Since its initial outbreak in Wuhan, Hubei Province, China, the Coronavirus Disease-19 (COVID-19) pandemic has progressed at an unprecedented pace, spreading to 234 countries and territories [1], with the first case of COVID-19 occurring in Daegu, South Korea, in February 2020 [2]. On March 11, 2020, the World Health Organization (WHO) declared COVID-19 a global pandemic [3]. The spread of COVID-19 is still ongoing worldwide. Liu et al. [4] analyzed the demographic spread pattern of COVID-19 by age and gender in Hong Kong. They reported differences in the spread pattern by the epidemic period and population characteristics. In Korea, COVID-19 infections were centered in the capital regions, and it is believed that regional characteristics, such as population structure, may have facilitated the influx of COVID-19, as the capital region is a region with high population density and high connectivity to the surrounding commercial and business areas [5]. On the other hand, as the number of COVID-19 outbreaks in the capital region has increased, it was reported that the center of COVID-19 spread diversified outside the capital region and expanded to local cities over time [6], which is expected due to the diversification of infection routes and increase in spreading among cities as the COVID-19 pandemic prolonged.

Various policies have been implemented worldwide to prevent the spread of COVID-19 [7], and the restricted daily routines significantly impact people's lives, causing radical changes in their lifestyles. Studies reporting changes in daily routines after COVID-19 reported problems, such as decreased physical activity, increased sleep time, and weight gain, due to prolonged social isolation [8, 9]. A US study reported changes in food consumption behaviors and eating habits after COVID-19, including a 34.5% increase in frozen food purchases, 31.8% increase in snack purchases, 28% increase in juice and soda purchases, and 55.2% increase in food worries [10]. In contrast, a Korean study reported a 35.0% increase in takeout orders, 34.4% increase in delivery, 26.4% increase in cooking, and 22.6% increase in HMR foods after COVID-19 [11].

On the other hand, diet is one of the basic factors related to health, and many causes of chronic diseases are related to dietary habits [12]. Hence, unfavorable changes in dietary habits after COVID-19 may lead to changes in the prevalence of chronic diseases [13]. A Korean study analyzing the data from the 2020 Korea National Health and Nutrition Examination Survey reported a significant increase in chronic diseases in 2020 compared to before COVID-19, with a 6.0% increase in the obesity prevalence, 1.8% increase in hypertension prevalence, 1.9% increase in diabetes prevalence, and 2.8% increase in hyperlipidemia prevalence [14].

The world is currently fighting COVID-19, and many detrimental health effects have been reported during the pandemic. In Korea, a large body of literature has been published comparing before and after COVID-19 dietary habits, chronic disease prevalence, and mental health prevalence. On the other hand, there is a lack of literature on before and after COVID-19 comparisons across regions. In addition, due to the difference in the scope of the COVID-19 outbreak between metropolitan and non-metropolitan areas, it is expected that the after-COVID-19 dietary habits and food consumption behaviors will differ across regions. Therefore, this study compared the before and after COVID-19 dietary factors and the prevalence of chronic diseases across regions in preparation for the post-COVID-19 era.

This study compared dietary factors and the prevalence of chronic diseases and mental health by region before and after COVID-19. Compared to before COVID-19, the rate of eating out in the Seoul metropolitan area and Provinces after COVID-19 decreased, and the frequency of eating out with household members in the Seoul metropolitan area increased.

