The weight of summer: Children's fat and body mass index gain accelerate during summer

Authors: WEAVER, R - University Of South Carolina; FINNEGAN, OLIVIA - University Of South Carolina; WHITE III, JAMES - University Of South Carolina; KIELY, KEAGAN - University Of South Carolina; ARMSTRONG, BRIDGET - University Of South Carolina; BEETS, MICHAEL - University Of South Carolina; ADAMS, ELIZABETH - University Of South Carolina; BURKHART, SARAH - University Of South Carolina; SAVIDGE, MEGHAN - University Of South Carolina; RANDOLPH, GRIFFIN - University Of South Carolina; CULVERHOUSE, JOSH - University Of South Carolina; HOLMES, ANTHONY - University Of South Carolina; SARZYNSKI, MARK - University Of South Carolina; KACZYNSKI, ANDREW - University Of South Carolina; BOHNERT, AMY - Loyola University; MORENO, JENNETTE - Children'S Nutrition Research Center (CNRC)

Submitted to: Childhood Obesity
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/20/2026
Publication Date: 5/26/2026
Citation: Weaver, R.G., Finnegan, O., White III, J.W., Kiely, K., Armstrong, B., Beets, M.W., Adams, E.L., Burkhart, S., Savidge, M.C., Randolph, G., Culverhouse, J., Holmes, A., Sarzynski, M.A., Kaczynski, A.T., Bohnert, A., Moreno, J.P. 2026. The weight of summer: Children's fat and body mass index gain accelerate during summer. Childhood Obesity. https://doi.org/10.1177/21532176261454484.
DOI: https://doi.org/10.1177/21532176261454484

Interpretive Summary: This study shows that elementary school children gain weight much faster during summer than during the school year—and importantly, most of that extra weight is actual body fat, not just normal growth, with about 36% of yearly BMI gain and over 90% of body fat gain happening in just three summer months. In simple terms, when kids are out of school, changes in daily routines (such as less structure, different eating habits, and lower activity) appear to put them at much higher risk for unhealthy weight gain, making summer a key window for prevention. This finding aligns closely with USDA's 2026 research priorities, which emphasize improving human health through food quality and precision nutrition alongside strengthening agricultural systems, and with the USDA ARS Human Nutrition Action Plan (2024–2029), which focuses on understanding diet, physical activity, and their role in preventing obesity and other chronic diseases across the lifespan. Together, this work highlights a clear opportunity for USDA-supported research and programs to target seasonal risk periods like summer with nutrition, food access, and behavioral interventions—directly supporting national action plans aimed at reducing childhood obesity and improving long-term health.

Technical Abstract: Accelerated summer body mass index (BMI) gain in children is well Accelerated summer body mass index (BMI) gain in children is well documented, but no studies have examined if these increases are accompanied by increases in percent body fat (%BF). Data from the Healthy Seasons (N=418; mean age=7.1 years) observational cohort was examined. Children's height, weight, and %BF were measured during January, May, August, and December from 2023 to 2025. Monthly BMI gain was 0.067 during the school year and 0.119 during the summer (difference in change=0.052, 95% confidence interval [CI]=0.025, 0.079), indicating that the 3 months of summer represent 36% of annual BMI gain. Monthly %BF gain was 0.009 during the school year and 0.456 during the summer (difference in change=0.446, 95% CI=0.355, 0.538), indicating that the 3 months of summer represent 93% of annual %BF gain. This study suggests that summer BMI gain is accompanied by increases in %BF, suggesting summer is a critical period for obesity prevention efforts.