The National Health and Nutrition Examination Survey data suggest a plateauing of childhood obesity in the United States, with no trend among children younger than 11 years since 1999.1,2 However, studies have challenged this finding and cautioned that the overall static trend masks significant differences across subgroups.3,4 There is not much consensus about whether and how childhood obesity prevalence has changed for various age, sex, race/ethnicity, and socioeconomic subgroups.2,4 Available data on 2 nationally representative cohorts of US kindergarten-aged children 12 years apart provide a valuable opportunity to document changes in weight gain and obesity among young children and underlying sex, socioeconomic, and racial/ethnic disparities between 1998 and 2010.
Sponsored by the National Center for Education Statistics, the Early Childhood Longitudinal Study kindergarten class (ECLS-K) consists of 2 separate nationally representative cohorts recruited as kindergarteners during the 1998 to 1999 and 2010 to 2011 school years (mean age, 5.7 years). Both cohorts first surveyed children in the fall of the kindergarten year, with additional waves in later grades. Only kindergarten data were available at the time of analysis; therefore, our analyses used only the fall kindergarten data from both cohorts. In both cohorts, body mass index (calculated as weight in kilograms divided by height in meters squared) was calculated using height and weight measurements taken by trained staff using standardized procedures described elsewhere.5,6 Measures of children who were overweight or obese (body mass index ≥ 85th percentile) and children who were obese (body mass index ≥ 95th percentile) were computed using the 2000 sex- and age-specific Centers for Disease Control and Prevention growth charts.
The study used existing deidentified data and was exempt from human participant review. Children’s race/ethnicity and sex were obtained from the parent and/or school records. Children were classified into quintiles of a composite socioeconomic scale constructed by the ECLS-K,5 capturing parents’ education, occupation, and household income.
The proportion of children who were overweight or obese and the proportion of children who were obese were estimated overall and separately by sex, socioeconomic quintile, and race/ethnicity. Socioeconomic disparity was measured as the difference in proportion between the highest quintile and other quintiles. Racial/ethnic disparities were measured by the difference in proportion between non-Hispanic black and non-Hispanic white children and Hispanic and non-Hispanic white children.
Weighted estimates were generated in Stata version 12.1 (StataCorp Inc) to adjust for the multistage sampling design of the ECLS-K. Our analysis sample (rounded to the nearest 10 per the National Center for Education Statistics data-use restrictions) included approximately 17 000 and 15 560 kindergarteners who were representative of 3 442 716 and 4 003 224 US kindergarteners in the fall of 1998 and 2010, respectively.
The prevalence of childhood overweight and obesity increased significantly (P < .05) between 1998 and 2010, with a nearly 20% increase in obesity prevalence from 11.6% in 1998 to 1999 to 13.9% in 2010 to 2011 (Table). By subgroups, obesity increased significantly among both boys and girls, with no significant changes in sex disparity. Obesity decreased nonsignificantly for the highest quintile, increased nonsignificantly for the second-highest quintile, and increased significantly for the lowest 3 quintiles, with the largest increase observed for the middle quintile (11.2% to 15.3%; P < .05). Consequently, socioeconomic disparities increased significantly between the highest quintile and all other 4 quintiles. Obesity increased significantly only among non-Hispanic black children, with no significant changes in racial/ethnic disparities between 1998 and 2010.
Table. Estimated Prevalence of Overweight and Obese Children in the Early Childhood Longitudinal Study Kindergarten Class
To our knowledge, the ECLS-K is the only nationally representative data set on young school-aged children, with large sample sizes and standardized measurements of height and weight. In contrast to previous findings, ECLS-K data indicated that the prevalence of overweight and obese kindergarten-aged children increased substantially between 1998 to 2010. This was accompanied by a substantial increase in socioeconomic disparities because obesity decreased in children with higher socioeconomic backgrounds but increased significantly among children with lower socioeconomic backgrounds. These findings suggest that the news of overall static trends must be interpreted cautiously and underscore the need for understanding the drivers of these socioeconomic differences.
Corresponding Author: Ashlesha Datar, PhD, Center for Economic and Social Research, University of Southern California, 635 Downey Way, Los Angeles, CA 90089-3332 (firstname.lastname@example.org).
Published Online: May 18, 2015. doi:10.1001/jamapediatrics.2015.0172.
Author Contributions: Dr Datar had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Study concept and design: Datar.
Acquisition, analysis, or interpretation of data: Both authors.
Drafting of the manuscript: Datar.
Critical revision of the manuscript for important intellectual content: Both authors.
Statistical analysis: Datar.
Conflict of Interest Disclosures: None reported.
KM. Prevalence of obesity and trends in body mass index among US children and adolescents, 1999-2010. JAMA
. 2012;307(5):483-490.PubMedGoogle ScholarCrossref
KM. Prevalence of childhood and adult obesity in the United States, 2011-2012. JAMA
. 2014;311(8):806-814.PubMedGoogle ScholarCrossref
JA. Prevalence and trends in obesity and severe obesity among children in the United States, 1999-2012. JAMA Pediatr
. 2014;168(6):561-566.PubMedGoogle ScholarCrossref
RD. Increasing socioeconomic disparities in adolescent obesity. Proc Natl Acad Sci U S A
. 2014;111(4):1338-1342.PubMedGoogle ScholarCrossref
M. Early Childhood Longitudinal Study, Kindergarten Class of 1998-99 (ECLS-K): Combined User’s Manual for the ECLS-K Eighth-Grade and K-8 Full Sample Data Files and Electronic Codebooks. Washington, DC: US Department of Education; 2009.
et al. Early Childhood Longitudinal Study, Kindergarten Class of 2010-11 (ECLS-K:2011): User’s Manual for the ECLS-K:2011 Kindergarten Data File and Electronic Codebook. Washington, DC: US Department of Education; 2012.