Triglyceride-Glucose Index Mediates the Effect of General and Central Obesity on Cardiovascular Disease: A Prospective Cohort Study

Background: The mechanisms linking obesity to cardiovascular disease (CVD) are still not clearly dened. Individuals who are overweight or obese often develop insulin resistance, mediation of the association between obesity and CVD through the triglyceride-glucose (TyG) index (a simple surrogate of insulin resistance) seems plausible and has not been investigated. This study aimed to evaluate whether and to what extend the effect of general and central obesity is mediated by the TyG index. Methods: A total of 94,136 participants without CVD at baseline were recruited from the Kailuan study. The TyG index was calculated as ln [fasting triglyceride (mg/dL)×fasting glucose (mg/dL)/2]. Mediation analysis using a newly proposed 2-stage regression method for survival data was to explore the mediating effects of the TyG index on the association between obesity and CVD. Results: During a median follow-up of 13.01 years, we identied 7,327 cases of CVD. Mediation analyses showed that 45.60% of the total association (hazard ratio [HR], 1.24; 95% condence interval [CI], 1.18-1.31) between overweight and CVD was mediated through the TyG index (HR [indirect association], 1.10; 95% CI, 1.08-1.11), and the proportion mediated was 38.30% for general obesity. For central obesity, analysis by waist circumference, waist/hip, and waist/height categories yielded an attenuated proportion mediated of 33.76%, 37.10%, and 33.10% for obesity, taken normal weight as reference. Similar results were observed for stroke and myocardial infarction. Conclusions: This study found that the association between obesity and CVD was mediated by the TyG index, suggesting proper control of insulin resistance can be effective to signicantly reduce the effects of general and central obesity on CVD.


Study population
Data were deprived from the Kailuan study, which is a prospective cohort study conducted in the Kailuan community in Tangshan, China. The detailed study design and procedures have been published previously. [15][16][17] Brie y, during June 2006 to October 2007, a total of 101,510 participants (81,110 men and 20,400 women, aged 18-98 years) were enrolled in the baseline survey and have completed questionnaires and health assessments biennially since 2006. All the participants were followed up until their death or December 31, 2019. Participants were excluded if they had a history of stroke or myocardial infarction (MI) (3,669), or had missing data on BMI, WC, WHR, WHTR, FBG, or TG (3,705) at baseline. Ultimately, we included 94,136 participants in the current analysis ( Figure S1). The study was performed according to the guidelines of the Helsinki Declaration and was approved by the Ethics Committee of Kailuan General Hospital and Beijing Tiantan Hospital. All participants provided written informed consent.

Data collection
Demographic data, including age, sex, education, and income, as well as lifestyle behaviors, such as smoking status, drinking status, and medical history were collected via standardized questionnaires. Educational attainment was categorized as illiterate or primary school, middle school, or high school or above. The family income was categorized as <800 and ≥800 yuan/month. Smoking and drinking status were classi ed as never, former or current, according to selfreported information. Blood pressure was measured in the in the seated position using a mercury sphygmomanometer, the average of 3 readings were calculated as systolic blood pressure (SBP) and diastolic blood pressure (DBP).
Anthropometric parameters were measured by trained led workers. WC was measured in centimeters with nonstretchable tape held at the level of the naval while the subject was standing without clothing. Hip circumference was measured around the widest portion of the buttocks with the tape parallel to the oor. Height was measured to the nearest 0.1 cm using a tape rule, and weight was measured to the nearest 0.1 kg using calibrated platform scales.
Blood sample collections were conducted following an 8-to 12-h overnight fast. Fasting blood glucose (FBG) was measured with the hexokinase/glucose-6-phosphate dehydrogenase method. The CV using blind quality control specimens was <2.0%. Total cholesterol (TC), TG, high density lipoprotein cholesterol (HDL-C), low density lipoprotein cholesterol (LDL-C), creatinine, and high-sensitivity C-reactive protein (hs-CRP) were assessed by an auto-analyzer (Hitachi 747; Hitachi) at the central laboratory of Kailuan hospital. Hypertension was de ned as SBP ≥140 mm Hg or DBP ≥90 mm Hg, any use of the antihypertensive drug, or self-reported history of hypertension. Diabetes was de ned as FBG≥7.0mmol/L, any use of glucose-lowing drugs, or any self-reported history of diabetes. Dyslipidemia was de ned as any self-reported history or use of lipid-lowering drugs, or TC ≥6.2 mmol/L, TG ≥2.3 mmol/L, LDL-C ≥4.1 mmol/L, or HDL-C<1.0 mmol/L.

