Impact of Traditional vs Year-Round School Calendar on Childhood BMI

Investigators sought to assess change in BMI during the traditional months of the school year and summer vacation among children following a year-round or traditional school calendar.

Children who attended school on a year-round calendar were found to experience an increase in body mass index (BMI) during the traditional school year and a decrease during traditional summer vacation months, according to the results of a study published in Pediatric Obesity.

Researchers sought to compare changes in BMI z-score (zBMI) and cardiorespiratory fitness during the traditional school year and summer vacation months (August-May and May-August, respectively) among children attending school following a year-round calendar. The secondary aim of the study was to examine whether changes in zBMI and cardiorespiratory fitness differ by weight status and/or race.

Data were collected from 3 schools in a single district in South Carolina. One school followed a year-round school schedule, and 2 schools followed the traditional 9-month calendar.

All children at participating schools underwent measurements for height, weight, and cardiorespiratory fitness at the beginning and end of the traditional school year during the 2017/2018 and 2019/2019 academic school years and during the 2017, 2018, and 2019 summer vacation months.

Each of the 6 measurement periods included approximately 1000 students across all 3 schools.

Throughout the traditional school year, zBMI decreased by 0.002 per month for children following a traditional calendar and increased by 0.012 per month for children following a year-round calendar (representing a 0.015 greater increase in monthly zBMI change; 95% CI, 0.002-0.028).

During the summer months, children following a year-round calendar experienced a zBMI increase of 0.014 per month, and children following a traditional calendar experienced a zBMI decrease of 0.015 per month, indicating that year-round students experienced a gain of -0.029 less zBMI compared with students following a traditional calendar (95% CI, -0.041 to -0.018).

Secondary analyses stratified participant data by race and weight status at first measure. The investigators found that year-round students who began the study at a normal weight or with overweight or obesity did not experience any differences in monthly zBMI gain compared with their counterparts at the traditional calendar schools. During the traditional summer vacation months, though, the investigators found the opposite to be true: children of normal weight or with overweight or obesity at the year-round school gained -0.028, -0.026, and -0.033 less zBMI, respectively, compared with traditional calendar counterparts.

Black children in the year-round school group demonstrated a 0.016 greater zBMI increase during the months of the traditional school year compared with those in the traditional calendar school. During summer months, these children gained -0.039 less zBMI.

Study limitations include a small sample size, reliance on a single geographic location, and a lack of participant randomization, as well as a lack of measurement of children’s attendance at school. Behavioral mechanisms were only studied in a small subsample of participants.

“Future studies should attempt to include schools in more regions of the United States and internationally, examine multiple school years and summers, directly measure attendance at structured programming, and randomize children to receiving structure during the months of summer vacation,” the researchers concluded.


Weaver RG, Hunt E, Armstrong B, et al. Impact of a year-round school calendar on children’s BMI and fitness: Final outcomes from a natural experiment. Pediatr Obes. Published online March 25, 2021. doi:10.1111/ijpo.12789