The Weight of Summer: Children's Fat and Body Mass Index Gain Accelerate During Summer

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.