Calculation of obesity indexes and the TyG index
Obesity indexes in our study include BMI, WC, WHR, and WHTR. BMI was calculated as weight in kilograms dividing by the square of height in meter. WHR and WHTR were calculated as WC/hip and WC/height, respectively. According to the Guidelines and Prevention and Control of Overweight and Obesity in Chinese Adults [18], general obesity was de ned as a four-category variable: BMI<18.5 kg/m 2 for underweight, 18.5≤ BMI<25 km/m 2 for normal weight, 25≤ BMI <28 km/m 2 for overweight and BMI ≥28kg/m 2 for obesity. Central obesity was de ned as: (1) WC ≥90cm in men and ≥85cm in women; (2) WHR ≥0.9 in men or ≥0.8 in women; and (3) WHTR ≥0.6. The TyG index was calculated as ln (fasting TG [mg/dl] × FBG [mg/dl]/2). [19,20] Assessment of CVD Participants were followed up via face-to-face interviews at every 2-year routine medical examination until event of interest, December 31, 2019, or death. The outcomes in the present study were the rst occurrence of CVD, including stroke and MI. The database of CVD diagnoses was obtained from the Municipal Social Insurance Institution and Hospital Discharge Register and was updated annually during the follow-up period. An expert panel collected and reviewed annual discharge records from 11 local hospitals to identify patients who were suspected of CVD. Ascertainment of incident stroke and MI was described previously. [16,[19][20][21]  Statistical analysis Baseline characteristics are presented as the mean ± standard deviation or percentage. Differences between incident CVD and non-CVD participants were compared using Student's t test or Wilcoxon for continuous variables and chi-square test for categorical variables. Linear regression was used to assess the association between four obesity indexes and the TyG index.
Mediation analysis for the association of general and central obesity (exposure) with CVD (outcome) through the TyG index (mediator) was evaluated by the 2-stage regression method for survival data proposed by VanderWeele.
[24] In brief, 2 regression models are for to the data, one modeling the mediator and the other modeling the outcome; parameters estimates and standard errors of these 2 separate models are combined according to the formulas given therein to obtain estimates for effect size of mediation. We modeled the outcome (CVD) using Cox proportional hazards regression models, and the mediator (the TyG index) using linear regression. All models were adjusted for age, sex, education, income, smoking status, and drinking status, without inclusion of interaction terms.
The assuming association between variables is illustrated in Figure 1.[25] The VanderWeele's method decomposes the total effect of general and central obesity on CVD (expressed as the hazard ratio [HR] vs the reference normal weight) into 2 components: the natural indirect effect size (ie, the effect size of general and central obesity mediated through the TyG index, and natural direct effect size (ie, the effect size of general and central obesity not explained through the TyG index). [26,27] Because these estimates are based on observational data, we term the estimates as total, indirect, and direct associations. The proportion of the association of general and central obesity and CVD mediated through the TyG index as a measure of the contribution of the natural indirect association with the total association was calculated on the log-transformed HR scale as log (indirect association HR)/log(total association HR), since HRs are assistive on this scale. [28] Sensitivity analyses were performed to test the robust of the association. First, we excluded participants with baseline FBG ≥7 mmol/L. Second, the exposure and confounders were considered before the mediator at intervals of least two years (the exposure and confounders of baseline, the mediator of the second visit-up). Third, the confounders were considered before the mediator and exposure. Subgroup analysis strati ed by sex was also performed. All analyses were conducted using SAS version 9.4 (SAS Institute Inc., Cary, NC, USA). A two-sided P<0.05 was considered statistically signi cant.

