Changes in Body Mass During Elementary and Middle School in a National Cohort of Kindergarteners

OBJECTIVE: To analyze changes in BMI, according to gender and race/ethnicity, in a nationally representative cohort of children in the United States during their elementary and middle school years to identify critical periods of excess BMI gains. METHODS: The Early Childhood Longitudinal Study-Kindergarten Class monitored a nationally representative cohort of kindergarten students over 9 years (1998-2007). Height and weight measurements were available for 4240 white, 640 black, and 1070 Hispanic children in kindergarten and 1st, 3rd, 5th, and 8th grades. In each wave, we estimated the proportions of children with BMI values in each quartile of the Centers for Disease Control and Prevention reference-population distribution according to gender and race/ethnicity. We conducted nonparametric tests of differences in BMI distributions over time within racial/ethnic groups and across racial/ethnic groups in each wave. Piecewise linear growth models were estimated to test for specific time periods during which the largest gains in BMI percentiles occurred. RESULTS: Overall, nearly 40% of children started kindergarten with a BMI in the top quartile of the growth charts (BMI > 75th percentile). This proportion increased significantly during the elementary school years, and the largest gains were between 1st and 3rd grades (5.8 percentage points), but there was no further increase during middle school. Increases in BMI percentiles over time were most notable among Hispanic children and black girls. CONCLUSIONS: The early school years might be a critical time for excess BMI gains, even among children with normal BMI values at kindergarten entry.