Body Mass Index Changes from Before to 3 Years After the COVID-19 Lockdown: A Retrospective Longitudinal Study in a Single Elementary School

Background/Objectives: The COVID-19 pandemic and related lockdown measures markedly disrupted children’s lives, raising concerns particularly about their weight. We investigated changes in body mass index (BMI) in children by grade and sex, from pre- to post-COVID-19 lockdown, and BMI recovery post-lockdown. Methods: We retrospectively reviewed the height, weight, and BMI of students from a single elementary school from 2019 to 2023, excluding 2020 (the year of the COVID-19 lockdown). We conducted longitudinal and cross-sectional studies to examine changes in BMI according to grade and BMI status pre-lockdown (2019) and post-lockdown (2021–2023). Results: In the entire student body, the BMI Z-score in 2021, 1 year after the COVID-19 lockdown, had increased significantly compared with that in 2019, 1 year pre-lockdown (p = 0.009). However, longitudinal studies in each grade yielded different results. Students who were in the first grade in 2019 experienced a significant decrease in BMI Z-score post-lockdown (p = 0.003). In contrast, students who were in other grades showed a significant increase in BMI Z-score post-lockdown, with those who were in third grade in 2019 showing the most significant increase (p = 0.027). Conclusions: The degree of BMI increase in children and adolescents due to the COVID-19 lockdown was inconsistent. Changes in obesity status post-lockdown varied depending on age and obesity levels pre-COVID-19 lockdown. Therefore, to manage and prevent obesity-related metabolic diseases in the post-COVID-19 era, diverse approaches and strategies tailored to age and obesity grades during the COVID-19 lockdown will need to be adopted.