INTRODUCTION

Metabolic syndrome (MetS) is characterized by a cluster of interrelated metabolic abnormalities, including abdominal obesity, dyslipidemia, hypertension, and hyperglycemia. It is widely recognized as a major risk factor for cardiovascular disease, type 2 diabetes, and premature mortality [1]. Over the past decade, the global prevalence of MetS has steadily increased, driven largely by population aging and the expanding burden of chronic diseases [2,3]. In Korea, approximately 1 in 4 adults (24.9%) currently meets the diagnostic criteria for MetS, and prevalence has continued to rise over the past 15 years (2007–2021) across both sexes and all age groups [2,4–6]. This sustained upward trend suggests that individual-level behavioral interventions alone may be insufficient and underscores the need for broader, population-based public health strategies [6].

Smoking has long been recognized as a key modifiable risk factor for metabolic and cardiovascular diseases [1,7–11]. Following the adoption of the World Health Organization’s Framework Convention on Tobacco Control in 2005, many countries implemented comprehensive tobacco control measures, including cigarette tax increases, expansion of smoke-free zones, and pictorial health warnings [12–14]. These policies primarily targeted conventional combustible cigarettes and contributed to a gradual global decline in smoking prevalence, from 22.8% in 2007 to 17.0% in 2017, according to World Health Organization estimates [12,13]. However, alongside these tobacco control efforts, novel nicotine delivery systems (NDS) rapidly expanded, including electronic cigarettes (ECs; nicotine-containing liquid-type e-cigarettes) and heated tobacco products (HTPs; heat-not-burn devices). ECs, developed by Hon Lik in China in 2003 and commercialized in 2004 [15], diffused quickly worldwide, fueled by marketing claims positioning them as “safer alternatives” to cigarettes or as “effective smoking cessation aids” [16–18].

Korea has mirrored these global patterns in tobacco control and market evolution. After the 2015 cigarette tax increase and the introduction of smoke-free zones and pictorial warnings, the national smoking rate declined steadily [6,19]. Nevertheless, despite this decline, cigarette sales increased slightly from 3.656 billion packs in 2020 to 3.714 billion packs in 2022, while the market share of HTPs rose from 10.6% to 14.8% during the same period [6,20]. This divergence between declining smoking prevalence and increasing cigarette sales, together with the expanding market share of HTPs, suggests that overall nicotine consumption may not have decreased substantially. Instead, the growing uptake of EC/HTP use may reflect compensatory smoking behaviors and an increase in dual use [6,21,22]. Against this backdrop of rapidly diversifying tobacco use patterns, concerns have emerged regarding the cardiometabolic consequences of EC/HTP use and dual use.

Although EC/HTP products were initially promoted as harm-reduction devices, accumulating toxicological and epidemiological evidence increasingly challenges this perception [9,23–26]. Toxicological studies indicate that EC aerosols contain nicotine, volatile organic compounds, heavy metals, and fine particulate matter capable of inducing oxidative stress, systemic inflammation, and endothelial dysfunction—pathways closely linked to metabolic dysregulation [6,9,23,27]. Moreover, the long-term health effects of solvents, flavoring agents, and other chemical additives commonly used in ECs remain incompletely characterized [9,23,27]. Consistent with these mechanistic concerns, recent population-based studies have reported associations between EC/HTP use or dual use and MetS, as well as other adverse cardiometabolic outcomes, including prediabetes, diabetes, and cardiovascular disease [8,10,22,24,28–31].

These shifts in tobacco use behaviors are particularly pronounced among younger adults, who increasingly engage in EC/HTP use or dual use, whereas conventional cigarette smoking remains more prevalent among middle-aged and older populations [8,19,32,33]. Such generational differences may translate into distinct metabolic health trajectories, as lifestyle factors, stress responses, and health priorities vary across life stages [16]. Compounding these trends, the coronavirus disease 2019 (COVID-19) pandemic substantially altered health-related behaviors, including tobacco use, physical activity, alcohol consumption, and stress, all of which are closely linked to metabolic risk [16–18]. Despite these converging influences, limited research has examined how pandemic-era changes in tobacco use behaviors, particularly the expansion of ECs and HTPs, have affected the risk of MetS.

