The Health Examinees (HEXA) study is a part of the Korea Genome Epidemiology Study (KoGES) funded by Korea Centers for Disease Control and Prevention . National Health Insurance Corporation (NHIC) covered the entire Korean population for general health screening, and beneficiaries aged over 40 years can biannually receive the national health examination program . Participants of the HEXA study were prospectively recruited at the 38 health examination centers from 2004 to 2013 at 38 health examination centers and training hospitals located in 8 regions based on the infrastructural advantage of the national health checkup services funded by Korea Centers for Disease Control and Prevention .
Figure 1 shows the flow chart of the study population. We included HEXA participants aged 40 to 69 years in the analyses and restricted to the Health Examinees-Gem (HEXA-G) participants who were defined as follows: we excluded (1) 8 sites (n = 9,370) that only participated in the pilot study years 2004–2006, (2) 8 sites (n = 12,205) that did not meet the HEXA biospecimen quality control criteria (i.e., different testing protocols), and (3) 5 sites (n = 8,799) that had participated in the study for less than two years. A total of 139,348 participants were included in the HEXA-G data. Among HEXA-G participants, we excluded 1,391 participants who had no information on anthropometric measurements of height, weight, waist circumference, and hip circumference. An additional 3,762 participants had no information on blood pressure or biochemical measurements of the blood specimen, such as fasting glucose, triglyceride, and high-density lipoprotein. We conducted all analyses among the 134,195 participants who remained after exclusion.
Participants were interviewed by trained interviewers and responded to a structured questionnaire on general characteristics and past medical history. Biochemical assessments and anthropometric measurements were also conducted for all participants. Blood specimens were taken after 8 hours of fasting at enrollment and transported to the clinical laboratory for blood tests using plasma to examine the levels of glucose, triglyceride, and high-density lipoprotein. Height was measured using digital freestanding stadiometers (BSM, InBody Co, Seoul, Korea) with the participants' head in the Frankfort horizontal plane and read up to one decimal place. Weight was measured using digital scales (BSM, InBody Co, Seoul, Korea) in units of 10g. Waist and hip circumferences were obtained by a measuring tape of a horizontal plane and were read up to one decimal place. In detail, waist circumference was measured at the midpoint between the lower margin of the last palpable rib and the top of the iliac crest, and hip circumference was measured around the widest part of the buttocks.
Systolic and diastolic blood pressure was manually measured using the stethoscope and mercury sphygmomanometer on one arm in the sitting position according to the standard operating procedure by trained medical staff. Blood pressure was measured at least twice, and the second blood pressure measurement should be taken at least 1 minute after the first measurement. If the difference between the two records of blood pressure was more than five mmHg, additional measurements were taken until the last two records of blood pressure are similar. Then, the last two records of blood pressures were recorded. Blood pressure was measured in both arms and re-measured if the difference of blood pressure between both arms was more than ten mmHg. Blood pressure was measured on the other arm only if there were arm injury, previous breast surgery, the venous or arterial tube, or plaster bandage. We determined blood pressure as the average of the two readings.
Definition of terms
Body mass index, waist circumference, waist to hip ratio, and waist to height ratio were calculated using directly measured anthropometric values. Additionally, we considered the conicity index as a measure of central adiposity with the below equation .
Participants meeting at least one of the following criteria were considered to have the metabolic abnormality: defined hypertension , those who had systolic blood pressure higher than 140 mmHg, diastolic blood pressure higher than 90 mmHg, or those who reported taking antihypertensive medication; hyperglycemia , those who had a fasting blood glucose higher than 126 mg/dL or who reported taking antidiabetic medication; and dyslipidemia, those who had a triglyceride level higher than 150 mg/mL, high-density lipoprotein cholesterol lower than 40 mg/dL, or those who reported taking medication for dyslipidemia. Low-density lipoprotein cholesterol is commonly used for diagnoses of hyperglycemia, but low-density lipoprotein was not examined from blood samples. Therefore, we defined dyslipidemia using the level of triglyceride and high-density lipoprotein cholesterol.
We calculated the mean and standard deviation for demographic, anthropometric, blood pressure, and biochemical characteristics. The inclusion of a large population in this study would reduce the meaningfulness of statistical significance for differences in the general characteristics between sexes. For this reason, we did not present a p-value in the descriptive analysis of Tables 1 and 2. Receiver operating characteristic (ROC) curves were plotted for obesity indices to identify the best obesity index that discriminates the presence of metabolic abnormality. The area under the receiver operating characteristic curves (AUCs) was used as a summary measure of accuracy to evaluate the performance of obesity indices for the discrimination of participants with metabolic dysfunctions. Youden's J statistics  was used to determine the optimal cut-off values for the obesity indices. Youden's index was calculated using the below equation.: