The Association Between Triglyceride-Glucose Index, Cardio-Cerebrovascular Diseases, and Death in Korean Adults: A Longitudinal Study Based on the NHIS-HEALS Cohort


 Background: The triglyceride-glucose (TyG) index is a reliable indicator of insulin resistance. We aimed to investigate the TyG index in relation to cardio-cerebrovascular diseases (CCVDs) and mortality.Methods: This retrospective study included 114,603 subjects. The TyG index was categorized into four quartile groups by sex: Q1, <8.249 and <8.063; Q2, 8.249 ‒ <8.614 and 8.063 ‒ <8.403; Q3, 8.614 ‒ < 8.998 and 8.403 ‒ <8.752; and Q4, ≥8.998 and ≥8.752, in men and women, respectively. To calculate hazard ratios (HRs) and 95% confidence intervals (CIs) for the primary outcomes (CCVDs and all-cause mortality), Cox proportional hazards regression models were adopted.Results: Compared to Q1, fully adjusted HRs (95% CIs) for the primary outcomes of Q2, Q3, and Q4 were 1.063 (0.982‒1.152), 1.112 (1.026‒1.206), and 1.153 (1.060‒1.254) in men and 1.099 (0.986‒1.226), 1.049 (0.941‒1.169), and 1.069 (0.960‒1.190) in women, respectively. HRs (95% CIs) for cardiovascular diseases (CVDs) of Q2, Q3, and Q4 were 1.117 (0.971‒1.285), 1.191 (1.036‒1.369), and 1.237 (1.071‒1.427) in men and 1.239 (1.018‒1.509), 1.188 (0.976‒1.446), and 1.248 (1.027‒1.517) in women, respectively. Conclusions: The elevated TyG index were was positively associated with CCVDs and all-cause mortality in men and predicted the higher risk of CVDs in both sexes.


Introduction
Cardiovascular diseases (CVDs) and cerebrovascular diseases (CbVDs), which include coronary heart disease, peripheral arterial disease, and deep vein thrombosis in addition to other diseases of the heart and brain, are the leading cause of death worldwide [1]. CVDs and CbVDs are the second and fourth most common causes of death in Korea, respectively [2]. These cardio-cerebrovascular diseases (CCVDs) are a large burden on public health and a considerable number can be avoided by early detection and active management of their risk factors such as a healthy lifestyle.
Insulin resistance plays an important role in the development of diabetes mellitus and may be closely associated with atherosclerotic CCVDs, malignant neoplasms, and mortality [3][4][5]. The early detection of insulin resistance is important in reducing the burden of chronic diseases. The hyperinsulinemiceuglycemic glucose clamp method is the gold standard to measure insulin resistance [6]. However, it is di cult to apply this technique to a real-world clinical setting because of its complex measurement procedure. The homeostasis model assessment of insulin resistance (HOMA-IR) is frequently used in the clinical setting but also has limited applicability because insulin levels must be measured [7]. The triglyceride-glucose (TyG) index is a novel reliable indicator to detect the early phase of insulin resistance and type 2 diabetes [8 -10]. Furthermore, several studies have reported that the TyG index was superior to HOMA-IR in predicting type 2 diabetes in the Korean population [10]. In part, the dietary pattern of the Korean populations who consume high levels of carbohydrates and low levels of fat may contribute to the higher predictability of the TyG index.
This study aimed to examine whether the TyG index was associated with primary outcomes (CCVDs and all-cause mortality) in Korean adults, based on the Korean National Health Insurance Service (NHIS)-National Health Screening (HEALS) cohort. In addition, we further investigated the association between the TyG index and each secondary outcome after stratifying the primary outcomes into CVDs, CbVDs, CCVD-related death, and all-cause death.

Study population
The NHIS-HEALS cohort included 514,794 subjects who were randomly selected from the 5. for CVDs or I60-I69 for CbVDs) (n = 55,095) between 2002 and 2011; 6) subjects who had any history of diabetes, heart disease, CVDs, or CbVDs according to the self-reported questionnaires of the national health check-up program between 2002 and 2010 (n= 2,686); 7) subjects who were prescribed lipidlowering agents (such as statin, omega-3 fatty acids, niacin, or cholesterol sequestrants) between 2002 and 2010 (n = 30,615); 8) subjects who had incomplete data for the confounders (n = 6036); or 9) subjects whose study duration was 30 days or less (n = 94). After these exclusions, 144,603 subjects (78,021 men and 66,582 women) were included in this study. The primary end point of this study is to compare the occurrence rates of CCVDs and all-cause mortality by TyG quartile groups after enrollment (2009)(2010). CCVDs included CVDs (I20-I25) and CbVDs (I60-I69) based on the ICD-10 code. CCVDs were de ned when the main diagnosis (I20-I25 or I60-I69) was recorded at least twice in outpatients or once in hospitalized patients. As secondary outcomes, we considered the subgroup analyses for each CVD, CbVD, CCVD-related death, and all-cause deaths. In the case of deceased patients, it was classi ed by the cause of death recorded in the death certi cate. If subjects died from fatal CCVD and the direct cause of death was recorded as CVD or CbVD on his/her death certi cate, they were de ned as CCVD-related deaths.
The research start date was de ned as the day of the rst health check-up at baseline (2009-2010). If a subject experienced CCVDs or death, the end date is the earlier date of the events. Otherwise, the end date is the last of the two following cases: the date of the last outpatient clinic visit or the last health check-up.

De nition of covariates
Risk factors that can attribute to CCVDs or deaths were controlled such as age, body mass index (BMI), systolic blood pressure (SBP), low-density lipoprotein cholesterol (LDL-C), lifestyle (cigarette smoking, alcohol consumption, and physical activity), and economic status (monthly household income). These confounding factors were surveyed at baseline (2009-2010).
BMI (unit, kg/m 2 ) was calculated as body mass (kg) divided by squared height (m). Lifestyle and household income levels were collected from self-reported questionnaires. Smoking status was classi ed as ever smokers (who had smoked cigarettes in the past) and never smokers (who had never smoked cigarettes). Alcohol consumption was categorized into rare (less than once a week), sometimes (once to twice a week), and often (three or more times a week). Physical activity was divided into rare (less than once a week of any kind of exercise), sometimes (individuals who did not meet the de nition of rare or regular physical activity), and regular ( ve or more times of walking or moderate-intensity exercise a week; three or more times of vigorous-intensity a week; four or more times of walking, moderate-intensity physical activity or vigorous-intensity exercise a week). Economic status was classi ed into three groups, based on the self-reported monthly household income: low, 0 to 30 th percentile; middle, 31 st to 70 th percentile; and high, 71 st to 100 th percentile.

Statistical analysis
Continuous and categorical variables were presented as mean ± standard deviations (SDs) and the number of subjects (%), respectively. To check the group difference, analysis of variance (ANOVA) tests for continuous variables and chi-square tests for categorical variables were used. To investigate the association between the TyG index and outcomes (CVD, CbVD, and deaths), outcome-free survival rates were estimated and compared using the Kaplan-Meier method and the log-rank test. The hazard ratios (HRs) and 95% con dence intervals (CIs) were calculated to examine the association between the TyG index and the primary outcomes (CCVDs and all-cause mortality) based on Cox proportional hazards regression models after adjusting for confounders. Three Cox proportional hazards regression models were built: 1) Model 1, age only; 2) Model 2, age, smoking status, alcohol consumption, and physical activity; 3) Model 3, BMI, SBP, LDL-C, in addition to the variables in Model 2. All p-values were two-sided and statistical signi cance was set as <0.05. The statistical analyses were performed using the statistical package SAS enterprise version 7.1 (SAS Inc., Cary, NC) and R studio version 3.3.3.

Results
In total, 144,603 subjects (78,021 men and 66,582 women) were included and the median study duration was 5.97 years. Table 1 presents the baseline characteristics of the subjects according to the TyG quartile by sex. Mean age increased in men but decreased in women with the higher quartile group. In both sexes, BMI, SBP, glucose, TG, and percentage of ever smokers increased with increased quartile while the percentage of