Nevertheless, tobacco control policies in Korea remain largely centered on conventional combustible cigarettes, and most prior epidemiologic studies have similarly focused on the health risks of cigarette smoking [6]. Given the rapid diversification of tobacco product use behaviors, reliance on binary smoking classifications may obscure meaningful differences in health risk. Although EC/HTP use is widely perceived as a relatively safer alternative to cigarette smoking, empirical evidence regarding their metabolic health effects remains limited and inconsistent [8,23].

Therefore, this study used nationally representative data from the 2020–2022 Korea National Health and Nutrition Examination Survey (KNHANES), a period coinciding with the COVID-19 pandemic during which both lifestyle and tobacco use behaviors underwent substantial change. The analysis examined associations between multiple tobacco use behaviors —dual use, EC/HTP use, cigarette-only use, past smoking, and non-smoking—and the likelihood of MetS among Korean adults. In addition, age-stratified analyses were conducted to assess whether these associations differed across life stages during the pandemic period.

METHODS

This study was a cross-sectional analysis using data from the KNHANES, conducted by the Korea Disease Control and Prevention Agency between 2020 and 2022. The KNHANES employs a complex, stratified, multistage probability sampling design to generate a nationally representative sample of the Korean population. Sampling weights provided by the KNHANES were applied to all analyses to account for unequal probabilities of selection and to ensure population-level representativeness.

The study period (2020–2022) was selected to capture behavioral changes occurring during the COVID-19 pandemic, which may have influenced both tobacco use behaviors and metabolic risk factors. Of the 20 714 individuals who participated during this period, 3307 participants younger than 19 years and 2048 participants with missing data on key variables were excluded. The final analytic sample therefore consisted of 15 359 adults. Variables analyzed in this study encompassed socio-demographic characteristics, health behaviors, and health examination data. Socio-demographic information included household composition, household income, education level, employment status, and physical activity. Health behaviors—including tobacco product use, alcohol consumption, and mental health status—were assessed using self-administered questionnaires. The health examination component included anthropometric measurements, blood pressure, pulse rate, and body composition assessments.

Variables

The primary exposure variable of interest was tobacco use behavior, categorized into 5 mutually exclusive groups: dual users (concurrent use of combustible cigarette smoking and either ECs or HTPs), EC/HTP users (use of ECs or HTPs without current combustible cigarette smoking), cigarette-only users, ex-smokers, and non-smokers. Cigarette-only users were defined as individuals who responded “daily” or “occasionally” to the question, “Do you currently smoke conventional cigarettes?” Ex-smokers were those who answered, “I used to smoke, but I do not smoke now,” and non-smokers were defined as individuals who answered, “I have never smoked.” EC/HTP users were defined as individuals who reported either “daily” or “occasional” use of HTPs in response to the question, “Do you currently use HTPs?”, or who answered “yes” to the question, “Have you used nicotine-containing liquid-type e-cigarettes in the past month?”, and who reported no current combustible cigarette smoking. For analytic purposes, use of either ECs or HTPs was classified as EC/HTP use, consistent with the KNHANES survey structure and previously published operational definitions. Dual users were defined as individuals who reported both current combustible cigarette smoking and EC/HTP use.

The outcome variable was MetS, defined according to the National Cholesterol Education Program Adult Treatment Panel III (NCEP ATP III) criteria, with waist circumference thresholds adapted to recommendations from the Korean Society for the Study of Obesity (2022) [10]. MetS was defined as the presence of 3 or more of the following 5 components: abdominal obesity, elevated blood pressure, elevated fasting glucose, hypertriglyceridemia, and reduced high-density lipoprotein cholesterol (HDL-C). Abdominal obesity was defined as a waist circumference ≥90 cm for male and ≥85 cm for female. Elevated blood pressure was defined as systolic blood pressure ≥130 mmHg, diastolic blood pressure ≥85 mmHg, or current use of antihypertensive medications. Elevated fasting glucose was defined as a fasting plasma glucose level ≥100 mg/dL or current use of antidiabetic medications. Hypertriglyceridemia was defined as a triglyceride level ≥150 mg/dL or current use of lipid-lowering medications. Low HDL-C was defined as <40 mg/dL for male and <50 mg/dL for female.

