What We Know about Obesity Development During Adolescence:
Findings from the NHLBI Growth and Health Study
Sue Y. S. Kimm, M.D., M.P.H., Nancy W. Glynn, Ph.D.
University of Pittsburgh
The Role of Sociodemographic, Psychologic and Behavioral Predictors of Weight Outcomes, Dietary Behaviors and Physical Activity
Although poverty has often been viewed as a major contributor to the high prevalence of obesity in African American women, the findings from NGHS indicate racial differences in the association between socioeconomic status and obesity. For 9 or 10 year old NGHS black girls, obesity risk did not vary across household income or parental education. In a logistic regression model, when income, parental education, and the number of parents in the household were included, only greater television viewing was associated with higher likelihood of being obese in black girls, but not in white girls. For white girls, television viewing was inversely associated with household income and parental education. However, when TV viewing was included in the model with both SES and energy intake, parental education and the number of parents in the household, but not television viewing, were significantly associated with obesity risk for white girls. This association became somewhat attenuated with age. For black girls, from age 14 and onward, household income, but not parental education, became significantly and inversely associated with adiposity.
Several demographic factors were associated with obesity in 9 or 10 year old girls with some racial differences. For black girls, the prevalence of obesity was not related to parental employment. For white girls, the odds of obesity were higher for girls with an unemployed parent/guardian in the household. The likelihood of being obese increased by 14% for each 5-year increase in mothers/female guardians' age. Obesity was less common for girls with more siblings, with the odds for obesity decreasing by 14% for each additional sibling in the household.
Psychosocial and Cultural Factors
Measures of self-perception with the Harter scale were assessed longitudinally in the NGHS cohort. From inception, there were racial differences in the association between adiposity and the scores of "physical appearance", "social acceptance" and "global self worth" domains. Adiposity in general impacted negatively on the scores of these 3 domains for white girls. The magnitude of the effect was somewhat less in black girls. Perceived physical attractiveness began to decrease for 9 or 10 year old white girls, even at extreme thinness, and declined linearly across the entire spectrum of adiposity. This finding suggests the presence of an underlying drive for thinness even among prepubertal white girls. For black girls, their perceived attractiveness decreased with adiposity levels around the 70th percentile of the sum of skinfolds (SSF) at the triceps, subscapular and suprailiac sites. The slope for the decline in appearance scores was more precipitous at the higher end of SSF distribution for white girls, whereas the rate of decline in the score for black girls did not vary at the higher end of the SSF distribution. The racial difference was most striking for scores of perceived "social acceptance" for black girls whose scores remained unchanged across the entire spectrum of SSF. For white girls, there was a significant inverse association between perceived social acceptance and adiposity. These findings suggest that in contrast to white girls, black girls did not feel socially rejected even at very high levels of adiposity. Perhaps, this is a manifestation of greater tolerance for obesity in African American culture.
While global self-worth scores showed little change in black girls ages 9-14, they decreased in white girls. Physical appearance scores for both races declined between ages 9 and 14. Social acceptance scores increased for both races between ages 9 and 14. The scores for these 3 subscales decreased with increasing BMI during early adolescence. However, these trends in physical appearance and social acceptance were less pronounced in black girls than in white girls.
Approximately 600 girls were surveyed at one of the NGHS sites at baseline with the "drive for thinness" subscales of the Eating Disorder Inventory. Black girls reported significantly (p=0.0001) higher scores of "drive for thinness" than white girls. Their mean score was 5.19 as compared to 3.24 of white girls. The score of this domain was significantly associated with adiposity for both black and white girls after adjustment for maturation, education, income, physical appearance (Harter scale), self-esteem (Harter scale) and criticism (beta =2.14 p=0.023 for black; beta =4.06 p=0.003 for white). While these findings were limited only to a subsample and the subscales were reworded to be more age-appropriate, nevertheless, they indicate that "drive for thinness" and adiposity are directly associated. These findings seem somewhat counter intuitive. Perhaps, this relationship may reflect a psychological consequence of weight gain rather than a driving force for the maintenance of thinness. No longitudinal information from NGHS is currently available.
At baseline, emotion-induced eating behavior was assessed using selected items from the Nutrition Pattern Questionnaire developed for the NGHS protocol. This "scale" consists of 7 items concerning emotion-induced eating. Black girls scored significantly (p<0.0001) higher on emotion-induced eating than did white girls. There was an inverse (p=0.0001) relationship between emotion-induced eating and BMI. In white girls, this scale was significantly associated with increased sucrose intake, but this was not present in black girls.
