Subjects
The research was conducted in the Pingguoyuan community in Beijing, China. A public recruitment announcement was first published on the community bulletin board with the consent of the community management committee. The subjects voluntarily participated in the study, which aimed to recruit community residents. Bedridden patients, or patients suffering from mental disorders or severe systemic diseases (including myocardial infarction, coronary artery disease, cerebrovascular events, severe liver insufficiency, severe renal insufficiency, immune system diseases, endocrine and metabolic disorders [except for Type 2 diabetes]) were excluded from this study. Initially, all the subjects received physical examinations at the community medical center. From September 2007 to January 2009, a total of 1828 subjects aged ≥18 years were recruited. In the end, 1680 subjects were enrolled in this study, and the enrolled subjects were followed up for a second time between September 2017 and October 2018 via a face-to-face questionnaire by two cardiovascular physicians. An average follow-up time of 9.5 years (8.7-10.5 years) was completed by 1325 subjects, during which 191 major adverse cardiovascular events (MACEs) occurred, and 355 subjects were lost to follow-up. The proportion of subjects lost to follow-up was approximately 21.1%, and the follow-up rate was 78.9%. The entire protocol of this study was approved by the ethics committee of the PLA General Hospital, and each subject signed an informed consent form.
Clinical data collection
Medical history was collected by cardiovascular specialists with unified training. All the subjects were investigated to determine the following: cardiovascular history, hypertension history, diabetes history, family history, smoking history, drinking history, and medication history. A standardized questionnaire was used to investigate the subjects’ name, sex, nationality, age, education level, occupation, marital status, family address, contact information and other information. Blood pressure, height, weight, and waist and hip circumference were measured by trained doctors. When measuring the height and weight, shoes, socks and caps were removed, and the height measurements were accurate to 1 cm, and the weight meter measurements were accurate to 0.1 kg. To measure the waist and hip circumference, any coat was removed and belts were loosened, and the tape measure was placed between the upper part of the hip bone and the lower part of the chest. The measurements were made while the subject was exhaling and their abdomen was relaxed, and the readings were accurate to 1 cm. Prior to measuring their blood pressure, the subjects rested in a sitting position for 5 minutes. Then, blood pressure was measured twice with an interval of 1-2 minutes, and the mean value was calculated.
Blood was drawn from subjects with an empty stomach by specially trained nurses. Patients who had not been diagnosed with diabetes mellitus were examined by an oral glucose tolerance test (OGTT). Fasting blood glucose and postprandial blood glucose, serum TC, TG, HDL-C, LDL-C and uric acid (UA) were measured with a Roche Diagnostics GmbH kit (Mannheim, Germany) using a Roche 6000 automatic biochemical analyzer (Roche 6000). Serum creatinine (CR) was determined by a Roche enzyme assay kit on the Hitachi 7600 automatic analyzer (Hitachi, Tokyo, Japan). The sdLDL "Seiken" kit (Denka Seiken Co. Ltd, Tokyo, Japan) was used to detect the plasma sdLDL levels on the Hitachi 7180 automatic biochemical analyzer (Hitachi, Japan) [6]. According to the standards of the WHO lipid reference laboratory, all the blood samples were analyzed in the same laboratory.
Definition of variables
Smoking is defined as smoking at least 1 cigarette a day for at least 1 year. Exercise is defined as at least 1 hour of each exercise performed 3 to 5 times a week. Hypertension is defined as: mean systolic blood pressure (SBP)≥140 mmHg and/or mean diastolic blood pressure (DBP) ≥90 mmHg, and/or regular use of antihypertensive drugs. Diabetes is defined as: intravenous fasting blood glucose level ≥7.1 mmol/L, and/or 2 hours after eating blood glucose level ≥ 11.1 mmol/L, or use of hypoglycemic drugs or insulin. BMI = weight(kg) / height2(m2); Non-HDL-C=TC(mmol/L)-HDL-C(mmol/L); eGFR= 141× min (Scr/κ,1)α × max (Scr/κ,1)-1.209 ×0.993Age × 1.018 [female] (or ×1.159 [male]); Scr is blood creatinine (mg/dL); κ Values are: 0.7 for women and 0.9 for men; α is -0.329 for women and -0.411 for men.
The definition of MACE comprised nonfatal myocardial infarction, newly diagnosed CHD (identified by coronary artery imaging or receiving coronary revascularization), stroke (ischemic or hemorrhagic) and cardiovascular mortality.
Statistical analyses
The categorical variables are expressed as numbers and percentages, and the continuous variables are expressed as the mean ± standard deviation or median (quartile). T test was used for normally distributed data, while the nonparametric Mann-Whitney U test was used for nonnormally distributed data, and the chi-square test used for classified data. For the comparison between groups, analysis of variance was used when the data were homogeneous, and Kruskal-Wallis test was used for the comparison between groups when the data were not normally distributed or the variance was not uniform. The quartiles of the sdLDL-C levels were labeled as Q1, Q2, Q3 and Q4. The subjects were divided into four groups according to whether or not they had hypertension: H1: high levels of sdLDL-C and hypertension; H2: high levels of sdLDL-C and nonhypertensive; H3: hypertensive and low levels of sdLDL-C; H4: low levels of sdLDL-C and nonhypertensive. Spearman correlation analysis and multiple linear regression analysis were used to evaluate the correlation between the plasma sdLDL-C levels and biomarkers. The relationship between the sdLDL-C levels and MACEs was analyzed by Cox proportional hazard regression model. SPSS for Windows version 20.0 (SPSS, Chicago, IL, USA) was used for statistical analysis, P < 0.05 was considered statistically significant.