Aim: During the coronavirus disease 2019 pandemic, the governments of many countries responded to high levels of infection with lockdowns. As a result, some children were reported to experience weight gain. The aim of the present study was to examine the impact of school closures on body mass index (BMI) in Japanese children. Methods: This was a retrospective study of students enrolled in the participating schools (6- to 11-year-old elementary school students and 12- to 14-year-old junior high school students) between 2015 and 2020. Using school health check-up data, annual changes in the BMI standard deviation score (ΔBMI-SDS) were calculated. We compared ΔBMI-SDS in 2019–2020 with the corresponding control years. Results: 19 565 children with complete data were included in the analysis. Median ΔBMI-SDS in 2019–2020 were 0.24–0.35 in elementary school boys, 0.10–0.13 in junior high school boys, −0.02 to 0.15 in elementary school girls and −0.14 to −0.10 in junior high school girls. In comparison with every control year, ΔBMI-SDS in 2019–2020 were significantly higher in elementary school boys (control years: −0.07 to 0.14) and junior high school boys (control years: −0.04 to 0.06), and significantly lower in junior high school girls (control years: −0.06 to 0.09). Conclusion: BMI-SDS increased significantly in elementary and junior high school boys, but decreased significantly in junior high school girls. The pandemic appears to have had an impact on Japanese children that was different from other countries.
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