The study population were from the Kailuan study, which is an ongoing prospective cohort study conducted in Tangshan, China. The details on Kailuan study have been previously described. Briefly, since June 2006, a total of 101,510 participants (81,110 men and 20,400 women, aged 18-98 years) were enrolled in the first survey from 11 hospitals and underwent questionnaire assessments, clinical examinations, and laboratory tests. All participants were followed biennially to update their status on the aforementioned parameters. In the present study, we excluded participants with history of myocardial infraction or stroke (n=3,669) at baseline, ultimately, a total of 97,841 participants were included in the current analysis. 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 the participants agreed to take part in the study and provided written informed consent.
Incident CVD ascertainment
Incident CVD was a composite of first stroke or myocardial infarction. Assessment of CVD has been described previously.[19-21] The database of CVD diagnoses was obtained from the Municipal Social Insurance Institution and Hospital Discharge Register and was updated annually. An expert panel collected and reviewed the annual discharge records from 11 hospitals in Kailuan community to identify patients who were suspected of CVD. Incident stroke was diagnosed based on neurological signs, clinical symptoms, and neuroimaging tests, including computed tomography or magnetic resonance, according to the World Health Organization criteria. Myocardial infarction was diagnosed according to the criteria of the World Health Organization on the based on the clinical symptoms, changes in the serum concentrations of cardiac enzymes and biomarkers, and electrocardiographic results.[20, 23]
Risk factors assessment
Baseline risk factors were collected via standardized questionnaire by trained staff, including age, sex, education level, income, physical activity, smoking, alcohol intake, medical history (hypertension, diabetes, and dyslipidemia). Educational level was classified as illiterate or primary school, middle school, and high school or above. Income was categorized into > 800 and ≤ 800 yuan/month. Smoking and alcohol intake habits were stratified into never, former or current. Physical activity was classified as ≥80 minutes per week, <80 minutes per week, or none. Body mass index (BMI) was calculated as weight in kilograms divided by the height in meters squared, and were categorized as overweight (BMI 25.0 to <28.0) and obsess (BMI ≥28.0). Blood pressure was measured in the in the seated position using a mercury sphygmomanometer, and the mean results of three measurements of the systolic blood pressure (SBP) and diastolic blood pressure (DBP) were recorded. Hypertension was defined as SBP ≥140 mm Hg or DBP ≥90 mm Hg, any use of the antihypertensive drug, or a self-reported history of hypertension. Diabetes was defined as fasting blood glucose (FBG) ≥7.0mmol/L, any use of glucose-lowering drugs, or a self-reported history of diabetes. Dyslipidemia was defined as any self-reported history or use of lipid-lowering drugs, or total cholesterol (TC) ≥ 5.17 mmol/L or triglyceride (TG) ≥ 1.69 mmol/L or low density lipoprotein cholesterol (LDL-C) ≥ 3.62 mmol/L or high density lipoprotein cholesterol (HDL-C) ≤ 1.04 mmol/L. Metabolic syndrome was defined according to the ATP-III criteria.
Fasting blood samples were collected in the morning after an 8- to 12-h overnight fast and transfused into vacuum tubes containing EDTA. Plasma was separated from blood immediately and stored at 4°C. All the blood samples were analyzed using an auto-analyzer (Hitachi 747, Hitachi, Tokyo, Japan) on the day of the blood draw. The biochemical indicators tested included FBG, serum lipids (TC, TG, LDL-C, HDL-C, TC/HDL-C, TG/HDL-C, non-HDL-C, remnant cholesterol [calculated as TC-LDL-C-HDL-C]), serum creatinine, high-sensitivity C-reactive protein (hs-CRP), white blood cell count, neutrophil count, and platelet count. The TyG index was calculated as ln (fasting TG [mg/dl] × FBG [mg/dl]/2). Estimated glomerular filtration rate (eGFR) was calculated by the Chronic Kidney Disease Epidemiology Collaboration creatinine equation.
We divided the study time into 4 age groups (<55, 55 to <65, 65 to <75, and ≥75 years) and participants contributed to advancing age groups over time until the occurrence of incident CVD or censoring (death or the end of the follow-up), and calculated CVD incidence rates. The baseline characteristics are presented as mean ± standard deviation (SD) or frequency with percentage as appropriate. Differences in the characteristics across 4 age categories were tested using analysis of variance or the Kruskal-Wallis test for continuous variables according to distribution, and using chi-square for categorical variables.
We used stratified Cox proportional hazards regression models with the counting process method, stratified by the 4 age groups. We estimated adjusted hazard ratios (HRs) with 95% confidence interval (CIs) for per SD increment of each biomarker and for clinical categories for risk factors with clinical cutoff points. The adjusted model included sex, educational level and family income. In additional analyses, we examine the associations between risk factors and incident CVD in the models that included covariates mentioned-above plus the following additional covariates (physical activity, smoking, drinking, BMI, hypertension, diabetes, dyslipidemia, SBP and DBP). The proportionality assumption was met for Cox models. The population-attributable risk for clinical risk factors was calculated with a method previously described. Likelihood ratio tests evaluated interactions between individual risk factors and age groups.
All analyses were performed using SAS version 9.4 (SAS Institute, Cary, North Carolina). All statistical tests were 2-sided, and P < 0.05 was considered statistically significant.