In Korea, social distancing policies have been implemented to prevent COVID-19 infections, including limiting private gatherings, limiting the number of people, restricting events and gatherings, and restricting the business hours of restaurants and other public facilities [5]. Chang et al. [23] reported that people also voluntarily reduced their outside activities for reasons, such as fear of COVID-19 infection and social criticism, resulting in fewer private outings and more time spent at home, leading to a preference for home-cooked meals rather than eating out, and more frequent consumption of delivered food rather than cooking home-cooked meals, which may have contributed to the decrease in eating out. On the other hand, the frequency of eating out with household members in the Seoul metropolitan area increased. Emergency disaster relief funds refer to the support the state or local governments’ provide to stabilize people's livelihood and promote consumption in response to COVID-19 [24]. Hong [25] reported that consumption expenditure decreased by 16.3% in 2020 due to COVID-19, but it increased by 10.4% due to the emergency disaster assistance paid by the government or local governments from mid-May. Of these payments, 27.9% were used to replace the consumption that would have occurred without the payments, and approximately 72.1% of the payments generated new consumption. According to the Ministry of the Interior and Safety, 90.1% of households in South Korea received the COVID-19 emergency disaster benefit, and the most common industries where the emergency disaster benefit was used were “supermarket/grocery” at 26.3% and “restaurant” at 24.3% [26]. Kim et al. [27] reported that households purchased groceries and ate at restaurants, which increased to 27.0% after receiving the disaster assistance, suggesting that the impact of the disaster assistance may have contributed to the consumption of eating out.

The comparison of before and after COVID-19 delivery/takeout behaviors conducted in this study showed a decrease in individuals delivery/takeout use in the Provinces, with Chang et al. [23] reporting that the increase in delivery and home delivery meant that people avoided meeting with acquaintances due to high anxiety about contracting COVID-19. On the other hand, for those who reported using deliveries/takeout, the frequency of deliveries/takeout has increased in the Seoul metropolitan area, with Kim [29] reporting that 75.1% of respondents reported an increase in the amount of time they spent at home with their families during COVID-19. In addition, there was a 43.0% increase in grocery purchases and a 78.3% increase in money spent on food deliveries, in addition to maintaining interpersonal relationships centered on family [23]. Furthermore, when meeting up with friends after COVID-19, they reported an increase in the proportion of people ordering home-delivered food rather than eating out due to the anxiety about going out. These reasons may have contributed to the increased frequency of delivery/takeout with individuals or household members.

The cost of eating out and deliveries/takeout has been increasing since before COVID-19 because the price index for food items in the Consumer Price Index has been rising steadily to 98.254 in the fourth quarter of 2018, 99.413 in the fourth quarter of 2019, and 100.2 in the fourth quarter of 2020 (base: 2020 = 100) [28]. These results suggest that macro factors, such as rising inflation, may have contributed to the increase in individual food expenses.

A comparison of breakfast frequency and nutrition label utilization in this study also showed regional differences before and after COVID-19. The breakfast skipping rate, according to the 2020 National Health and Nutrition Examination Survey, was 28.9% in 2018, 31.3% in 2019, and 34.6% in 2020 [30], which is the same as the result of a steady increase in the breakfast skipping rate in this study. Yun and Oh [31] reported an increase in the breakfast skipping rate after the COVID-19 pandemic, which may be attributed to irregular eating due to restrictions on school attendance and work caused by COVID-19. On the other hand, it is unlikely that non-COVID-19 influences can be excluded entirely because the breakfast skipping rates have been increasing consistently since before COVID-19. As breakfast skipping can increase the risk of overweight and obesity, as well as the risk of type 2 diabetes [32, 33], it is necessary to monitor the breakfast skipping rates and develop measures, such as non-face-to-face nutrition education programs that can be conducted during the pandemic. In addition, this study showed a significant increase in the use of nutrition labels between 2018-2019 in all regions but no significant change in 2020 after COVID-19. Moreover, the use of nutrition labels, according to the 2020 National Health and Nutrition Examination Survey, was reported to be slightly lower than before: 32.8% in 2020 after COVID-19, compared to 30.3% in 2018 and 33.5% in 2019 before COVID-19 [30]. The increase in online purchases of processed foods during COVID-19 [34] and the tendency for shoppers to place more emphasis on taste and less on nutrition when purchasing processed foods online [35] may have contributed to the after-COVID-19 decline in nutrition label use. On the other hand, nutrition labeling is effective in promoting healthier food choices, and nutrition labeling has been reported to reduce the body mass index and prevent chronic diseases, such as diabetes, cancer, and cardiovascular disease [36], so specific measures should be taken to promote online nutrition labeling.