Baseline characteristics
The nal sample included 94,163 participants. The mean age of the population was 51.24±12.37 years. During a median follow-up of 13.01 years, we identi ed 7,327 (7.78%) cases of incident CVD, including 5,898(6.27%) stroke and 1,622 (1.72%) MI. baseline characteristics of the study population are presented in Table 1. There was a signi cant difference between the incident CVD and non-CVD group in age, sex, education, income, smoking, drinking, hypertension, diabetes, dyslipidemia, medications, SBP, DBP, FBG, TC, TG, HDL-C, LDL-C, and hs-CRP. The BMI, WC, WHR, WHTR, proportion of obesity and the TyG index was higher in participants with CVD than that without CVD.

Correlation between the obesity indexes with the TyG index
Both the general central obesity indexes were signi cant with the TyG index ( Figure S2). R2 from the linear regression for the correlation between BMI and the TyG index was 0.1088, indicating 10.88% of the variation in the BMI was explained by variation in the TyG index in this model. Similar results were observed for WC, WHR, and WHTR.

Mediation analysis
Results on the decomposition of the total association of general and central obesity with the risk of CVD into direct and indirect associations mediated by the TyG index is shown as Table 2 and Figure 2. Mediation analysis showed that the risk of CVD increased 24% (HR [total association], 1.24; 95% con dence interval [CI], 1.18-1.31) for overweight vs the reference normal weight, which increased to 59% (HR, 1.59; 95% CI, 1.50-1.69) for the obesity group, of which 45.60% and 38.30% was mediated through the TyG index, the HR for the indirect association was 1.10 (95% CI, 1.08-1.11) and 1.17 (95% CI, 1.14-1.19) for overweight and obesity respectively.
On scrutinizing the relationship between central obesity and CVD, the results yielded a total association HR of 1.43 (1.36-1.50) for WC ≥90cm in men or ≥85cm in women, 1.29 (95% CI, 1.22-1.36) for WHR ≥0.90 in men or ≥0.80 in women, and 1.38 (95% CI, 1.29-1.48) for WHTR ≥0.60, compared with their counterparts with normal weight, and the proportion mediated was 33.76%, 37.10%, and 33.10% respectively (Table 2 and Figure 2). In the subtype analyses of CVD, similar results were yield for stroke and MI (Table S1 and S2).

Sensitivity and subgroup analysis
The results of the sensitivity analysis performed in this study showed that excluding participants with baseline FBG ≥7mmol/L (n=7,642) slightly attenuated the indirect association of general and central obesity with CVD, the proportion mediated by the TyG index was 32.79% for overweight, 26.64% for general obesity, and 24.63%, 30.22%, 22.2% for WC, WHR and WHTR as the index of central obesity. Sensitivity analyses by adding a lag time (a two-year lag time in this study) between the general and central obesity and the TyG index, in addition to applying a two-year period between the confounders and general and central obesity presented the same pattern of results, with a decreased proportion mediated compared with the main results ( Table 3, Table S3 and S4). Subgroup analyses revealed similar results across men and women (Table S5).

