Sue Y. S. Kimm, M.D., M.P.H., Nancy W. Glynn, Ph.D.
University of Pittsburgh
The Role of Diet and Activity in Obesity Development During Adolescence
The NHLBI Growth and Health Study (NGHS) is a longitudinal study designed to assess factors associated with progressive weight gain and development of obesity in black and white girls during the transition between childhood and young adulthood. NGHS was initiated in 1985 because of the notion that the higher mortality rate from cardiovascular disease was in part explicated by the high prevalence of obesity among African American women. Cross-sectional data indicated that prepubertal black girls were leaner than white girls, but it was during adolescence when the racial divergence in obesity development took place. Thus, the NGHS is a 10-year cohort study of 1213 black and 1166 white girls, aged 9 or 10 at entry, who were followed annually until ages 18 or 19. The follow up rate was 91% for black girls and 88% for white girls.
Racial divergence in adiposity
Puberty was associated with a significant gain in adiposity for both racial groups with the largest gain in adiposity seen at the time of pubescence for both groups, an approximate increase of 8.0 mm of SSF for white girls and 10.8 mm for black girls. At age 9, there were no racial differences in adiposity as measured by the sum of skinfolds (SSF) at the triceps, subscapular and suprailiac sites. Longitudinal regression analysis showed that adiposity for black girls became significantly greater at age 12, after adjusting for pubertal maturation stages. 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.
Effect of Dietary Intake and Patterns
Energy intake was significantly higher for black girls at every age, except at age 9 years, with racial differences in the average daily energy intake ranging from 127 kcal at age 10 to 216 kcal at age 17.
At baseline (ages 9 or 10), there was no significant relationship between daily energy intake and adiposity or BMI. However, there was a significant (p<0.01) direct relationship between saturated fat intake and BMI for black girls. For white girls, there was a significant (p=0.002) direct relationship between total fat intake and BMI.
Longitudinal regression analysis revealed that daily caloric intake was inversely associated with adiposity.
Effect of Levels of Physical Activity
Levels of habitual activity (HAQ) were significantly lower in black girls even at ages 9 or 10 (p=0.008). Scores for HAQ declined steadily from baseline until the end of the study, ages 18 or 19. The decline was more precipitous between ages 9 or 10 to 15 or 16. The overall decline in HAQ was 83% for the cohort, but there was a significant racial difference with the decline greater in black girls. Their median HAQ score declined by 100% while that for white girls declined by 64%. The median score for the daily activity scores declined, but not as precipitously as that of HAQ during the same time by 35% for the cohort. At baseline, neither the AD nor HAQ scores were significantly related to adiposity. However, television watching was significantly associated with adiposity in 9 or 10 year old girls when adjusted for energy intake. When the multivariate model included parental education, household income, the number of parents in the household and caloric intake as adjustment variables, television watching was a significant risk factor for obesity only for black and not for white girls. Longitudinal regression analysis revealed a significant (p<0.0001) relationship between BMI and the decline in HAQ after adjusting for race, energy intake, cigarette smoking, age at menarche and childbirth.
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