Age and Gender Differences in COVID-19 Lockdown Impacts on Childhood BMI: A Japanese Longitudinal Study

The COVID-19 pandemic has globally disrupted children's lifestyles, with emerging evidence indicating an increase in the prevalence of childhood obesity. However, longitudinal analyses focusing on age-specific recovery patterns remain scarce, particularly in Asian populations.

This study employed a retrospective longitudinal design utilizing mandatory school health examination data from Kurume City, Japan, collected from April 2018 to April 2021. The dataset comprised 4244 measurements from the pre-pandemic, pandemic lockdown, and post-pandemic periods. We examined BMI and BMI-SDS trends using linear mixed-effects models (n = 711 children with complete three-timepoint data) and individual BMI-SDS trajectories using repeated measures ANOVA, stratified by gender and grade level.

Population-level analysis using linear mixed-effects models showed significant BMI increases during lockdown (boys: 16.60 ± 2.41 to 17.08 ± 2.78 kg/m², p = 0.04; girls: 16.39 ± 2.31 to 16.78 ± 2.53 kg/m², p = 0.03). Boys demonstrated significant BMI-SDS increases from pre-pandemic to pandemic (p = 0.019) and post-pandemic (p < 0.001) periods, whereas girls showed no significant changes. Individual trajectory analysis revealed that children in Grade 2 (ages 7-8 years) during lockdown demonstrated sustained BMI-SDS increases up to 10 months post-pandemic in both genders (p < 0.01), with no significant sex differences (p = 0.807). In contrast, children in Grades 3-5 exhibited significant gender differences (p < 0.05), with boys showing greater vulnerability and more variable recovery patterns.

The COVID-19 lockdown significantly affected childhood weight trajectories, with notable differences based on age and sex. Younger children exhibited sustained vulnerability, whereas older children demonstrated the capacity for recovery. These findings underscore the necessity of age-specific intervention strategies during health emergencies.