Discussion
In this large prospective population-based cohort study, we found that the association of general and central obesity with CVD was mediated through the TyG index. Similar patterns were observed for stroke and MI. The trend remained robust among strati ed analyses and multiple sensitivity analyses. These ndings suggested public health efforts aiming at the reduction of body weight might decrease the sequelae of insulin resistance and the burden of CVD.
Obesity is frequently associated with elevated risk of CVD. Both observational [29] and Mendelian randomization approaches[30] have consistently shown the association between general obesity and CVD risk. In addition to the important clinical implications of BMI assessment, studies also proved that central obesity (measured by WC, WHR, and WHTR) is also an important risk factor for various obesity-related chronic diseases.
[31] Consistent with these study, our study also demonstrated a signi cant relationship of general and central obesity with the risk of CVD, even after adjustment for the TyG index. Obesity can increase CVD morbidity and mortality directly and indirectly. Direct effects are mediated by obesity-induced structural and functional adaptions of the cardiovascular system to accommodate excess body weight, as well as by adipocyte effects on in ammation and vascular homeostasis, leading to a pro-in ammatory and pro-thrombotic milieu. Indirect effects are mediated by concomitant CVD risk factors. Previous study showed that obesity is a well-established risk factor of insulin resistance, individuals who are overweight or obese are more likely to develop insulin resistance indicating early impaired glucose metabolism.
[32] Epidemiological studies have shown a signi cant association of insulin resistance with CVD independent of diabetes, which was greater in the presence of general obesity.
[33] Therefore, insulin resistance might be a potential important mediator of the association between obesity and the risk of CVD.
Our study results provide epidemiologic support for the biologically plausible hypothesis that insulin resistance plays an important role in the pathway between obesity and CVD, the proportion mediated of the TyG index was 45.60% for t overweight, 38.30% for general obesity, 33.76%, 37.10, and 33.10% for central obesity measured by WC, WHR, and WHTR, respectively. Insulin resistance often clusters with various classical risk factors such as lipid abnormalities, glucose intolerance, and high blood pressure, which have been con rmed to mediate the effect of general and central obesity on CVD in several previous studies. [34,35] While the role of insulin resistance in the relationship between obesity and CVD has not been established well previously. A retrospective cohort analysis of 6,078 participants aged over 60 years revealed the TyG index mediated the effect of BMI on CVD event, HR for the indirect effect through the TyG index was 1.01, but the proportion mediated was not provided in this study, and the obesity index was only restricted to BMI.
[36] In contrast, our prospective analysis with a large sample size (N=94,136) can provided a stronger statistical power, focused on a series of indices on both general and central obesity, and used a newly developed method to quantify the proportion mediated of the TyG index. This approach offers several advantages over conventional methods. In particular, it can accommodate interactions between BMI and its mediators by incorporating interaction terms into the outcome model and using a separate regression model for each exposure-mediator association.
[37] In addition, conventional methods often overlook mediator-outcome confounders and this can introduce bias in the direct and indirect effect estimates. [35] We adjusted for confounders for both obesity-CVD and mediator-CVD in regression that parameterized those association.
Our results together with the previous ndings indicated that controlling the TyG index may can be effective to reduce the effects of general and central obesity on CVD.
Our result also revealed that the TyG index explained more to the association between general obesity and CVD than central obesity, which was indirectly supported by the evidence that central obesity plays a more importance role than BMI in determining the risk of CVD.
[31] In the sensitivity analysis, we found after excluded those with FBG ≥7mmol/L, the proportion mediated was attenuated slightly, indicating general and central obesity contributed more to the risk of CVD among individuals with normal FBG levels. This is in line with the results from the National Sample Cohort (NSC) database, which showed a higher BMI on the risk of CVD event was signi cantly weaker among individuals with diabetes than those with without diabetes.
[38] It is conceivable that the association between obesity and CVD mediated through the TyG index can in part be explained by the development of diabetes. For public health programs, the mediating role of the TyG index in the association of general and central obesity and CVD indicates that it is important to emphasize body weight management and to encourage individuals to focus on their body fat distribution as well as the level of FBG.
Overweight and obesity (general and central obesity) have harmful effects on CVD risk, and the underlying mechanism has been thoroughly investigated.
[39] Adipose tissue increases basal lipolysis and releases free fatty acids (FFA), interleukins and cytokines that drive cardiac dysfunction by accelerating atherosclerotic processes and modifying factors associated with in ammation and endothelial and coagulation dysfunction.
[40] The increase in FFA due to obesity can trigger insulin resistance, which further inhibits insulin signalling and insulin-stimulated glucose uptake in skeletal muscles and increases glucose delivery by the liver.
[41] Furthermore, in ammatory factors associated with obesity promote the processes of lipolysis and hepatic triglyceride synthesis, as well as hyperlipidemia induced by increased fatty acid esteri cation. [9] Hence, the TyG index is a plausible link between obesity and the risk of CVD.
Our ndings have clinical and public health implications. In fact, behavioral interventions which are intended to properly manage and control weight gain are successful only for a short time.
[42] Since the majority of weight-loss drugs have no effectiveness and e ciency, and numerous surgical procedures are performed for obese individuals, the global occurrence of CVDs has been regarded as a most important concern and challenge.
[43] E cient clinical and public health interventions aiming at imperative reductions in the predominance of some risk factors is necessary, since the prevalence of obesity is still growing during the past decades. The ndings of our study supported the indirectly causal pathway way from obesity to insulin resistance and CVD, declaring that the proper control of insulin resistance can be effective to signi cantly reduce the effects of general and central obesity on CVD.

