Study population
The dataset used in this study (Ansan-Ansung cohort) was obtained after review and evaluation of our research plan by the Korea Centers for Disease Control and Prevention (http://www.cdc.go.kr/CDC/eng/main.jsp). This study used data from the Korean Genome and Epidemiology Study (KoGES), a longitudinal prospective cohort study conducted by the Korean Centers for Disease Control and Prevention (KCDC) to examine the prevalence of and risk factors for chronic diseases in Korea. The KoGES consists of six large prospective cohort studies categorized into population-based and gene-environment model studies. All data in this study were derived from the KoGES Ansan and Ansung study, one of the population-based KoGES cohort studies. All participants enrolled in the study voluntarily, and all provided written informed consent. This study was conducted in accordance with the Declaration of Helsinki. The study protocol was approved by the Ethics Committee of the Korean Health and Genomic Study at the Korea National Institute of Health. Details of the KoGES and its sampling method have been reported elsewhere. The KoGES included men and women who were 40–69 years old and lived in Ansan (urban area) or Ansung (rural area) during the baseline survey period from 2001–2002. The cohort was surveyed biennially until 2013–2014. Among the 10,030 participants in the baseline survey, we excluded 1,351 (13.5%) who had been previously diagnosed with type 2 diabetes or satisfied the American Diabetes Association (ADA) diagnostic criteria for type 2 diabetes on the baseline survey. Of the remaining participants, we excluded those who were lost to follow-up (n = 888). We also excluded participants who satisfied one or more of the following criteria: missing data or current use of lipid-lowering medication (n = 183). After these exclusions, 7,608 participants (3,662 men and 3,946 women) were included in the final analysis. A flow chart of the selection process is shown in Figure 1.
Study definitions and outcomes
The fasting plasma glucose, glycosylated hemoglobin (HbA1c), and post two-hour plasma glucose levels after a 75-g oral glucose tolerance test (OGTT) were evaluated biennially in all participants until 2013–2014. Based on the ADA criteria, new-onset type 2 diabetes was defined as any of the following: a fasting plasma glucose level ≥ 126 mg/dL; a plasma glucose level ≥ 200 mg/dL at two hours after a 75-g OGTT; an HbA1c ≥ 6.5%; or current treatment with oral anti-diabetic medications or insulin therapy. Non-HDL cholesterol level was calculated as [total cholesterol – HDL cholesterol] mg/dL. The formula for Homeostasis model assessment of insulin resistance (HOMA-IR) was as follows: [fasting glucose (mg/dL) * fasting insulin (μIU/mL)/405].
Measurement of anthropometric and biochemical parameters
Participant height and body weight were measured to the nearest 0.1 cm and 0.0 kg, respectively, while they wore light indoor clothing without shoes. Their smoking status, drinking behaviors, and physical activity levels were obtained from self-reported questionnaires that all study participants completed during the interview period. Smoking status was divided into three categories: current smokers, ex-smokers, and never smokers. We categorized alcohol drinking status as current drinker or not. Regular exercise was defined as moderate-intensity physical exercise at least three times a week. We defined one episode of exercise as any physical activity that lasted for at least 30 minutes. The systolic and diastolic blood pressure values were assessed three times in the right upper arm using a standard mercury sphygmomanometer (Baumanometer, Baum, Copiague, NY, USA), and the mean of the second and third blood pressure readings was used for analysis.
Biochemical parameters, including fasting serum glucose, HbA1c, 60-minute OGTT, 120-minute serum glucose, and lipid levels (total cholesterol, triglycerides [TG], and HDL-C) were measured enzymatically using a 747 Chemistry Analyzer (Hitachi 7600, Tokyo, Japan). The HbA1c level was assessed using high-performance liquid chromatography (VARIANT II; Bio-Rad Laboratories, Hercules, CA). The plasma insulin concentration level was determined using a radioimmunoassay (LINCO kit, St. Charles, MO, USA).
Statistical analysis
Non-HDL cholesterol quartiles were categorized as follows: Q1, ≤121 mg/dL; Q2, 122–142 mg/dL; Q3, 143–166 mg/dL; and Q4, ≥ 167 mg/dL. Depending on the normality of the distributions of continuous variables, the baseline characteristics of the study population according to non-HDL cholesterol quartile were compared using one-way analysis of variance or the Kruskal-Wallis test. The chi-square test was used to compare categorical variables. The continuous data are presented as mean (standard deviation [SD]) or median (interquartile range [IQR]). Categorical data are shown as frequency. The lowest quartile (Q1) was defined as the reference group. The hazard ratio (HR) with 95% confidence interval (CI) for incident type 2 diabetes was calculated using multivariate Cox proportional hazards regression models after adjusting for potentially confounding variables. The cumulative incidence of type 2 diabetes was represented using a Kaplan-Meier curve. Log-rank tests were conducted to determine the differences in the cumulative incidence of type 2 diabetes among the groups. All analyses were conducted with SAS 9.4 (SAS Institute Inc., Cary, NC, USA). All statistical tests were two-sided, and statistical significance was defined as p < 0.05.