Participant characteristics included sex (male or female), age (19–29, 30–39, 40–49, 50–59, and ≥60 years), education level (elementary school or lower, middle school, high school, and college or higher), household income (quartiles Q1–Q4, with Q1 indicating the lowest and Q4 the highest income level), and survey year (2020–2022). Health behavior variables included high-risk drinking (defined as ≥7 drinks per occasion for male or ≥5 drinks for female, at least twice per week), muscle-strengthening exercise (yes/no; ≥2 days per week), and perceived stress (classified as high or low based on self-reported daily stress levels). Occupational status was categorized as white collar, blue collar, or unemployed, and residential region was classified as capital area, metropolitan city, or non-metropolitan province.

Statistical Analysis

All analyses incorporated sampling weights provided by the KNHANES to account for the complex survey design. Weighted frequencies and percentages were calculated, and differences in participant characteristics across groups were examined using the Rao–Scott chi-square test, which adjusts standard chi-square statistics for complex sampling.

Associations between tobacco product use behaviors and MetS were evaluated using survey-weighted logistic regression models that accounted for the complex sampling design. Non-smokers served as the reference group. Multivariable models were adjusted for sex, age group, survey year, household income quartile, education level, occupation type, residential region, high-risk alcohol consumption, muscle-strengthening exercise, and perceived stress level. Adjusted odds ratios (ORs) and 95% confidence intervals (CIs) were estimated.

Subgroup analyses were conducted by survey year and age group to assess whether associations between tobacco product use behaviors and MetS differed across these strata. In addition, temporal changes in tobacco product use behaviors from 2020 to 2022 were examined to describe behavioral trends during the COVID-19 period.

All statistical analyses were performed using SAS version 9.4 (SAS Institute Inc., Cary, NC, USA), and a 2-sided p-value <0.05 was considered statistically significant.

Ethics Statement

This study used publicly available data from KNHANES. All KNHANES data were anonymized and de-identified prior to release. Therefore, ethical approval and informed consent were not required.

RESULTS

A total of 15 359 adults were included in the analysis, of whom 5906 (38.5%) met the criteria for MetS and 9453 (61.5%) did not (Table 1). Within the MetS group, 30.7% were dual users, 28.8% were EC/HTP users, and 41.6% were cigarette-only users. In contrast, the corresponding proportions in the non-MetS group were 69.3%, 71.2%, and 58.4%, respectively. When stratified by health behaviors, the prevalence of MetS was 43.0% among high-risk drinkers, 36.5% among participants who did not engage in muscle-strengthening activities, and 32.8% among those reporting high perceived stress. With respect to socio-demographic characteristics, 39.5% of males had MetS, and prevalence increased markedly with age, from 8.2% among participants aged 19–29 years to 57.7% among those aged ≥60 years. The prevalence of MetS was also higher among individuals in the lowest household income quartile (Q1; 48.2%), those with an elementary school education or lower (62.9%), and blue-collar workers (42.4%).

Table 2 presents the associations between tobacco use behaviors and MetS. Compared with non-smokers, dual users showed a significantly higher likelihood of MetS (OR, 1.48; 95% CI, 1.12 to 1.94; p=0.006), followed by cigarette-only users (OR, 1.19; 95% CI, 1.01 to 1.41; p=0.034). In contrast, EC/HTP users (OR, 1.21; 95% CI, 0.91 to 1.61; p=0.187) and ex-smokers (OR, 1.11; 95% CI, 0.97 to 1.26; p=0.141) did not demonstrate statistically significant associations with MetS.

Among health behavior covariates, high-risk drinking and high perceived stress were positively associated with MetS, whereas participation in muscle-strengthening exercise was inversely associated with MetS. Age also showed a strong positive association with MetS prevalence. All estimates were adjusted for the demographic and behavioral covariates described in the Methods section.

Figure 1 illustrates changes in the distribution of tobacco use behaviors from 2020 to 2022. The proportion of dual users remained relatively stable across survey years, whereas the proportion of EC/HTP users increased from 10.9% in 2020 to 16.8% in 2022. Conversely, the proportion of cigarette-only users declined gradually over the same period. Overall, the distribution of tobacco use behaviors differed significantly by survey year (p=0.044).