A comparison of chronic disease prevalence showed differences before and after COVID-19 by region, but slightly different results were obtained. Hence, caution is needed when interpreting the results. The prevalence of obesity increased from 34.6% in 2018 and 33.8% in 2019 before COVID-19 to 38.3% in 2020 after COVID-19, according to the 2020 National Health and Nutrition Examination Survey [30]. By contrast, the prevalence of obesity decreased in this study. Lee et al. [14] used data from the National Health and Nutrition Examination Survey to compare the prevalence changes before and after the COVID-19 outbreak, comparing 2017-2019 and 2020, and reported that the prevalence of obesity, particularly among men, increased in 2020 compared to before COVID-19. The data from Korea Community Health Survey used in this study were analyzed by excluding the data from 2019, where the weight was measured directly, as opposed to 2018 and 2020, where weight was self-reported, because of differences in the methodology of the anthropometric survey. Therefore, the prevalence rates in 2018 and 2020 were compared, and the possibility of error in interpreting the results cannot be excluded.

This study showed that the prevalence of hypertension in the Provinces continued to increase. According to the 2020 National Health and Nutrition Examination Survey, however, the prevalence of hypertension decreased from 23.1% in 2018 before COVID-19 to 22.2% in 2019. After COVID-19, it increased slightly to 22.9% in 2020 [32]. Lee et al. [14] reported a slight decrease in the prevalence of hypertension in 2019 compared to before COVID-19 (2017-2018) and an increase in the prevalence of hypertension in men in 2020 compared to before COVID-19 (2017-2019). This differed from the present results, which showed that the prevalence of hypertension in the province continued to increase. The Korea Community Health Survey data used in this study is a self-reported survey, which may have limitations in interpreting the results caused by possible errors in the response accuracy and measurement methods.

Similar to the present results in all regions, the prevalence of diabetes according to the 2020 Korea Health and Nutrition Examination Survey was 9.2% in 2018, 9.5% in 2019, and 10.7% in 2020 [30]. Prior studies reported an increased time spent at home during COVID-19, decreased physical activity, and increased sleep duration [8, 9]. In addition, the use of major meal delivery apps in Korea has increased since COVID-19 [37]. Most delivered food consisted of fast food such as pizza, hamburgers, chicken, and sweetened beverages [38]. Increased fast food consumption is associated with obesity and type 2 diabetes [39, 40]. Hence, nutrition education programs are needed to foster healthy eating habits during the pandemic. On the other hand, the prevalence of diabetes has continued to increase even before COVID-19. Therefore, it is difficult to attribute the increase entirely to COVID-19.

This study compared the mental health prevalence. High subjective stress levels increased in metropolitan areas. Major depressive disorder decreased in Metropolitan cities and Provinces, and sleep duration increased in all regions. Jeong et al. [41] compared the prevalence of mental health before and after the COVID-19 outbreak using data from the National Health and Nutrition Examination Survey, 2016-2019 and 2020. They reported a 40% increase in the prevalence of depression and a 30% increase in high stress among men. This was similar to the present results, in which the high stress level in the Seoul metropolitan area increased in 2020 compared to 2018-2019, before COVID-19. On the other hand, there was a difference from the results of this study, which showed a decrease in major depressive disorder in Metropolitan cities and Provinces before COVID-19. In August 2020, to prevent the COVID-19 pandemic in Seoul metropolitan area, Korea raised social distancing to level 2 only in the Metropolitan area. In September 2020, the social distancing measure limited to the Seoul metropolitan area was further strengthened to level 2.5 [42]. The survey period of the 2020 Korea Community Health Survey was from August 16, 2020, to October 31, 2020 [15]. The coincidence of the survey period with the pandemic may have led to higher stress levels due to COVID-19 among people living in Seoul metropolitan area. On the other hand, various stresses, such as specific life events and cognitive expectations, can affect depression [43]. As COVID-19 progresses for a long time, the 'corona blue' phenomenon of feeling depressed due to COVID-19 is spreading [44]. Nevertheless, this study showed a decrease in major depressive disorder in Metropolitan cities and Provinces. In this regard, Van et al. [45] reported that people might consider themselves to be less depressed compared to those who are more distressed. Hence, depression may be buffered by the energy and sense of togetherness that people feel when connected to a common event. Moreover, COVID-19 may have reduced the time pressure obligations, which may have benefitted mental health. On the other hand, the prevalence of major depressive disorder has been decreasing continuously since COVID-19. Therefore, it cannot be attributed conclusively to COVID-19, and caution should be taken when interpreting the results. In addition, the survey period of the 2020 National Health and Nutrition Examination Survey was conducted for about a year, from January to December 2020 [30]. Accordingly, the effects of COVID-19 may have been more pronounced than the 2020 Korea Community Health Survey, which was conducted for approximately two months, which may have contributed to the difference in results. The sleep duration increased after COVID-19 in all regions, and previous research by Yuan et al. [46] suggested that work-from-home and lockdown policies caused by COVID-19 increased the sleep duration. Another potential cause is that sleep may be affected by business hours restrictions, non-essential venue closures, and lockdowns.

