· The prevalence of obesity of the total subjects was increased during during COVID-19 lockdown and male aged 12–15 years showed highest increase.

· Decreased physical activity and increased sedentary time mostly affected adolescent who are growing and active.

Obesity is a condition of excess body fat that negatively affects an individual's health or well-being. Childhood obesity is linked to immediate and long-term health risks [1]. Furthermore, childhood obesity persists into adulthood and results in increased mortality and morbidity in later life [2,3]. As childhood obesity are related with an increased intake of nonhealthy diets, a sedentary lifestyle, including physical inactivity and excessive screen time. Thus, health professionals with governments and policymakers adopted healthy lifestyle programs in family, school, and community to prevent childhood obesity. As a result, developed countries reported that the prevalence of childhood overweight and obesity has stabilized around the early 2000s [4-6]. In the United States, the rate of obesity among children 2–19 years of age increased from 5.2% to 17.1%, from 1974 to 2004 and which did not continue to increase from 2005 to 2012 [4]. In Korea, there was no significant change of obesity prevalence in children from 2001 to 2014 [6].

However, the coronavirus disease 2019 (COVID-19) pandemic has caused a tremendous change in lifestyle behaviors [7-9]. Many countries have adopted the "lockdown" strategy as a means to avoid crowding. The Korea Disease Control and Prevention Agency implemented "social distancing" to prevent disease transmission [10]. These orders included guidelines to limit the number of people in private gatherings, prohibit largescale events, restrict access to public facilities, close schools, limit business hours at night, and to define quarantine orders. Social distancing enforced children and adolescents are restricted to staying at home, further reducing opportunities for physical activities and exercise. Furthermore, most housebound children and adolescents are exposed to easily accessible junk food composed with high sugar and fat food sources.

Some studies have demonstrated that the prevalence of childhood obesity increased during the COVID-19 pandemic compared to that in the pre-COVID-19 period [11-15]. However, most studies are based on clinic and small population. Thus, the objectives of this study were as follows: (1) to investigate whether there was a significant change in the prevalence of overweight and obesity of Korean adolescents aged 12 to 19 years in country based in 1 year during the COVID-19 pandemic; (2) to examine whether the changes are different according to sex, age group, and socioeconomic status (SES); and (3) to examine what kind of changes most affect obesity prevalence difference between 2019 and 2020.

In this study, we found that the obesity prevalence of adolescents aged 12–18 years in 2020 (post-COVID-19) increased to 12.1% compared with 11.1% in 2019 (pre-COVID-19) using data of the Korea Youth Risk Behavior Webbased Survey. We also found that main increase of obesity adolescents was middle school boys and with high and middle SES. In this group, we identified decreased physical activity and increased sedentary time played major role.

This study demonstrated difference in obesity prevalence, physical activity, diet, stress, and sleep time that occurred before and after the COVID-19 pandemic among a representative sample of Korean adolescents. Our findings are in line with several studies on COVID-19-related weight changes in children and adolescents [19-22]. In Korean adolescents the overall BMI increased from 21.4 kg/m² in 2019 to 21.5 kg/m² in 2020. While, there was gender and age group difference; male adolescents aged 12–15 years only increased the total BMI from 21.3 kg/m² in 2019 to 21.8 kg/m² in 2020, male adolescents aged 16–18 years showed no change, female adolescents showed slight decrease.

One interesting finding in this study is, the overall obesity increased in all age group and both sexes, however, marked increased prevalence of obesity was shown male adolescents aged 12–15 years (mainly middle school boys). In USA MPDHnet study including 46,151 participants aged 2–20 years, they found greater increase obesity prevalence in boys, younger age, and some race comparing 2019 with 2020 [19]. In China COINLICS study including 10,082 participants aged 19.8±2.3 years, the youths' average BMI increased from 21.8 to 22.1 kg/ m², with obesity increasing from 10.5% to 12.6% [20]. They also demonstrated younger age and male showed greater increase. In Germany, 3-month change in zBMI was lower in adolescents aged 12–18 years compared younger children but not between sexes [12]. In addition, several other COVID-related studies also reported a remarkable weight gain in elementary school children or children aged 7–12 years [15]; Therefore, our findings are in line with other studies. Korean male aged 12–15 years is rapid pubertal growth phase, while others are late puberty and most reached final height heights and relative stable weight [23]. In Korean female, the mean age of menarche is 12.6 years and most reached their final height [24]. In our study, the ORs of adolescents' obesity in 2020 compared to 2019 is 1.119 (95% CI, 1.079–1.161). The ORs of male aged 12–15 years was 1.296 (95% CI, 1.218–1.378). Thus, the odds ratio for obesity of Korean adolescents before and during the COVID-19 pandemic increased by 11.9%. The main portion of increased prevalence was male aged 12–15 years (29.6%).

