Childhood obesity is a growing epidemic in the United States with approximately 1 in 5 children being obese. Although prevalence estimates for childhood obesity are regularly updated, little is known about the current incidence of childhood obesity among a nationally representative population, since initial estimates were provided by the Early Childhood Longitudinal Study Kindergarten (ECLS-K) Class of 1998.
We estimated the annual incidence of obesity and the cumulative incidence of obesity over 5 years using the contemporary ECLS-K Class of 2011 (analytic sample = 7019). Height and weight were measured at eight time points between kindergarten (2010) and fourth grade (2015). We defined normal weight (<85
percentile), overweight (85
percentile), and obesity (>95
percentile) using Centers for Disease Control and Prevention sex-and age-specific growth curves.
At kindergarten entry (mean age 5.6 years), 15.5% of children were overweight and 14.7% of children were obese; in fourth grade (mean age 10.1 years) overweight prevalence had increased to 17.3% and obesity prevalence had increased to 20.1%. Kindergarteners who were overweight were 6 times as likely as normal-weight kindergarteners to become obese by fourth grade (5-year cumulative incidence, 30.5% vs. 5.2%). In early elementary school, annualized obesity incidence proportions were higher over summer breaks (5.6% and 5.4% during summer breaks from Kindergarten to first grade and first grade to second grade, respectively) than during the school year (3.8% and 3.8% during Kindergarten and first grade school years, respectively). Incident obesity was higher among boys compared girls, non-Hispanic black and Hispanic children compared to non-Hispanic white children, and among those from the poorest socioeconomic quintiles compared to the highest socioeconomic quintile.
Incident obesity between the ages of 5 and 10 years was more likely to have occurred during summer breaks and primarily among children who had entered kindergarten overweight. Childhood obesity prevention efforts should focus on the prevention of overweight prior to kindergarten and investigate causes of summer weight gain during elementary school.