Incidence of childhood obesity in the United States.

BACKGROUND: Although the increased prevalence of childhood obesity in the United States has been documented, little is known about its incidence. We report here on the national incidence of obesity among elementary-school children.

METHODS: We evaluated data from the Early Childhood Longitudinal Study, Kindergarten Class of 1998-1999, a representative prospective cohort of 7738 participants who were in kindergarten in 1998 in the United States. Weight and height were measured seven times between 1998 and 2007. Of the 7738 participants, 6807 were not obese at baseline; these participants were followed for 50,396 person-years. We used standard thresholds from the Centers for Disease Control and Prevention to define "overweight" and "obese" categories. We estimated the annual incidence of obesity, the cumulative incidence over 9 years, and the incidence density (cases per person-years) overall and according to sex, socioeconomic status, race or ethnic group, birth weight, and kindergarten weight.

RESULTS: When the children entered kindergarten (mean age, 5.6 years), 12.4% were obese and another 14.9% were overweight; in eighth grade (mean age, 14.1 years), 20.8% were obese and 17.0% were overweight. The annual incidence of obesity decreased from 5.4% during kindergarten to 1.7% between fifth and eighth grade. Overweight 5-year-olds were four times as likely as normal-weight children to become obese (9-year cumulative incidence, 31.8% vs. 7.9%), with rates of 91.5 versus 17.2 per 1000 person-years. Among children who became obese between the ages of 5 and 14 years, nearly half had been overweight and 75% had been above the 70th percentile for body-mass index at baseline.

CONCLUSIONS: Incident obesity between the ages of 5 and 14 years was more likely to have occurred at younger ages, primarily among children who had entered kindergarten overweight. (Funded by the Eunice Kennedy Shriver National Institute of Child Health and Human Development.).