Racial Divergence in Adiposity During Adolescence: The NHLBI Growth and Health Study
Authors: Kimm Sue Y. S. , Barton Bruce A. , Obarzanek Eva , McMahon Robert P. , Sabry Zak I. , Waclawiw Myron A. , Schreiber George B. , Morrison John A. , Similo Shari , Daniels Stephen R. 
Published: August 2004

Background.
Black women are particularly vulnerable to obesity, with a prevalence rate of &gt;50%. The higher mortality and morbidity from cardiovascular disease, stroke, and diabetes have been attributed, in part, to their obesity. In recent years, a particular public health concern is the increasing secular trend in obesity with an even greater racial disparity, especially in girls and women. Between the early 1960s and late 1980s, the prevalence of obesity tripled in young black girls 6 to 11 years of age, while it doubled in white girls. Similarly, both overweight and obesity in adolescent girls 12 to 17 years of age also increased, with a greater increase again seen in adolescent black girls. This secular trend in obesity with a greater increase in black girls signals a potentially grave future chronic disease burden on black women, which is already higher than in white women. The increasing occurrence in children and adolescents of noninsulin-dependent diabetes, traditionally viewed as an adult-onset condition, may be a consequence of the currently high prevalence of obesity in American youth. Not surprisingly, this condition is seen more frequently among black youths.
Prepubescent black girls are generally leaner than age-comparable white girls, but by 20 years of age, black women are considerably heavier than are white women. Thus, it is assumed that the racial disparity in adiposity evolves during adolescence. However, the specific age at which this occurs and underlying factors are yet to be identified because of the current paucity of longitudinal cohort data.

Objectives.
In 1985, the National Heart, Lung, and Blood Institute (NHLBI) initiated a 10-year longitudinal multicenter study (the NHLBI Growth and Health Study [NGHS]) to investigate the development of obesity in black and white girls during adolescence and its environmental, psychosocial, and cardiovascular disease risk factor correlates. The purpose of this report is to examine the natural history of adiposity and weight accretion during adolescence in a biracial cohort of girls to investigate the evolution of the racial divergence in adiposity and to examine the relationships between increases in adiposity and pubertal maturation, energy intake, and physical activity.

Participants and Setting.
A total of 2379 black (51%) and white (49%) girls, 9 to 10 years of age, were recruited from public and parochial schools in Richmond, California, and Cincinnati, Ohio, and from families enrolled in a large health maintenance organization in the Washington, DC area. Participant eligibility was limited to girls and their parents who declared themselves as being either black or white and who lived in racially concordant households.

Design and Statistical Analysis.
The NGHS is a multicenter prospective study of black and white girls with annual visits from 9 to 10 years of age through 18 to 19 years of age. The follow-up rate was 89% at the 10th annual visit. Skinfold measurements were obtained at the triceps, suprailiac, and subscapular sites with Holtain calipers. Sexual maturation was assessed by trained registered nurses. The onset of menarche was ascertained annually by questionnaire. All clinical assessments were conducted using a common protocol by centrally trained staff. Longitudinal regression (generalized estimating equations) models were used to examine the relationship between adiposity and race, age, pubertal maturation, daily energy intake, and physical activity.

Main Outcome Measures.
The main outcome measure was the sum of skinfolds (SSF) at the triceps, subscapular, and suprailiac sites as an index of adiposity for comparison between the 2 racial groups. Body mass index (BMI; weight in kilograms divided by height in meters, squared) distributions were examined by age and race.

Results.
Racial differences in SSF, unadjusted for maturation, were evident at 10 years of age. For each chronological age, there was a higher proportion of black girls with more advanced pubertal maturation than white girls. The 15th percentiles for SSF were similar and remained thus throughout the study. The median for SSF for black girls, although similar to the median SSF of white girls at 9 years of age, became greater for black girls at 12 years of age (36 mm vs 32.5 mm) and at age 19 years the difference was 6 mm (49.5 mm vs 43.5 mm). In contrast, the difference in the 85th as well as the 95th percentile values for SSF were substantially higher in black girls at all ages (9 mm and 10 mm, or 18% and 15%, respectively, at age 9 years) and these racial differences widened with age (20 mm and 26 mm, or 25% and 24%, respectively, by age 19 years).
The racial difference in the median BMI increased from 0.4 to 2.3 kg/m2 between ages 9 and 19 years. Unlike SSF at the 15th percentile, the BMI for lean 9-year-old black girls was ∼3% higher than whites. As with SSF, for heavier girls, BMI at the 85th percentile even at age 9 years was 11% greater in black girls and became 23% greater by age 19 years. Differences in BMI at the 95th percentile also increased from 3.6 to 8.1 kg/m2 between ages 10 and 18 years.
After adjusting for stages of maturation in multivariate longitudinal regression models, adiposity for black girls became significantly greater at age 12 years compared with white girls. The largest gain in adiposity for both groups was seen at the time of pubescence, an approximate increase of 8.0 mm in SSF for white girls and 10.8 mm for black girls. The next milestone for a gain in adiposity occurred around menarche with an increase in SSF of 5.0 mm for white girls and 3.4 mm for black girls. Additionally, there was a significantly greater accrual of adiposity with earlier achievement of menarche, ie, a gain of 3.7 mm for white girls and 3.0 mm for black girls for each year. Although the effect of puberty on the gain in adiposity was similar for both races, for each chronological age, there was a greater accrual of adiposity in black girls because they matured earlier than white girls. Energy intake was significantly and inversely associated with increasing adiposity but not with levels of physical activity.

Conclusion.
The time of the largest accrual of body fat occurred around the 2 major pubertal milestones, the onsets of puberty and menarche. Even after adjusting for pubertal maturation, after age 12 years, black girls were significantly fatter than were white girls. Earlier menarche conferred an additional risk for greater gain in adiposity for both racial groups. Primary prevention of obesity, therefore, should commence with fostering the maintenance of normal growth in young girls before the initiation of pubertal maturation because increased adiposity is associated with earlier menarche. Next, and more importantly, pediatricians should be particularly vigilant with growth monitoring during the critical milestones of pubertal development, a vulnerable time for a large accrual of adiposity. Greater emphasis needs to be placed on preventive efforts in black girls to minimize their risk for developing obesity during adolescence.