There were racial differences in maternal influences on body satisfaction. Black mothers were less tolerant than white mothers of body build among moderately heavy 9 or 10 year old daughters. However, black mothers were more tolerant than white mothers of body build for their heaviest daughters.
Black girls had higher body satisfaction scores than did white girls (P<0.01). However, maternal disapproval of their build and habitus had little effect on daughters( body satisfaction (2.2% of variation explained). Body satisfaction scores decreased with increasing BMI, yet, they decreased less for black girls than for white girls. The largest proportion of variation in daughters’ body satisfaction (21%) was explained by race, BMI, household income and race-BMI interaction.
Higher levels of parental education tended to be consistently related to more favorable nutrient intakes for both racial groups. Girls with parents with higher levels of education had lower fat intake and higher intakes of vitamin C, calcium and potassium.
Dietary Patterns and Dieting Practices
Attempts at weight reduction were seen even among 9 or 10 year old girls, but with racial differences. At ages 9 and 10, almost one half of NGHS girls were trying to lose weight. More of the thin black girls than white girls (16-19% vs. 5-7% in the 1st quartile of BMI) reported trying to gain weight, suggesting a lower tolerance by black girls for being too thin. Logistic regression analysis identified high BMI, the mother telling her child she was too fat and body dissatisfaction as the major factors associated with "trying to lose weight". Chronic dieting, however, was only associated with a high BMI and the mother telling the daughter "she was too fat".
The NGHS examined "weight-related" eating practices (11 items) which are defined as those traditionally targeted for behavior modification in weight reducing programs. Black girls were more than twice as likely as white girls to engage in these weight-related eating practices. Although there was an inverse association between SES and these practices, even after adjustment, black girls remained more likely to engage in these eating practices than white girls. In general, girls who frequently practiced one of these behaviors had higher energy intake than those who practiced such behaviors infrequently.
Physical Activity Levels
Habitual physical activity levels declined by a dramatic 83% during adolescence in the NGHS cohort and was found to be inversely associated with parental education, but primarily in white girls whose parents had attended high school only. White girls whose parent/parents had some college had a greater decline in activity than those whose parent/parents had 4 or more years of college. This effect became attenuated at older ages. Self-reported obstacles to participating in physical activity were queried annually for 3 consecutive years among those girls who exercised "sometimes" or "rarely" (i.e., </=1-2 times a week) from NGHS Study Years 8-10.
The perceived barrier scores as well as the barrier items remained remarkably stable without racial differences among 16 or 17 to 18 or 19 year old sedentary NGHS girls. Despite the greater decline in activity and greater prevalence of inactivity, black girls reported fewer barriers than did white girls (2.2 for black and 2.7 for white girls, P<0.001). The scores remained unchanged for all 3 years and the items did not vary across the years, nor across socioeconomic status. Lack of time was the most frequently cited barrier to activity participation (65% of black and 81% of white girls). Fatigue was the second most frequently cited item for 48% of black and 59% of white girls. Lack of interest was another barrier for both black (37%) and white (35%) girls. The first two barrier items were not corroborated by the amount of house work or household chores done, as there were no differences between those who cited this barrier and those who did not. Similarly, there were no differences in the average daily hours of sleep between those who cited "fatigue" and those who did not. However, those answering "yes" to lack of interest in exercising were more likely to admit to "rather be doing other things than exercise".
In conclusion, the composite findings from NGHS show that while there is a consistent inverse relationship between obesity and SES, this relationship is present primarily in white girls. For black girls, at younger ages, SES appeared to have less influence on the likelihood of being obese. Additionally, there were also racial differences in some of the behaviors associated with obesity development. For instance, despite the far greater decline in physical activity among black girls, SES had almost no impact on this decline, whereas it was inversely associated with the decline in white girls. Psychosocial parameters indicate that although black girls manifest the stigma of obesity in their perception of physical attractiveness and self-worth, they appeared remarkably resistant to the stigma of social rejection. Since some of these environment factors traditionally associated with obesity risk indicate racial differences, the ethnic differences seen may signal cultural differences. As culture is a concept which can be neither readily categorized nor measured, the challenge remains for all of us to gain further mechanistic insights into the complex interplay between ethnicity and disease risk and its associated risk factors.
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