The limitation of this study was that the Korea Community Health Survey and Consumer Behavior Survey for Foods data are independent data and must be interpreted independently of the research results. The data collection method may affect the measurement value, so the possibility of error cannot be excluded when interpreting the results. In addition, this study was analyzed by administrative division into Seoul metropolitan area, Metropolitan cities, and Provinces, which is not a detailed analysis by city, county, and district according to the concentration of COVID-19 cases. On the other hand, there were differences between regions, with 0.73% in the Metropolitan areas, 0.56% in Metropolitan cities, and 0.47% in Provinces responding “yes” to “experience of COVID-19 isolation or hospitalization” in Korea Community Health Survey data. A follow-up study should undergo a detailed analysis of the differences between regions before and after COVID-19 by selecting regions by city, county, or district according to the density of confirmed COVID-19 cases. Since the outbreak of COVID-19 in Korea in February 2020, it has been difficult to compare some dietary factors before and after COVID-19 and to observe the changes in chronic diseases based on the 2020 data. Furthermore, continuous observation and investigation through future follow-up studies will be needed.

Despite these limitations, this study is meaningful because it analyzed and considered the changes in dietary habits before and after COVID-19, chronic diseases, and mental health prevalence by region using the data from the Korea Community Health Survey and Consumer Behavior Survey for Foods. In the future, it will be necessary to reflect on the differences in dietary habits and changes in the prevalence of chronic diseases and mental health by region in policies aimed at narrowing the health gap between regions.

This paper provided evidence of regional health inequalities by analyzing the changes in diet, chronic disease, and mental health prevalence between regions before and after COVID-19. This study used the data from the Korea Community Health Survey (n = 686,708) and the Consumer Behavior Survey for Foods (n = 19,109) conducted in 2018-2020 among adults aged 19 and older. The participants were categorized by the administrative region into the Seoul metropolitan area, Metropolitan cities, and Provinces, with 2018-2019 defined as before COVID-19 and 2020 as after COVID-19. The diet, chronic diseases, and mental health problems were measured through self-reported questionnaires. Compared to before COVID-19, the rate of eating out for participants residing in the Seoul metropolitan area and Provinces after COVID-19 decreased. Moreover, the frequency of eating out with household members in the Seoul metropolitan area increased. On the other hand, delivery and takeout use rates decreased in the Provinces, and the frequency of delivery/takeout with individuals and household members increased significantly in the Seoul metropolitan area. After COVID-19, the prevalence of obesity decreased in all regions, and the prevalence of diabetes continuously increased in all regions. Hypertension prevalence increased significantly only in the Provinces. High subjective stress levels increased significantly in the Seoul metropolitan area, and the sleep duration significantly increased in all areas. Major depressive disorder was significantly reduced in the Metropolitan cities and Provinces. The regional differences in the prevalence of chronic diseases and mental disorders before and after the onset of COVID-19 were confirmed, along with changes in eating habits. The differences in dietary habits by region after COVID-19 were reflected in future policies to address the regional health inequality. In addition, a large-scale observational survey is needed to identify dietary factors before and after COVID-19 and the changes in the prevalence of chronic diseases and mental health.