Introduction

The COVID-19 outbreak has had considerable global consequences. Over 200 million confirmed cases and 4 million deaths were reported within 18 months of the onset of the pandemic1. South Korea had the first case confirmed on January 20, 2020. The virus quickly spread, resulting in 6284 confirmed cases and 42 deaths in less than 1 month2. Thus, the South Korean government implemented rigorous social distancing measures from February 29, 2020, to mid-2022 to prevent the spread of the outbreak3. These measures involve contact tracing, quarantine, social distancing, and remote working, which cause significant lifestyle changes.

Owing to social distancing, people spent more time indoors and prolonged sedentary behavior compared to pre-pandemic times4,5. According to a recent systematic review, 64 studies reported an increase in sedentary behavior during the respective COVID-19 pandemic closures in various populations, including children and patients with a variety of medical conditions6. Sedentary behavior is associated with a range of adverse health outcomes, including changes in the circulatory levels of sex hormones and considerable weight gain5,7,8. Obesity has emerged as a leading contributor to poor health9, with significant impacts on the quality of life, and an increased risk of serious medical complications, such as cardiovascular disease, musculoskeletal disorders, and various types of cancers10.

All 40 studies from different countries included in a systematic review reported changes in body weight during the pandemic, and most showed weight gain among participants11. The obesity rate among South Korean adults (Body Mass Index (BMI) ≥ 25) steadily increased from approximately 31% in 2005 to 34–35% in 2016, but remained unchanged until 2019. After the COVID-19 outbreak, however, the obesity rate of Korea rose to 39% in 2020 according to the Korea Disease Control and Prevention Agency (KDCA)12. Previous studies have identified a sedentary lifestyle as a risk factor for obesity13. The increasing prevalence of sedentary behavior during the COVID-19 pandemic may have contributed to the observed changes in obesity rates in South Korea. To the best of our knowledge, no previous investigations have explored the impact of sedentary time on obesity while considering the effects of the pandemic. Therefore, this study aimed to investigate the synergistic effects of the COVID-19 pandemic and sedentary lifestyle on obesity in South Korean adults.

Methods

Study design and participants

The study samples were obtained from the Korea National Health and Nutrition Examination Survey (KNHANES) from 2018 to 2020. This survey provides comprehensive data on health status and behaviors, such as smoking status, dietary patterns, and daily sedentary time. The KNHANES is conducted by the national institution KDCA using a complex, stratified, multistage, probability-cluster survey with rolling sampling designs to represent the non-institutionalized civilian population in South Korea14. We certify that all applicable institutional and governmental regulations concerning the ethical use of human volunteers were followed during the course of this research. All procedures were performed in accordance with relevant guidelines. Further details of the surveys can be found by consulting items in the reference section15.

Of the 23,461 individuals who participated in the surveys, those aged < 19 years were excluded to focus on adults only (n = 19,228). No observation was excluded because of missing information on BMI and sedentary time. Finally, 9476 adults were included in the analysis after excluding observations with missing information on covariates, including educational level, marital status, household income, occupational classification, subjective health status, stress level, alcohol consumption, regular exercise, breakfast frequency, and eating out frequency.

Variables

As the first COVID-19 patient was diagnosed on January 20, 2020, and social distancing was implemented from the beginning of 2020, this study examined the change in the obesity rate of the participants between the years 2018–2019 (before COVID-19) and 2021 (during COVID-19)16.

Studies have shown that higher sedentary time increases the risk of cardiovascular disease, cancer, obesity, and type 2 diabetes17. Considering that > 20% of Korean adults were sedentary for > 12 h, sedentary lifestyle was categorized as yes (≥ 12 h) and no (< 12 h)7. The sedentary time was self-reported in response to the question: "On average, how long did you sit during the past 7 days?".

Demographic characteristics were participants' age (19–39, 40–59, ≥ 60) and sex. Socioeconomic factors included educational level (middle school or less, high school, and college or higher level), marital status (married, separated or divorced, or never married), household income quartiles (1: low; 2: lower middle; 3: upper middle; 4: high), and occupational classification (white collar, pink collar, blue collar, and none).

Health-related characteristics included self-reported health status (high, medium, or low), stress level (high, medium, or low), alcohol consumption (heavy or light), regular exercise (yes or no), breakfast frequency (> 5 times a week, 1–4 times a week, and less than once a week), and frequency of eating out (> 5 times a week, 1–4 times a week, and less than once a week).

