There was a striking amount of shifting across BMI categories during the middle school years. Shifts in BMI category were not related to school intervention status, gender, race/ethnicity, baseline or end of study pubertal status, or head-of-household education. Furthermore, shifts in BMI were not explained by changes in height in this group of children studied during the pubertal transition. Of particular interest, more than one-third of overweight sixth-graders shifted to the healthy weight range, and nearly one-third of obese sixth-graders shifted to a lower BMI category by the end of eighth grade. Conversely, of youth who were in the lower end of the healthy range in sixth grade, more than one-quarter moved to the upper end of the healthy range. Similarly, of youth who were in the upper end of the healthy range or in the overweight category in the sixth grade, more than one-quarter increased BMI category.
Sizeable shifts in BMI category in children during childhood have been noted in studies of younger children and those outside the United States.2–4
followed more than 6000 children in the United Kingdom from ages 7 to 11 years and documented shifts in this younger cohort that were similar to those observed in the current study. In a U.S. study of 451 African American and white rural children who were in the fourth through sixth grade at baseline and followed over 28 months,6
rates of overweight and obesity were stable, but rates of incidence and remission of obesity and overweight were approximately equal. Findings from the current investigation, however, significantly extend those of previous studies. Given the substantial racial/ethnic disparities in US rates of pediatric obesity,18
it is crucial to determine whether shifts in BMI are seen across ethnic minority groups. The HEALTHY sample includes large numbers of Hispanic as well as African American and white youth, and thus the current study provides evidence that shifts in BMI category occur across racial/ethnic groups.
The present investigation also documents that BMI shifts are associated with significant changes in cardiometabolic risk factors after adjustment for school intervention status, race/ethnicity, pubertal stage in sixth and eighth grade, and household education. The magnitude of observed changes is similar to those reported in pediatric weight management programs. For example, Savoye et al11
followed an ethnically diverse inner-city group of 209 obese children and adolescents who participated in a yearlong family-based weight control program or a usual care control group and were evaluated 1 year later. Intervention leading to modest weight decreases −2.8 kg/m2
) was associated with decreases in total cholesterol, LDL cholesterol, and fasting insulin of −8.0 mg/dL, −4.4 mg/DL, and −4.7 uIU/mL, respectively. Changes of comparable magnitude were observed as a function of decreasing BMI category in HEALTHY youth (−17.2 mg/dL, −14.2 mg/dL, and −2.3 uU/mL for total cholesterol, LDL, and fasting insulin, respectively).
Other research suggests that the observed changes are clinically meaningful. Cross-sectional studies in youth9,19
have documented a strong relation between BMI and cardiometabolic risk. For example, Weiss et al9
documented that each element of the metabolic syndrome is exacerbated with increasing obesity. Similarly, prospective studies have documented a strong linear relationship between BMI and metabolic risk factors in childhood and adverse adult outcomes, including metabolic syndrome20
and coronary heart disease.21,22
In summary, available research findings, although not directly comparable to current data, suggest that the observed shifts in cardiometabolic risk are clinically significant.
Consistent with previous findings,12
the current results also demonstrate convincingly that staying obese or severely obese is associated with significant risk. For example, youth who remained obese or severely obese had significant increases in fasting insulin, with eighth-grade levels of 29.7 uU/mL (insulin levels of ≥30 uU/mL have been used to define elevated risk23
), whereas obese or severely obese youth who decreased BMI had significant decreases in fasting insulin in comparison with those who remained obese or severely obese (average eighth-grade fasting insulin was 14.0 uU/mL). Similarly, youth with stable obesity had increases in waist circumference of 5.5 cm compared with decreases of 4.2 cm for youth who were obese or severely obese in the sixth grade but decreased to the healthy weight range. Finally, staying in the healthy weight range over time was associated with the most favorable levels across risk factors.
Changes in risk factors varied as a function of gender such that boys had greater increases in SBP and greater decreases in HDL than girls. Furthermore, boys and girls had different patterns of triglyceride changes, with boys tending to have more unfavorable changes in response to increases in BMI and girls benefitting more from decreases in BMI. These findings echo those observed in ALSPAC,12
which also documented that BMI changes were associated with more adverse risk factor changes in boys than in girls. Thus, boys may be particularly vulnerable to the negative cardiometabolic consequences of obesity.
The current study has significant strengths including the large, multiethnic sample from across the United States and assessments collected by trained, certified staff by using standardized procedures. Nevertheless, there also are limitations. All children participated in a study designed to mitigate risk for type 2 diabetes. Thus, the schools and youth who participated may not be representative of all high-risk US schools or students. All youth participated in health screenings and parents were given reports about child health. Consequently, observed decreases in BMI category may reflect greater awareness of the importance of health behavior among overweight or obese participants and their family members. Next, we used BMI as a proxy for adiposity. However, other research has shown that the association between BMI and cardiovascular risk factors is similar to that between directly assessed fat mass and risk factors,12
indicating the validity of BMI as an index of body fat.
In summary, results document considerable mutability in BMI categories during the middle school years, with clinically meaningful changes in cardiometabolic risk. The shifts in BMI and associated changes in risk were not associated with school intervention status and were robust to adjustment for salient demographic parameters and potential confounders. Thus, there is compelling evidence for the relevance of universal obesity prevention efforts that target middle-school-aged children across all BMI categories to enhance downward shifts in BMI category for overweight and obese children and mitigate increases in BMI category among children in the healthy weight range.