Discussion
This study demonstrated that there was a positive association between the TyG index primary outcomes (CCVDs or all-cause mortality) in men, but not in women. After stratifying the primary outcomes into CVDs, CbVDs, CCVD-related death, and all-cause death, a higher TyG index predicted a higher risk of CVDs in both sexes while it was not signi cantly associated with CbVDs, CCVD-related death, and all-cause death.
Insulin stimulates glucose metabolism and suppresses fatty acid utilization and lipolysis as an energy source. Insulin resistance is de ned as an impaired tissue response to insulin stimulation, resulting in the dysfunction of glucose and lipid metabolism [3]. These metabolic alterations lead to chronic hyperglycemia, dyslipidemia (in particular, increased free fatty acid release in adipose tissue and hepatic TG production), oxidative stress, and in ammation that induce endothelial dysfunction and atherosclerotic change. Eventually, insulin resistance increases CCVDs and premature death through these pathophysiologic mechanisms [3][4][5]. Thus, early detection and a more intensive management of insulin resistance are very important in reducing the likelihood of these events. Even though several methods to measure insulin resistance have been proposed, the hyperinsulinemic-euglycemic glucose clamp is considered the standard [6]. However, the hyperinsulinemic-euglycemic glucose clamp is not practical for use in real-world clinics because of complex and invasive measurements. Another reliable surrogate marker of insulin resistance is HOMA-IR. However, the disadvantage is that blood insulin levels are not routinely checked for patients without diabetes in the primary clinical setting [7].
The TyG index is newly proposed as a marker to measure insulin resistance. TG and glucose levels, which are used as variables to calculate the TyG index, are widely and commonly measured in clinics. The Korean Society of Lipid and Atherosclerosis recommends that the lipid pro le (total cholesterol, TG, HDL-C, and LDL-C) be checked every 4-6 years for adults to screen for dyslipidemia and CCVD risk [13]. Many guidelines recommend the annual measurement of fasting glucose levels to screen for diabetes or prediabetes [14,15], while there are few recommendations for the measurement of fasting insulin levels to screen for them. Furthermore, Asian populations may consume more foods with a higher carbohydrate content than Western populations, which increases their blood TG and glucose levels [16]. Carbohydraterich diets raise the possibility of developing hypertriglyceridemia and impaired fasting glucose, which are related to the risk of CCVDs [17,18]. Epidemiological studies support the proposal that the TyG index is a better indicator of insulin resistance in Asian populations than HOMA-IR [10,19]. In addition, a lot of studies report that the TyG index was more closely associated with atherosclerosis, arterial stiffness, subclinical cerebral vessel diseases, and coronary arterial diseases than HOMA-IR [20-23].
The primary outcome of this study is CCVDs and all-cause mortality. CCVDs are composed of CVDs (such as angina pectoris, myocardial infarctions) and CbVDs (such as non-traumatic cerebral hemorrhage, cerebral infarction, and cerebral arterial diseases, etc.). The statistical signi cance between the TyG index and primary outcomes are different according to statistical methods (signi cant only in women from the log-rank test and only in men from Cox proportional hazards model). Here we observed a very interesting result. In terms of the association between the incidence of the primary outcomes and TyG index in men, there was no signi cance from the log-rank test (Figure 2). However, it began to be signi cant after adjusting for age in the Cox proportional hazards regression model (Supplementary Table). This phenomenon is the opposite in women. That is, the log-rank test showed a signi cant association, but the result from the fully adjusted Cox proportional hazards regression model showed non-signi cance. This might be explained by the different age distribution over TyG index by sex. As the TyG quartile increases, the mean age decreases in men but increases in women. Because the log-rank test does not take any confounding factors into account, the signi cance in women might include the compounded effect of age and TyG index. An investigation into the con icting relationship between age and the TyG index by sex is worthwhile. However, the elevated TyG index increased CVDs in both sexes, while it was not independently associated with individual events such as CbVDs, CCVD-related deaths, and all-cause deaths. These ndings signify that the TyG index may be a good indicator in predicting CVDs such as ischemic heart diseases. Results from this study are consistent with a previous study by Park et. al. who reported that an elevated TyG index forecasted the risk of ischemic heart disease in Korean population [24]. Zhao et al. reported that the TyG index is an independent marker for cardiovascular events [25]. However, these studies have a smaller population size and did not stratify the study population according to sex. Our study supports the previous studies but more clearly clari es the association between the TyG index and individual events. The TyG index was signi cantly associated with the primary outcomes (CCVDs and all-cause mortality) in men, but not in women. However, further statistical analyses demonstrated that an elevated TyG index was an independent indicator of CVD development in both sexes while there was a null association between TyG index and other outcomes (CbVDs, CCVD-related death, and all-cause death).
