 Newborns who were African American, Asian, or Hispanic had greater central fat deposition (by subscapular skinfold) than did Caucasians. Similarly, for the same total FM, race differences in fat distribution are evident as early as prepuberty,31 and these differences persist throughout adulthood. Smaller hip circumferences have been documented in females who are Asian at all pubertal stages compared with those who are white or Hispanic,35 and trunk subcutaneous fat is greater in females who are Asian compared with whites.36 Differences have been described in subcutaneous FM and fat distribution in adults who are Asian compared with whites,37 as have race differences in the course of sex-specific fat distribution with the progression of puberty.38 Differences between blacks and whites in the distribution of subcutaneous adipose tissue and in the density of FFM (1.113 g/cm3 vs. 1.100 g/cm3 in blacks and whites, respectively) reduce the validity of some body composition measurement techniques,39 including anthropometry and air displacement plethysmography. As an alternative, the dual energy X-ray absorptiometry method is less sensitive to the density of FFM issue, and magnetic resonance imaging allows for quantification of fat depots and therefore fat distribution. Factors that account for the observed racial/ethnic differences in body composition are not well understood, and despite these observed racial/ethnic differences, no race/ethnicity-specific growth charts for children have been developed.
Fat-free Mass
Although the primary focus of this guide is the measurement of FM, understanding the non-fat compartment, FFM, and changes in its subcomponents, including skeletal muscle, bone, and TBW, can contribute to understanding the challenges related to in-vivo estimates of body components and how they influence the accuracy of available measurement methods.
These challenges are addressed in a review by Toro-Ramos et al. that summarizes the evidence specific to body composition during fetal development and infancy through age 5 years.8
Overall, FFM increases during growth and development, is relatively stable throughout adulthood, and declines during senescence. Total body bone mass increases with age during growth, reaching peak bone mass between ages 20 and 30 years, then decreases with age after this peak. In general, there is a rapid accretion of skeletal muscle during growth development, with marked sexual dimorphism developing during adolescence. Skeletal muscle mass is then relatively stable during adulthood up to about age 30 to 40 years, after which it begins to decrease. Total body water is an important component of FFM and body weight. A newborn has a higher TBW relative to body weight (i.e., 81 to 83% at age 1 week) than at any other age thereafter. Total body water then decreases throughout childhood, from 79% at 1 year to about 75 to 76% at 10 years.10,40-42 Steady state estimates in normal adults of about 73% are achieved during late adolescence.