Impact of a year-round school calendar on children's BMI and fitness: Final outcomes from a natural experiment

Background Structure may mitigate children's accelerated summer BMI gain and cardiorespiratory-fitness (CRF) loss. Objectives Examine BMI and CRF change during school and summer for year-round and traditional calendar school children. Methods Three schools (N = 2279, 1 year-round) participated in this natural experiment. Children's BMI z-score (zBMI) and CRF (PACER laps) were measured from 2017 to 2019 each May/August. Mixed effects regression estimated monthly zBMI and CRF change during school/summer. Secondary analyses examined differences by weight status and race. Spline regression models estimated zBMI and CRF growth from kindergarten-sixth grade. Results Compared to traditional school, children attending a year-round school gained more zBMI (difference = 0.015; 95CI = 0.002, 0.028) during school, and less zBMI (difference = -0.029; 95CI = -0.041, -0.018), and more CRF (difference = 0.834; 95CI = 0.575, 1.093) monthly during summer. Differences by weight status and race were observed during summer and school. Growth models demonstrated that the magnitude of overall zBMI and CRF change from kindergarten-sixth grade was similar for year-round or traditional school children. Conclusions Contrary to traditional school children zBMI increased during the traditional 9-month school calendar and zBMI decreased during the traditional summer vacation for year-round school children. Structured summer programming may mitigate accelerated summer BMI gain and CRF loss especially for overweight or obese, and/or Black children.

Automated summary

Compared to traditional schools, children attending year-round schools had higher BMI gains during the school year, but lower BMI gains and higher cardiorespiratory fitness improvements during the summer. There were observed differences in weight status and race during both summer and school periods. Growth models showed similar overall changes in zBMI and cardiorespiratory fitness from kindergarten to sixth grade for both year-round and traditional school children.