Understanding accelerated summer body mass index gain by tracking changes in children's height, weight, and body mass index throughout the year

Authors: WEAVER, R - University Of South Carolina; WHITE III, JAMES - University Of South Carolina; FINNEGAN, OLIVIA - University Of South Carolina; ARMSTRONG, BRIDGET - University Of South Carolina; BEETS, MICHAEL - University Of South Carolina; ADAMS, ELIZABETH - University Of South Carolina; BURKART, SARAH - University Of South Carolina; DUGGER, RODDRICK - University Of South Carolina; PARKER, HANNAH - University Of South Carolina; VON KLINGGRAEFF, LAUREN - University Of South Carolina; BASTYR, MEGHAN - University Of South Carolina; ZHU, XUANXUAN - University Of South Carolina; BANDEIRA, ALEXSANDRA - University Of South Carolina; REESOR-OYER, LAYTON - University Of South Carolina; PFLEDDERER, CHRISTOPHER - University Of South Carolina; MORENO, JENNETTE - Children'S Nutrition Research Center (CNRC)

Submitted to: Childhood Obesity
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/27/2023
Publication Date: 4/19/2023
Citation: Weaver, R.G., White III, J.W., Finnegan, O., Armstrong, B., Beets, M.W., Adams, E.L., Burkart, S., Dugger, R., Parker, H., Von Klinggraeff, L., Bastyr, M., Zhu, X., Bandeira, A.S., Reesor-Oyer, L., Pfledderer, C.D., Moreno, J.P. 2023. Understanding accelerated summer body mass index gain by tracking changes in children's height, weight, and body mass index throughout the year. Childhood Obesity. https://doi.org/10.1089/chi.2023.0029.
DOI: https://doi.org/10.1089/chi.2023.0029

Interpretive Summary: Previous research has shown that children increase their body mass index (BMI) at a faster rate during the summer compared to the school year. The extent that these patterns are driven by exposure to the earth's seasonal light-dark cycle or differences in the obesogenic nature of the school year and summer environment is unclear. The goal of this study was to identify the mechanisms driving children's seasonal BMI gain. Children's height and weight were measured monthly during the school year, and once in summer (July–August). School year vs. summer vacation analyses showed accelerations in children's weight and BMI during summer vacation, but change in height remained relatively constant during summer vacation compared with school. Seasonal analyses showed summer had the greatest increases in weight and BMI while height was relatively constant across seasons. Compared with school months without a break, months with a break showed higher weight change and change in BMI, but height gain was constant. Fluctuations in sleep timing and screen time may explain increases in weight and BMI during out-of-school times. Overall, we found greater evidence that changes in weight and BMI were driven by exposure to an out-of-school environment rather than by exposure to the earth's seasonal light-dark cycle. Interventions targeting consistent sleep and reduced screen time during breaks from school may be warranted no matter the season.

Technical Abstract: Drivers of summer body mass index (BMI) gain in children remain unclear. The Circadian and Circannual Rhythm Model (CCRM) posits summer BMI gain is biologically driven, while the Structured Days Hypothesis (SDH) proposes it is driven by reduced structure. Our objective was to identify the mechanisms driving children's seasonal BMI gain through the CCRM and SDH. Children's (N=147, mean age=8.2 years) height and weight were measured monthly during the school year, and once in summer (July-August). BMI z-score (zBMI) was calculated using CDC growth charts. Behaviors were measured once per season. Mixed methods regression estimated monthly percent change in children's height (%Hchange), weight (%Wchange), and monthly zBMI for school year vs. summer vacation, seasonally, and during school months with no breaks vs. school months with a break >= week. School year vs. summer vacation analyses showed accelerations in children's %Wchange (change=0.9, Standard Error (SE)=0.1 vs. change=1.4, SE=0.1) and zBMI (change=-0.01, SE=0.01 vs. change=0.04, SE=0.3) during summer vacation, but %HCHANGE remained relatively constant during summer vacation compared with school (change=0.3, SE=0.0 vs. change=0.4, SE=0.1). Seasonal analyses showed summer had the greatest %Wchange (change=1.8, SE=0.4) and zBMI change (change=0.05, SE=0.03) while %Hchange was relatively constant across seasons. Compared with school months without a break, months with a break showed higher %Wchange (change=0.7, SE = 0.1 vs. change=1.6, SE=0.2) and zBMI change (change=-0.03, SE=0.01 vs. change=0.04, SE=0.01), but %Hchange was constant (change=0.4, SE=0.0 vs. change=0.3, SE=0.1). Fluctuations in sleep timing and screen time may explain these changes. Evidence for both the CCRM and SDH was identified but the SDH may more fully explain BMI gain. Interventions targeting consistent sleep and reduced screen time during breaks from school may be warranted no matter the season.