The present study takes advantage of California’s mandated school-based BMI screening to examine trends in prevalence of high BMI from 2001 to 2008. The large sample size of this dataset is a unique strength, providing ample power to identify significant trends over time, and disparities in trends. To our knowledge, this is the first study to document a population-based decline in prevalence of high BMI after 2005, among most boys and for white girls. Even Hispanic boys, long leading in prevalence of obesity, declined in prevalence after peaking. However, Hispanic girls demonstrate no decline after peaking, and African American and American Indian girls had the highest prevalence across BMI categories in 2008, the last year for which data were available. Differing patterns of plateau vs. decline by race/ethnicity suggest that the alarming disparities in prevalence of high BMI in 2008 are expected to widen.
Overall prevalence of high BMI in 2008 in California’s dataset was similar to or slightly higher than prevalence among NHANES youth age 6–19 years in 2007–08: 13.3% at BMI ≥ 97th
percentile in both datasets; 19.8% in California and 18.7% in NHANES at BMI ≥ 95th
percentile; and 38.0% of California youth vs. 34.7% of NHANES youth had BMI ≥ 85th
Across BMI categories, we demonstrate a larger magnitude of disparities than those seen in NHANES data,2
largely because California’s non-Hispanic white youth had lower prevalence of high BMI. This pattern replicates regional differences in obesity seen among adults in the CDC BRFSS data,16
with non-Hispanic whites in the West having lower prevalence of obesity than whites in any other region, while Hispanics in the West have higher obesity prevalence than that seen nationally.16
Thus, higher prevalence in California is expected given a higher proportion of individuals of Hispanic ethnicity in California relative to the U.S. population as a whole. While this study is limited to California, it represents a large proportion of U.S. children: current census data reveal that 1 in 8 children in the U.S. live in California.
These findings are a call to action for policies and interventions tailored for use in high-risk populations. School- and after-school-based programs have demonstrated improvements in weight status among African American17–19
and Hispanic youth.20, 21
More such interventions are needed and more work must be done to address weight status in understudied American Indian youth, who demonstrated the greatest increases in prevalence of high BMI in the present study, and are at high risk for weight-related comorbidities.9, 22
Future interventions should build on the work done and lessons learned in the NHLBI-funded Pathways study.23, 24
Interventions in early childhood will also be critical as national data demonstrate increasing obesity prevalence among low-income American Indian preschool children as well.25
We demonstrate that disparities increase with increasing severity of obesity. However, while disparities are greatest for BMI ≥ 99th
percentile, concerns around severe obesity apply to all youth. All race/ethnic subgroups experienced the greatest proportional increase in prevalence for BMI ≥ 99th
percentile. This has also been demonstrated in NHANES data for African American, Hispanic, and non-Hispanic white youth.4
In the present study, no subgroups except white and Asian boys showed a decline after peaking in prevalence of severe obesity. This is of significant consequence because the adverse effects of high BMI worsen as severity of obesity increases, with respect to risk for future complications and economic impacts,3, 5, 26
as well as risk for morbidity in adolescence.27–31
Reversing childhood obesity will require concerted public health efforts, similar to the multi-faceted approach taken to reduce smoking.32–34
Interventions to be considered might include: restricting advertising of unhealthy products both in schools and during television programming targeted at children;35, 36
taxing sugar sweetened beverages, which have been causally linked to obesity;37
banning the sale of sugar-sweetened beverages and snacks high in fat or sugar during the school day (such policies in California may be related to declines in obesity seen after 2005);38
and increasing the quality and quantity of physical education.39–42
Providing levers to allow low-income communities to benefit first and most from such policies may address disparities.
While school-based BMI screening provides objective data on a vast number of youth, data quality is unknown. There is no surveillance of Fitnessgram test administration, and data collection methods and integrity likely vary (which will decrease precision of prevalence estimates), and may vary by school (which might bias estimates of prevalence, although less likely to bias estimates of trends over time). Additionally, not all students enrolled in a school complete the Fitnessgram. The test, most often administered during PE, is more likely to miss students not taking PE. Because greater participation in PE has been associated with improved BMI,39
missing data might result in an underestimate of prevalence in the present study.
Examining four BMI cutpoints could be considered multiple hypothesis testing. Applying Bonferroni adjustments with a p-value of 0.0125 (0.05 divided by 4) would suggest greater disparities: among girls, only non-Hispanic whites would demonstrate a decline post peak; non-Hispanic white boys would remain the only group to decline across BMI cutpoints, and American Indian boys would show no decline at any cutpoint. Finally, using a cutpoint of BMI at the 99th
percentile is problematic as percentiles greater than the 97th
are beyond the range of the data from which parameters for estimating BMI z-scores (based on CDC growth charts) were derived. Therefore, “extrapolation beyond this range should be done with caution.”43
However, given the potential public health impact of increasing rates of severe obesity, highlighting this problem is critical.
The encouraging first signs of a decline in the obesity epidemic demonstrated in the present study are tempered by concerns for increasing disparities. The groups of greatest concern are first, African American and American Indian girls who have not demonstrated any reduction in prevalence of high BMI; second, American Indian boys and Hispanic girls, whose rates plateaued rather than declined after peaking; and finally Hispanic boys, who showed only very small declines, and not for the most severe obesity. Clinicians’ voices supporting policy approaches that focus on preventing and reducing prevalence of high BMI among these groups will be critical in reversing childhood obesity.