National school lunch program participation and sex differences in body mass index trajectories of children from low-income families

Objectives: To investigate participation patterns in the National School Lunch Program (NSLP) among lowincome children from kindergarten to fifth grade and to examine the ways in which participation influences sex differences in the trajectories of body mass index (BMI) through the eighth grade. Design: Longitudinal, secondary data analysis. Setting: Sample of low-income US children who entered kindergarten in 1998. Participants: Girls (n=574) and boys (n=566) from lowincome families who participated in the Early Childhood Longitudinal Study, Kindergarten Cohort. Main Exposure: Participation in the NSLP. Main Outcome Measures: Temporary and persistent patterns of NSLP participation, and age-specific and sex-specific BMI raw scores calculated at 5 data points. Results: Among the low-income children who attended schools that participated in the NSLP, both the children who persistently participated in the program and those who temporarily participated in the program displayed similar socioeconomically disadvantaged factors. Nonlinear mixed models indicated a larger rate of change in BMI (ie, an increase) among low-income, participating girls than among low-income, nonparticipating girls; however, mean BMIs did not significantly differ between low-income girls who participated and those who did not participate. No significant differences were observed among low-income boys. Conclusions: Results suggest that participation in the NSLP is associated with rapid weight gain for lowincome girls but not for low-income boys.