According to the results of this study, during the COVID-19 period, the sitting time of Korean adolescents significantly increased, and the prevalence of obesity in-creased. This obesity prevalence demonstrated a tendency to increase in statistically significant manner after the pandemic, even when the effects of demographic covariates, such as sex, grade, and region, were analyzed using multivariate logistic regression. Similar trends were observed in the overweight analysis (Supplementary Materials).
To the best of our knowledge, this is the first study in Korea that addresses the changes in obesity rates by measuring the amount of physical activity during sitting time before and after the COVID-19 pandemic. Previous studies in Korea have demonstrated that the longer the sitting time for purposes other than the study, the higher the prevalence of obesity [17]. Another study in Korea showed that adolescents’ high weight tends to be associated with a low frequency of physical education classes, and adolescents who sit for more than two hours a day are more likely to be obese [18–19]. The finding that a decrease in physical activity due to increased sitting time increases obesity in adolescents is consistent with the results of our study.
Previous studies before the COVID-19 pandemic have shown that adolescents gain weight during summer vacation, suggesting that they have decreased physical activity, increased sitting behavior, increased access to harmful snacks, no plans, de-creased self-monitoring, and irregular sleep patterns. [20–22]. The lockdown period of COVID-19 can be considered as a type of vacation, and previous studies considered the lockdown period as an early summer vacation, suggesting that the child with obesity rate increases in proportion to the number of months of closure, resulting in rapid increase of new obesity cases [23–24]. In addition to school closures, there were restrictions on the large gatherings and business hours in public places and restaurants during the pandemic, which are believed to have created an environment that increased the obesity rate by limiting teenagers' physical activities. As classes were switched non-face-to-face due to Covid-19, the screen time of adolescents increased, which further exacerbated their sitting habits [25–26]. According to the "2021 adolescent Statis-tics" released by the Korea National Statistical Office, the average internet time of adolescents increased by 10 to 27.6 h in 2020 compared to 17.6 h in 2019 [27].
The negative relationship between a family's financial status and obesity prevalence in adolescents has been steadily reported in the past [28–29]. This study also con-firmed that the obesity prevalence in adolescents in the group with low-income household was higher than that in the group with high-income household. As the economy became more difficult due to COVID-19, many people lost their jobs, or their incomes decreased [30]. Therefore, it can be inferred that the prevalence of obesity in adolescents increased because of the increase in households whose family financial status deteriorated during the pandemic.
This study is meaningful in that it analyzes several variables, such as gender, grade, and housing income, including adolescents' sitting time, and investigates whether each variable affects the increase in the prevalence of overweight or obesity among adolescents during Covid-19. However, there are some limitations to the use of secondary data. First, memory bias may have existed because the data used in the study relied on the memory of the respondents and not observational data. Second, as the number of participants in the survey changed every year, it was not possible to confirm the change in individual students before and after the pandemic.