This study examined trends in race/ethnic and gender disparities in BMI during the transition from adolescence to young adulthood using nationally representative data that spanned the period from 1959–2002. We tested for significant differences in age trends in BMI by time period, and found that the widely-documented recent rise in BMI occurred during the adolescent ages in the 1990s, followed by a rise in BMI in the young adulthood ages a decade later in 2000. This pattern suggests that when BMI increased in the 1990s, that increase was greater for adolescents, and they carried a higher BMI with them into young adulthood ten years later. However, we cannot sort out how much of the increase in BMI among young adults beginning in 2000 was due to higher BMI among adolescents moving into the young adult ages or increasing BMI among young adults, but it is likely both processes were operating over time.
Our findings furthermore indicated that the age pattern of increasing BMI in the adolescent ages beginning in the 1990s and in the young adult ages in 2000 was more dramatic for females and blacks, particularly black females. These groups were therefore especially important in driving the rise in BMI/obesity over the last several decades. Black females, in particular, experienced the greatest rise in BMI since the 1990s. Why BMI would rise so dramatically for black females is not clear, but these findings portend adverse health consequences for black females if this trend persists across the life course and across birth cohorts into the future.
From a historical perspective, our trend analysis implies that the recent and dramatic increase in obesity among black females cannot be due to solely biological factors. It is more likely that recent changes in the physical and social environments in which black females reside are important causes of obesity. Perhaps race- and gender-specific social forces, also increasing during this period, such as rising unemployment, neighborhood and school segregation and crime, single motherhood, increasingly disadvantaged childhood conditions (e.g., absent fathers due to rise in mass incarceration) have served to increase stress and poor health behaviors related to obesity such as overeating and physical inactivity among this group [35
]. Interventions and clinical care for these populations needs to tailored to reflect these circumstances such as the work being done in the Bayview Child Health Center as highlighted in the New Yorker [37
This study contributes to the research on BMI by examining age and period trends in BMI during an increasingly vulnerable stage in the life course when adolescents transition to young adulthood. Recent research has identified this life-stage when young people leave the parental home and begin to establish their own health habits and behaviors in early adulthood as a period of poor health habits and behavioral choices, and a time in which individuals are more likely to be uninsured or underinsured with less access to primary and preventative care [5
]. Weight gain during this transition can set trajectories of BMI into adulthood with negative consequences for metabolic processes and cardiovascular health [20
]. Increasing BMI at any age is a serious health concern, but it is especially alarming early in life because of earlier and longer exposure to these health-threatening conditions and their associated morbidity and disability [6
Future research needs to focus on this early part of the life course to identify the factors that initiate trajectories of BMI increase among young people, an aspect our research did not address. Although obesity is a consequence of complex factors, the prevalence and rapid increase in BMI among young people over these decades is likely due to changes in the social and physical environment, including reduced funding for physical activity programs in public schools, changes in food advertising for children and adolescents, urban sprawl and lower neighborhood walk-ability, and rising income inequality, and race-specific factors discussed above, as well as reduced access to grocery stores in poor and minority neighborhoods [39
]. National studies such as Add Health with rich environmental data will provide key insights into the obesity epidemic of the last few decades among young people [40
Our results align well with other research that has combined multiple years of cross-sectional data to examine trends in obesity and BMI across the age spectrum [4
]. By also incorporating longitudinal data from Add Health, our results represent new findings on changes in BMI for the same individual during their transition to adulthood. Our results bring important attention to adolescence as a critical period for the development of obesity. Our findings help inform public health programs designed to curb this epidemic with interventions focused on the young, when BMI changes begin to emerge, rather than in later adulthood when health habits have been established and physical health is already compromised.
IMPLICATIONS AND CONTRIBUTION SUMMARY STATEMENT
Trends in BMI show the transition from adolescence into early adulthood is a critical life-stage for weight gain, during which the obesity epidemic emerged in the 1990s. Interventions to reduce health and social costs of obesity must occur during the transition to adulthood, before obesity trajectories are set in adulthood.