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.
Dr Solveig Cunningham and colleagues 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.
The research team used standard thresholds from the Centers for Disease Control and Prevention to define “overweight” and “obese” categories.
The team estimated the annual incidence of obesity, the cumulative incidence over 9 years, and the incidence density overall and according to sex, socioeconomic status, race or ethnic group, birth weight, and kindergarten weight.
When the children entered kindergarten, 12% were obese and another 15% were overweight.
The team found that in eighth grade, 21% were obese and 17% were overweight.
The researchers found that the annual incidence of obesity decreased from 5% during kindergarten to 2% between fifth and eighth grade.
Overweight 5-year-olds were 4 times as likely as normal-weight children to become obese, with rates of 92 versus 17 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.
Dr Cunningham's team concludes, "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."