· During the COVID-19 pandemic, the prevalence of overweight and obesity increased significantly, particularly among young males. Short sleep duration and increased sitting time were associated with an increased risk of overweight and obesity.

It has been nearly 2 years since the emergence of severe acute respiratory syndrome coronavirus 2 in Wuhan, China, in December 2019, and the end of the coronavirus disease 2019 (COVID-19) pandemic is difficult to predict. The unprecedented effects of the COVID-19 pandemic have changed our lifestyles dramatically. The social distancing mandates, including school closures, have specifically impacted the lives of Korean adolescents, given that they spend a large portion of their waking time at school. Several studies before the COVID-19 pandemic revealed increased obesity rates during academic breaks due to decreased physical activity, such as school-organized sports [1,2]. Researchers hypothesized that the structured timetable, with its organized exercise and controlled school meals, aided students in staying fit. Social distancing measures further reduced accessibility to other outdoor activities [3-13], consequently increasing sedentary behavior as time spent at home increased [8,13-15]. People were less likely to go shopping or eat out but were more likely to buy nonperishable processed foods [11,16]. Because of these, studies with limited participants have found that childhood obesity is on the rise during the COVID-19 pandemic [3,7,8,13,17-20]. The results varied depending on ethnicity [17], region [11,21], age [7,13], and sex [8], and studies have not explored nationwide databases or included lifestyle changes. In Korea, schools were fully closed from March–May 2020 and partially closed until June 2021. The duration of school closure (a total of 60 weeks) in Korea was longer than that in China (27 weeks) and the United States (US) (58 weeks) [22]. Only 2 single-center studies found an increase in obesity among Korean adolescents [19,20].

Thus, we carried out this study to identify the effects of the COVID-19 pandemic on lifestyle factors associated with overweight and obesity. We aimed to assess the changes in body mass index (BMI) z-score and obesity prevalence by gender and age, before and during the COVID-19 pandemic, and identify the lifestyle factors that contributed to these changes.

There were no significant differences in sex, household income, and residential area before and during the COVID-19 outbreak (Table 1). Overall, the prevalence of overweight and obesity increased from 10% to 10.5% and 11.5% to 12.7%, respectively. The BMI z-score also increased from 0.18 to 0.22.

Table 2 and Fig. 1 show the differences in mean weight, BMI, BMI z-score, and prevalence of obesity before and during the COVID-19 pandemic. During the pandemic, the prevalence of overweight and obesity increased significantly in males, from 11.0% to 12.1% and 14.4% to 16.3%, respectively. An increase in obesity was more evident in younger male participants and was significant in those younger than 15 years. In contrast, there were no significant differences in the prevalence of overweight and obesity in females except those aged 12 years, who showed an increased obesity prevalence from 8.5% to 8.7%. The average BMI z-score among males increased by 0.11, while that among females decreased by 0.04. The increase in BMI z-score was greater in younger males (by 0.3 in 12-year-olds) than in older males (by 0.01–0.02 in 16–18-year-olds). In females, consistent trends in weight/BMI changes were not observed. The BMI z-score in females increased between the ages of 12 and 13 and decreased between the ages of 14 and 18 during the COVID-19 pandemic.

Fig. 2 presents the comparison of lifestyle factors by sex before and during the COVID-19 pandemic. Fruit consumption was significantly reduced during the COVID-19 pandemic compared to that before the pandemic, but there was no difference in fast-food consumption. The proportion of participants who exercised more than once a week decreased significantly, particularly in males (-5.4% in males and -1.8% in females). In both sexes, mean sleep duration decreased by 0.05 hours. Notably, the mean sitting time increased by 0.14 hours in males, whereas it decreased by 0.1 hours in females.

Male participants from low-income households and those residing in rural residential areas primarily reported overweight/obesity (Fig. 3). In lifestyle factors, participants in the lowest sleep duration quartile (<5.9 hours per day) were at increased odds of overweight/obesity compared to those in the highest quartile (>7.9 hours per day) (OR [95% CI], 1.14 [1.08–1.20]). Those who spent more than 6.3 hours a day sitting (highest quartile) showed a 1.14 times higher OR of overweight or obesity than those who spent less than 3.6 hours a day sitting (lowest quartile).

In this study, we analyzed the changes in lifestyle factors and obesity in Korean adolescents before and during the COVID-19 outbreak. Our study showed that the prevalence of overweight and obesity increased significantly in males, especially those younger than 15 years, whereas there were no significant differences in females, except for those aged 12 years. Fruit consumption, exercise, and average sleep duration showed a decrease among both sexes, whereas average sitting time increased in males and decreased in females.

In our nationwide representative study among Korean adolescents, the prevalence of overweight (10% to 10.5%) and obesity (11.5% to 12.7%) increased during the COVID-19 pandemic. Several studies have reported an increase in the prevalence of childhood obesity or weight gain during the COVID-19 lockdown [3,7,8,13,17-20]. Among those studies, only 2 were large-scale studies. In a national retrospective survey in China, the prevalence of overweight/obesity (21.3% to 25.1%) and obesity (10.5% to 12.9%) increased in overall youths during the COVID-19 lockdown [13]. A recent cross-sectional study from the US reported an increased prevalence of childhood obesity from 13.7% to 15.4% during the COVID-19 pandemic [18].