Those who responded ‘very high’ or ‘high’ to the question ‘How do you perceive your health?’ were categorized as ‘high,’ while those who responded ‘medium’ were as ‘medium’ and ‘low’ or ‘very low’ as ‘low’ respectively. Men who drank more than seven cups of alcohol and women who drank more than five at least twice a week were categorized as ‘heavy drinkers’ according to the KNHNES guidelines18,19. Participants who engaged in 150 min of moderate or 75 min of vigorous physical activity per week were also classified as ‘yes’ to regular physical activity20. Finally, obesity was defined as BMI ≥ = 25 kg/m2 according to the Korean Society for the Study of Obesity (KSSO) guidelines21.

Statistical analysis

Chi-square tests were used to examine the differences in the general characteristics of the study population, while multiple logistic regression analysis to examine whether the risk of obesity depended on the main independent variables, the COVID-19 pandemic, or on a sedentary lifestyle after adjusting for all other potential confounding variables including age, gender, educational level, marital status, household income level, occupational classification, self-reported health status, stress level, alcohol consumption, regular exercise, breakfast frequency, and eating out frequency.

We also examined whether the combination of high sedentary time and COVID-19 is associated with a greater risk of obesity than the sum of their independent effects. The synergistic effect was assessed using three indices of additive interactions: relative excess risk due to the interaction (RERI), proportion attributable to the interaction (AP), and synergy index (SI)22.

The RERI (relative excess risk index) is defined as the additional risk due to interaction, calculated as the difference between the addition of odds ratios (ORs) due to a sedentary lifestyle before and after the COVID-19 pandemic: RERI = (OR11-OR10)-(OR01-1), where the first parenthesis measures the addition of ORs by a sedentary lifestyle (second subscript = 1) after the COVID-19 pandemic (first subscript = 1), while the second does so before the COVID-19 pandemic (first subscript = 0)23. AP (attributable proportion) is the proportion of obesity risk due to the interaction between individuals with both exposures (having a sedentary lifestyle after the COVID-19 pandemic): AP = RERI/OR11. Finally, SI (synergy index) measures the ratio of the excess risk from both exposures when there is an interaction (numerator) to the sum of the excess risks from both exposures when there is no interaction (denominator): SI = (OR11–1)/[(OR01–1) + (OR10–1)]).

P < 0.05 was considered statistically significant, and the results were reported as odds ratios (ORs) with 95% confidence intervals (CIs). All data analyses were performed using the SAS 9.4 software (version 9.4; SAS Institute Inc., Cary, NC, USA).

Results

Table 1 shows the general characteristics of the study population and their differences before and during the COVID-19 pandemic. The data included 6822 participants before and 2654 participants during the pandemic, out of a total of 9476 participants.

Table 1 General characteristics of study population (n = 9476).
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Of all the participants, 3351 (35.4%) were classified as obese and 6125 (64.6%) as normal according to their BMI. The obesity rate has increased from 34.1 to 38.6% during the pandemic. The proportion of participants with a sedentary lifestyle was 42.1% (3992), which increased from 40.9 to 45.2% during the pandemic. The Chi-square test showed that these differences were statistically significant.

Among all other confounding factors, self-reported health status decreased while stress levels increased during the pandemic. Interestingly, both the proportions of 'eating breakfast frequently' and 'eating out frequently' decreased.

Table 2 shows the direct association of obesity risk with both the COVID-19 pandemic and a sedentary lifestyle, after adjusting for all other confounding variables. First, the risk of obesity increased significantly during the pandemic (OR: 1.16, 95% CI 1.04–1.30). Second, it increased independently with a sedentary lifestyle (OR: 1.17, 95% CI 1.04–1.31).

Table 2 Factors associated with obesity.
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Results for other factors showed that heavy alcohol consumption and lower educational level were positively associated with obesity risk (OR: 1.28, 95% CI 1.11–1.47 for heavy alcohol consumption; OR: 1.45, 95% CI 1.20–1.76 for middle school or less education). Individuals reporting better health status had a lower risk of obesity (OR: 0.57, 95% CI 0.49–0.67 for high health status; OR: 0.63, 95% CI 0.54–0.72 for medium health status). Men were at a higher risk of developing obesity (OR: 2.29, 95% CI 2.05–2.56).

Table 3 shows the results on whether the combination of high sedentary time and COVID-19 had a greater risk of obesity than the sum of their independent effects. The participants were divided into four groups based on their sedentary lifestyle and COVID-19 exposure (first subscript = 1 during the COVID-19 pandemic, 0 otherwise; second subscript = 1 for sedentary lifestyle, 0 otherwise).

Table 3 Synergistic effect of Covid-19 and sedentary life style on obesity.
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The corresponding OR were 1 (OR00) for the comparison group "before the pandemic and no sedentary lifestyle,” 1.14 (OR01) for the group "before the pandemic and sedentary lifestyle,” 1.19 (OR10) for the group "during the pandemic and no sedentary lifestyle" and 1.47 (OR11) for the group "during the pandemic and sedentary lifestyle.” The results were adjusted for all of the confounding variables including age, gender, educational level, marital status, household income level, occupational classification, self-reported health status, stress level, alcohol consumption, regular exercise, breakfast frequency, eating out frequency. All results were statistically significant.