This study has several strengths to distinguish it from previous studies. First, the NHIS-HEALS cohort represents the entire Korean adult population. The NHIS provides reliable information on individual medical history (diagnostic code, laboratory results from national health screening program, lifestyle factors, anthropometric data, and death information based on a death certi cate) and socioeconomic data (based on insurance premiums and self-reported questionnaires). The data from the NHIS-HEALS cohort were based on real-world measurements in a clinical setting. The real-world data showed the true relationship re ecting many variables and environments that we did not expect. Second, an apparently healthy population aged 40 years or older who received a national health screening examination was enrolled in this study, while patients who were diagnosed with diabetes, cancers, or CCVDs were excluded from the analysis. In addition, to control the effects of medications, such as glucose-lowering and lipidlowering drugs, individuals who had taken these drugs between 2002 and 2010 were excluded. However, individuals treated with these drugs after 2010 were not excluded in the nal analyses because the aim of this study was to investigate the association between the initial TyG index and CCVDs or all-cause mortality regardless of medication during the follow-up period. In other words, we just tried to examine whether the elevated TyG index at baseline increases the CCVDs and all-cause mortality. These strict exclusion criteria may minimize the effects of underlying conditions on the primary outcome. Despite strict exclusion, many subjects (144,000 or more) were included in the nal analyses. Third, the follow-up duration (median 5.97 years) was relatively long. In the case of apparently healthy individuals, large population size and longer study duration allowed a more accurate relationship to be explored than those from previous studies. Fourth, we widely examined the outcomes, which were related to the TyG index. The primary outcomes were strati ed to several individual outcomes and sequentially investigated the relationships between them and the TyG index. In addition, many conventional risk factors including health behaviors, SBP, LDL-C, and economic status for CCVDs were adjusted in the Cox proportional hazards regression models. This conservative approach provides a reliable association between the TyG index and outcomes to minimize the function of these conventional risk factors as confounders.
There are several limitations that should be interpreted cautiously. We could not validate whether the TyG index directly correlated with insulin resistance because the NHIS-HEALS cohort did not contain information on serum insulin levels. However, the TyG index was already validated to indicate insulin resistance in previous studies [26,27]. The outcomes such as CVDs, CbVDs, and CCVD-related deaths were not directly collected by our research team. There is the possibility of misclassi cation or a different de nition of outcome events. The Korean health insurance system is uniquely different from other countries because all Koreans should subscribe to the NHIS and the Korean government collects and controls health information, insurance claim information, and reimbursement. For example, the NHIS assesses the coverage of insurance and determines the cost of the individual medical service, while Health Insurance Review and Assessment Service (HIRAS), another national agency, reviews the claim data and reimburses the hospital fees. Special diseases such as CCVDs, cancers, and rare diseases are more strictly monitored by these two national agencies because patients with special diseases pay 5% of their hospital fee and the NHIS pays the remaining medical cost. These strict monitoring systems minimize inaccurate diagnosis. In addition, to minimize the misclassi cation, we more conservatively de ned the CCVDs. CCVDs were de ned when the relevant diagnostic codes (I20-I25 or I60-I69) as the main diagnostic code were recorded more than once at hospitalization or at least twice at the outpatient visit. Even if there are misclassi cations, we believe that there would be very few.
In conclusion, men with an elevated TyG index were at higher risk for CCVDs or all-cause mortality.
Individuals with a higher TyG index were more likely to develop CVDs in both sexes while the TyG index was not signi cantly associated with CbVDs, CCVD-related death, and all-cause death.
Abbreviations ANOVA, analysis of variance; BMI, body mass index; CbVD, cerebrovascular disease; CCVD, cardiocerebrovascular disease; CI, con dence interval; CVD, cardiovascular disease; HEALS, health screening; HOMA-IR, homeostasis model assessment of insulin resistance; HR, hazard ratio; ICD, international classi cation of diseases; LDL-C, low-density lipoprotein cholesterol; NHIS, national health insurance service; SBP, systolic blood pressure; SD, standard deviation; TG, triglycerides; TyG, triglycerides-glucose Authors' contributions: All authors made substantial contributions to the conception or design of the work. All the authors made substantial contributions to the acquisition, analysis, or interpretation of data for the work. All authors contributed to the manuscript drafting or critically revised it for important intellectual content and reviewed and approved the nal manuscript. Hee-Taik Kang are the guarantor of this work and, as such, had full access to all data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. and physical activity (rare, sometimes, and regular), body mass index, systolic blood pressure, LDL-C, and economic status (low, middle, and high) Figure 1 Flowchart of inclusion and exclusion criteria