Strengths And Limitations
The strengths of the study include its prospective design, large community-based sample, and long follow-up period.
Furthermore, mediation analysis is performed using a newly proposed 2-stage regression method for survival data, which is a mathematically consistent decomposition of the total association into direct and indirect associations with clear interpretations. However, the study also has several limitations. First, we only collected information on stroke and MI, and we may have underestimated the prevalence and incidence rates of CVD, which has broader subtypes (e.g., heart failure, coronary artery disease). Second, this is not a national representative sample; the results need to be interpreted with caution, particularly with regard to generalizability. The prevalence of general and central obesity was higher in our study population than in the general Chinese population. Findings of this study may not be generalizable to other populations, as results were based on just the Kailuan community, in which approximately one-third of the participants were coal miners, who had different lifestyles and working environments than the rest of the participant. Third, although the possibility of unmeasured confounding cannot be ruled out, the magnitude of the observed effect sizes makes it unlikely that unmeasured confounding could completely explain our observed associations.

Conclusions
Our ndings suggested that the association of general and central obesity with risk of CVD was mediated through the TyG index. Proper control of insulin resistance can be effective to signi cantly reduce the effects of general and central obesity on CVD.

Declarations Ethics approval and consent to participate
The study was performed according to the guidelines of the Helsinki Declaration and was approved by the Ethics Committee of Kailuan General Hospital (approval number: 2006-05) and Beijing Tiantan Hospital (approval number: 2010-014-01). All participants were agreed to take part in the study and provided informed written consent.

Consent for publication
Not applicable

Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request

Competing interests
These authors declare that they have no con icts of interests.   b Decomposition of total associations into natural indirect and natural direct associations was done according to the 2stage regression method proposed by VanderWeele and performed with the SAS macro provided by ValerWeele. Con dence intervals were calculated according to the delta method procedure.

Source of Funding
All models were adjusted for age, sex, education, income, smoking status, and drinking status. Figure 1 Causal diagram on the possible other mechanisms which the association of general and central obesity and CVD is mediated. All statistical models were based on this structure and were adjusted for age, sex, education, income, smoking status and drinking status. Because blood pressure, cholesterol and other metabolic factors presented alternative pathways potentially mediating parts of the total association, these variables were not entered as covariates in our models. The possibility of unmeasured confounding, which can never be ruled out in observational research, is indicated with dashed arrows. Abbreviations: BMI, body mass index; CVD, cardiovascular disease; TyG index, triglyceride-glucose index; WC, Waist circumference; WHR, waist-hip ratio; WHTR, waist-height ratio.

Figure 2
Decomposition of the total association of general and central obesity and the risk of CVD into direct and indirect associations mediated by the TyG index. Abbreviations, BMI, body mass index; CI, con dence interval; CVD, cardiovascular disease; HR, hazard ratio; TyG, triglyceride-glucose index; WC, waist circumference; WHR, waist circumference to hip ratio; WHTR, waist circumference to height ratio. Compared with normal weight participants for general obesity and WC<90cm in men or <85cm in women, WHR<0.90 in men or <0.80 in women, and WHTR<0.60 as a reference for central obesity. All models were adjusted for age, sex, education, income, smoking status, and drinking status.

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