The additional risk due to the interaction of two risk factors, calculated as the difference between the addition of the ORs due to a sedentary lifestyle before and after the COVID-19 pandemic: RERI = (OR11-OR10)-(OR01-1), was estimated to be 0.17 (95% CI − 0.10 to 0.45). The proportion of disease due to the interaction between individuals with both exposures, i.e., AP = RERI/OR11, was estimated to be 12% (AP: 0.12, 95% CI − 0.16 to 0.39). Finally, the calculated measure (SI) indicating the impact of the interaction between sedentary lifestyle and the COVID-19 pandemic on obesity risk is 1.57 (95% CI 1.30–1.85). This ratio signifies that the combined risk of obesity due to both factors interacting is 1.57 times higher than the sum of their individual risks in the absence of interaction.

Discussion

This study showed that both COVID-19 and sedentary lifestyle were risk factors for obesity, even after adjusting for other confounding variables. Sedentary behavior adversely affected health through various mechanisms. It reduces lipoprotein lipase activity, muscle glucose level, protein transporter activities, insulin sensitivity, and vascular function by activating the sympathetic nervous system5,24. According to a previous study examining the sedentary lifestyle effect on body weight in relation to dietary habits, lower levels of physical activity had an even stronger association with the risk of obesity than dietary intake25.

During the COVID-19 pandemic, significantly higher average BMI and obesity prevalence rates were observed in U.S. adults26. The prevalence of obesity in South Korean adults in 2020 also increased by 4.2 percentage point compared with the average in 2017–201927. More importantly, the change in the prevalence of obesity during the pandemic was particularly evident among Korean adults aged 20–34 years, which increased dramatically by 10 percentage point28. This finding supports the motivation of this study, as most men in their 30 s were workers who were more likely to be affected by COVID-19 in terms of their sedentary lifestyles.

The current study showed that those who were forced to adopt a sedentary lifestyle during the pandemic had the highest risk of obesity compared to those who were exposed to either of the two individual risks. A previous study has suggested that a long period of sedentary time rather than several short periods of sedentary time of the same duration may be a greater risk factor for obesity8. Social distancing during the COVID-19 pandemic must have prolonged the sedentary time of South Korean adults, further supporting our finding that the combination of high sedentary time and COVID-19 is a greater risk factor for obesity than the sum of their independent effects. Furthermore, another study has illustrated that while physical inactivity and sedentary behaviors should be considered as separate entities with distinctive determinants, there exist synergistic negative health effects of them29. Considering the decreased physical activity during the pandemic30, these results also bolster our finding that there was a synergistic negative health impact of COVID-19 and a sedentary lifestyle.

This study showed a significant association between a sedentary lifestyle and obesity, deepened by the COVID-19 pandemic, even after accounting for significant confounders from the 2018–2020 KNHANES data. Extensive data on various lifestyle factors, such as breakfast and eating out frequencies, were included to provide reliable evidence for future obesity-related policies and programs31.

This study had limitations. First, the cross-sectional design of the data prevented us from demonstrating a thorough causal relationship between prolonged sedentary time and obesity during the COVID-19 pandemic. Second, there may have been measurement errors in self-reported sedentary time, which could have biased our results towards an insignificant effect. Despite these limitations, our study has several strengths. First, the Korea National Health and Nutrition Examination Survey (KNHANES) is conducted by a national institution using random cluster sampling, which ensures statistically reliable and representative data compared with surveys conducted by private institutions. In addition, many covariates, including age, gender, education level, marital status, household income level, occupation, stress level, subjective health status, alcohol consumption, regular physical exercise, breakfast and eating out frequency, were included to reduce potential confounding effects.

In conclusion, long-term obesity due to a sedentary lifestyle is an important public health concern. The results showed that both COVID-19 and sedentary lifestyle were significant risk factors for obesity in South Korean adults. The combined synergistic effects of the two risk factors were significant to be overlooked by adding their individual effects on obesity. Therefore, obesity prevention policies and programs need to implement more significant changes in health behaviors during the pandemic, while carefully considering the affected population and the side effects of social distancing policies.

Ethics statement

Ethical approval for the KNHANES data analysis study was obtained from the Institutional Review Board at the Seoul National University (IRB No. E2211/003-009).

Informed consent

Because the KNHANES data is secondary data that does not contain personal identifiers, the need for written consent was waived by the Institutional Review Board at the Seoul National University (IRB No. E2211/003-009).