We found that adolescents in the younger age group (12–15 years old) showed a significant increase in BMI z-score. Previous studies also showed age-associated differences in obesity risk after the COVID-19 pandemic among affected age groups. In China [13], high school aged adolescents gained more weight than older youth (undergraduate and graduate students). A study of 2–17-year-olds in the US revealed a relatively larger increase in obesity among those in the 5–12-year age group [18]. Additionally, among children 0–14 years old in a German study, obesity increased in the 10–12-year age group [7].

We also discovered that males were more likely to gain weight than females during the COVID-19 pandemic. Several studies have also revealed a sex difference in weight gain [7,8]. In a longitudinal study in Italy, adolescents gained 2.8 kg on average during the lockdown, with males gaining more weight than females [8]. A German study using retrospective interviews also reported that males were more likely to report weight gain than females [7]. We speculated that the increase in obesity during the COVID-19 pandemic is mainly due to a decrease in physical activity, a well-known risk factor for obesity [24]. In this study, physical activity, represented by exercise and sitting time, was higher in males than in females prior to the pandemic. While the amount of exercise decreased among both sexes during the pandemic, sitting time increased for males but decreased for females. An increase in sitting time was more profound in younger males (12–14 years) than older males (data not shown). In our logistic regression model, a sitting time longer than 6.3 hours per day was associated with an increased risk of obesity. Considering all the evidence, adolescents who were physically active (especially younger males) in their daily and school lives seemed to experience a more profound decrease in physical activity during the COVID-19 pandemic, making them more vulnerable to obesity.

Sleep duration decreased by 0.05 hours on average during the COVID-19 pandemic in Korean adolescents. Contrary to our findings, studies in Italy [25], Spain [9], Greece [3,25], and China [13] reported that children slept more during the COVID-19 lockdown. A retrospective study in the US discovered that bedtime was delayed by an average of 1.6 hours during the COVID-19 pandemic [5]. The postponed bedtime during school closure could have resulted in shorter sleep duration in our study population. Short sleep duration is also a risk factor for obesity [26,27], increasing the risk of obesity through increased calorie intake due to increased serum ghrelin and decreased serum leptin and reduced physical activity due to increased fatigue and tiredness [26,27]. In our study, adolescents who slept for less than 5.9 hours had higher odds of developing obesity than adolescents who slept for more than 7.9 hours. Our findings supported our hypothesis that reduced sleep duration during the COVID-19 pandemic contributed to an increase in obesity.

Fruit consumption decreased among both sexes during the COVID-19 pandemic. This could also reflect the effect of school closure on availability of fruits at school meals. However, in our logistic regression model, a decrease in fruit consumption was not associated with an increased risk of obesity. In contrast with our finding, other studies from China [28], Germany [7], and Greece [3] found that fruit consumption increased during the COVID-19 lockdown.

Regarding changes in other lifestyle factors, none were observed in fast-food consumption before and during the COVID-19 pandemic, and fast-food consumption was not associated with obesity in this study. Although low income and rural residence were associated with higher risk of obesity in this study, changes in the prevalence of obesity were absent in the low-income group and those residing in rural areas (data not shown) during the COVID-19 pandemic.

Our study had some limitations. Determining the exact time-to-time relationship between lifestyle factors and weight status was challenging due to the cross-sectional study design. Furthermore, because all data relied on self-assessment rather than actual measurement, anthropometric indices, such as height and weight, may have been inaccurate, and answers may have included recall bias. Nonetheless, this is the first nationally representative study that compared the prevalence of obesity according to sex and age and identified lifestyle factors associated with an increase in obesity before and during the COVID-19 pandemic.

In conclusion, the prevalence of obesity has increased in Korean adolescents during the COVID-19 pandemic, in males aged 12–14 years and from among those aged 12–18 years in age. The prevalence of obesity/overweight in females did not differ significantly before and during the COVID-19 lockdown; however, the BMI z-score decreased by 0.039. Furthermore, both sexes experienced adverse lifestyle changes, such as decreased fruit consumption, physical activity, and sleep duration as well as increased sedentary time. Short sleep duration and prolonged sitting time were positively associated with obesity risk in Korean adolescents. Given the possibility of a prolonged COVID-19 pandemic, health professionals should be aware of and prepare for an increase in obesity prevalence, particularly among males, as well related unfavorable trends in related lifestyle factors. To address deteriorating healthy lifestyle factors, such as decreased sleep duration and physical activity, adolescent public health strategies are required.

Since 2015, the Institutional Review Board of the Korean Centers for Disease Control and Prevention has waived the need for KYRBS ethics approval under the Bioethics and Safety Act, and the data have been made available to the public for academic use. Prior to the study, all participants provided informed consent. This study was approved by the Inje University Sanggye Paik Hospital's Institutional Review Board (approval number: SGPAIK 2021-08-003).