Table 3 shows the associations between tobacco use behaviors and MetS stratified by survey year (2020–2022). Dual users exhibited positive associations with MetS in all 3 survey years, with the strongest association observed in 2020 (OR, 1.54; 95% CI, 1.02 to 2.32; p=0.042), followed by slightly attenuated associations in 2021 and 2022. Cigarette-only users also demonstrated consistently positive associations with MetS across survey years. Among health behavior covariates, high-risk drinking and high perceived stress were positively associated with MetS, whereas engagement in muscle-strengthening exercise showed an inverse association.

Table 4 presents age-stratified associations between tobacco use behaviors and MetS. Dual users showed clear positive associations with MetS among adults aged 19–39 years (19–29 years: OR, 1.97; 95% CI, 1.03 to 3.77; p=0.040; 30–39 years: OR, 1.93; 95% CI, 1.11 to 3.35; p=0.020). Among cigarette-only users, a significant association was observed in the 50–59-year age group (OR, 1.40; 95% CI, 1.01 to 1.95; p=0.045). Ex-smokers demonstrated consistent positive associations with MetS among adults aged ≥40 years, whereas EC/HTP users did not exhibit clear or consistent age-specific patterns.

DISCUSSION

This study examined the association between various tobacco product use behaviors and MetS among Korean adults using nationally representative data from the 2020–2022 KNHANES. The findings indicate that tobacco product use, particularly dual use and cigarette-only smoking, was associated with a higher likelihood of MetS compared with non-smoking. Notably, age-stratified analyses showed that dual users aged 19–39 years exhibited the highest metabolic risk, suggesting that dual use may represent a particularly important metabolic risk behavior and a potential early indicator of future metabolic disorders among younger adults.

In this study, dual users had approximately 1.5-fold higher odds of MetS compared with non-smokers, consistent with findings from prior national and international studies [10,24]. Analyses of United States National Health and Nutrition Examination Survey data reported that dual users had approximately 1.35 times higher prevalence of MetS than non-smokers [24], while a Korean male cohort study observed nearly 2.8-fold higher odds of MetS among dual users [10]. Differences in the magnitude of these associations across studies may reflect variations in population characteristics, survey periods, analytic approaches, and the extent of market penetration of newer tobacco products. In Korea, the rapid increase in the market share of HTPs from 2.2% in 2017 to 14.8% in 2022 [20] may have contributed to distinct exposure patterns and cumulative nicotine intake among dual users, potentially amplifying metabolic risk [9,24].

This study also observed an elevated likelihood of MetS among EC/HTP users, although the association did not reach statistical significance. Previous studies from the United States and Korea similarly reported higher MetS risk among EC/HTP users compared with non-smokers, with adjusted ORs of 1.53 and 1.27, respectively [8,24]. Inconsistencies in statistical significance across studies may be attributable to the relatively small proportion of EC/HTP users, heterogeneity in device type, duration and intensity of use, and differences in survey timing and product composition. These findings underscore the need for further investigations using larger and more heterogeneous populations to more precisely characterize the metabolic health effects of EC/HTP use. In this context, recent evidence syntheses, including a World Health Organization policy brief and a systematic review and meta-analysis of human biomarkers and adverse events, indicate that evidence regarding the health impacts of HTPs remains mixed, reinforcing the need for cautious interpretation of harm-reduction claims and for independent long-term studies [25,26].

Although a definitive causal relationship between smoking and MetS has not been established, several plausible biological mechanisms have been proposed to explain their association. Smoking activates the sympathetic nervous system and induces oxidative stress and systemic inflammation, which can contribute to metabolic abnormalities such as insulin resistance, abdominal obesity, and hypertriglyceridemia. Nicotine further elevates blood pressure and heart rate and impairs endothelial function, thereby promoting hypertension and dyslipidemia, both key components of MetS [8,10]. Importantly, EC/HTP users may inhale more deeply and more frequently than conventional cigarette smokers, potentially delivering higher doses of nicotine per use episode [9,17]. In addition, EC aerosols contain nicotine, heavy metals, and volatile organic compounds that may disrupt lipid metabolism and exacerbate insulin resistance [11,24]. Taken together, these mechanisms suggest that dual users may be especially vulnerable due to concurrent exposure to toxicants from both combustible cigarettes and EC/HTPs, potentially intensifying inflammatory and oxidative pathways.

