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

Robert Glenn Weaver, Ethan Hunt, Bridget Armstrong, Michael W. Beets, Keith Brazendale, Gabrielle Turner-McGrievy, Russell R. Pate, Alberto Maydeu-Olivares, Brian Saelens, Shawn D. Youngstedt, Roddrick Dugger, Hannah Parker, Lauren von Klinggraeff, Alexis Jones, Sarah Burkhart, Layton Ressor-Oyer

Research output: Contribution to journalArticlepeer-review

3 Scopus citations


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.

Original languageEnglish (US)
Article numbere12789
JournalPediatric Obesity
Issue number10
StatePublished - Oct 2021


  • children
  • obesity
  • policy

ASJC Scopus subject areas

  • Pediatrics, Perinatology, and Child Health
  • Health Policy
  • Nutrition and Dietetics
  • Public Health, Environmental and Occupational Health


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