The other interesting finding is that only moderate-/vigorousintensity physical activity and average sedentary time showed marked difference especially in middle school boys before and after COVID-19. Most studies demonstrated the lockdown to prevent the spread of COVID-19, resulted in tremendous changes in lifestyle, including diet patterns, physical activity, and sleep patterns. Sedentary behavior including longer screen time along with physical inactivity is associated with a decrease in the basal metabolic correction factor by 10% to 50% [25]. Decrease in aerobic exercise induces cravings for high-calorie foods and higher levels of physical activity reduce BMI [26,27]. For every 1 hour of moderate-to-vigorous activity might decrease the risk of developing obesity by 10%. [28]. In the females aged 16–18 years, the prevalence of obesity increased the least (Table 2) and the increase of physical activity frequencies was the greatest (Supplementary Tables 1, 2). This group seemed to use spare time most actively in the health behavior change of this study population. Thus, education on ideal weight and proper weight control may contribute to improving the prevalence of obesity.

The lifestyle changes inevitably lead to weight gain during the lockdown [29]. In Korea like other countries, shutdown of sports facilities, reduced physical activity including school sports, and excess screen time including online classes affects lifestyle changes of adolescents inevitably lead to weight gain. However, the unhealthy eating including soda, SBB, and fast-food did not increased or even decreased. That might be due to adolescents at home are more supervised by their parents in Korea. Instead of eating fast food or junk food with friends in school and after school, the adolescents obviously ate most meals at home with their parents. Although, COVID-19-related decrease in physical activity and sedentary behavior has already been shown in most studies [7,9,30,31], there are mixed results concerning dietary behavior [7,30,32]. Some studies described increased total consumption and unhealthy binge eating, while other studies increased adherence to a healthy diet, homemade meals and increased eating with friends or family [29]. The diet patterns change during COVID-19 differ according to country, culture, economy, and nutrition education along with age, sex, and body weight status. A slight increase of obesity prevalence was observed in groups other than low SES, however, no significant changes of the prevalence was observed in low SES group (Table 2). The relationship of SES to child obesity prevalence is observed to have a negative or a positive association according to the socioeconomic development level of a country being studied [33,34]. The prevalence of obesity was highest in the low SES group in this study population. Therefore, it is conceivable that the obesity-related risk factors, which began to influence the prevalence of obesity in the high and middle SES groups after the pandemic, were already influencing the prevalence in low SES group.

The strengths of this study include a large nationally representative sample of the Korean adolescent population with the use of same protocols before and after COVID-19. However, this study has several limitations. First, the survey was conducted through self-reporting. Thus, the data, especially weight, may lead to an underestimation [18]. Second, this study subjects was adolescents aged 12–18 years, investigation of younger age including elementary school and preschool children should be needed. Finally, the variable of KYRBW data was qualitative rather than quantitative for diet variables and SES, thus its applicability to this context and the possibility of using alternative surveys are needed.

In conclusion, this study confirms that the environment created by the COVID-19 resulted in increased obesity prevalence in Korean adolescents aged 12–18 years. Korean middle school boys were most affected subjects by decreasing physical activity due to COVID-19 lockdown and school shutdown. This study presented unique strengths in the targeted population promptly for drawing strong conclusions. Health professionals and parents should aware physical activity is important for preventing obesity in adolescent period. Governments along with school administrators must develop social and in-school physical activity programs to prevent obesity from a long-term perspective.

The Research Ethics Committee of the Korea Centers for Disease Control approved the study protocol (No: 2022-05-001).