The observed vulnerability associated with tobacco use behaviors appears to differ across age groups. In age-stratified analyses, the highest metabolic risk was observed among dual users aged 19–39 years, whereas cigarette-only users showed positive associations primarily in their 50s and ex-smokers among adults aged 40 years and older. Dual users in their 20s and 30s demonstrated approximately a 1.5-fold higher likelihood of MetS compared with non-smokers, consistent with prior evidence indicating heightened metabolic vulnerability among younger dual users. Recent studies and World Health Organization reports have documented the increasing prevalence of EC/HTP use and dual product use among young adults [12,13,29], which aligns with the patterns observed in this study. Younger adults may be particularly susceptible to dual use because of higher sensitivity to nicotine reward, greater social acceptability of novel tobacco products, and marketing strategies that emphasize technological appeal [9,16,33]. This distinct age-related pattern of tobacco use underscores the need for age-specific smoking cessation interventions to prevent MetS [8,21,24]. Therefore, in the context of an evolving tobacco landscape, dual and EC/HTP users should be prioritized as key target populations for tobacco control efforts.

This study suggests that the risk of MetS may vary according to tobacco use behavior. However, MetS is influenced by multiple lifestyle and psychosocial factors, and the study period coincided with the COVID-19 pandemic, during which substantial changes in physical activity, dietary patterns, stress levels, and tobacco use behaviors occurred. Accordingly, the observed associations may partially reflect pandemic-related lifestyle changes rather than the independent effects of tobacco use alone. Further studies using post-pandemic data are therefore needed to disentangle these effects. Nevertheless, the present findings underscore the importance of addressing misconceptions that ECs and HTPs represent “less harmful alternatives” and support the need for comprehensive public health strategies that integrate education, prevention, and regulatory interventions.

This study has several strengths. First, it used a large, nationally representative dataset, enhancing the generalizability of the findings. Second, by incorporating multiple categories of tobacco product use, the study moved beyond binary smoking classifications and provided a more nuanced assessment of tobacco-related metabolic risk. Third, the inclusion of data collected during the COVID-19 pandemic and the application of age-stratified analyses offered insight into how metabolic risk associated with tobacco use may differ across life stages during a period of widespread behavioral change.

Several limitations should also be considered. Because of the cross-sectional design, causal relationships between tobacco use behaviors and the development of MetS cannot be inferred. In addition, detailed information on EC/HTP characteristics, including device type, duration and frequency of use, and nicotine concentration, was unavailable. Longitudinal or prospective cohort studies incorporating these factors are needed to clarify temporal relationships and underlying mechanisms. Furthermore, as the study period overlapped with the COVID-19 pandemic, unmeasured behavioral and psychosocial changes may have influenced the observed associations. Future studies using data from non-pandemic periods are therefore warranted to validate these findings.

This study examined the association between tobacco product use behaviors and MetS among Korean adults during the COVID-19 period using nationally representative data. The findings showed that all categories of tobacco product use were associated with a higher likelihood of MetS compared with non-smokers, with the strongest associations observed among dual users. These results suggest that dual use may contribute disproportionately to the burden of metabolic disorders. Notably, this pattern was particularly evident among adults in their 20s and 30s, highlighting distinct age-related differences in tobacco use behaviors and metabolic risk.

Data Availability

Notes

Conflict of Interest

The authors have no conflicts of interest associated with the material presented in this paper.

Funding

None.

Acknowledgements

None.

Author Contributions

Both authors contributed equally to conceiving the study, analyzing the data, and writing this paper.

Figure 1

Temporal variations in tobacco use behaviors (2020–2022). This figure presents the weighted distribution (%) of tobacco use behaviors—dual users, EC/HTP users, and cigarette-only users—among total tobacco users for each survey year (2020–2022). EC/HTP, electronic cigarettes/heated tobacco products. A chi-square test indicated a difference across survey years (p=0.044).

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Figure & Data

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