2.1. Study participants
This cross-sectional study used data from the KNHANES, a nationwide survey of non-institutionalized civilian residents of South Korea that has been conducted by the Korean Ministry of Health and Welfare since 1998. Briefly, this nationwide representative study uses a stratified, multistage probability sampling design to select household units [9]. The KNHANES aims to evaluate the nutritional and health status of the general population and subsequently provide representative and reliable statistical public health data that can be used as a basis for health care policies.
This analysis was based on data obtained from 5,428 respondents to the 2014 survey who were aged ≥ 20 years. From this initial population, we excluded the following individuals for the following reasons: no information about outpatient visits during the most recent 2-week period (n=3,670); no visits to a primary care center (n=536); and missing data for the ninth question on the Patient Health Questionnaire (PHQ-9) (n=11). The present study included 1,211 Korean adults.
2.2. Study variables
The PHQ-9 is a reliable and valid screening tool used widely to detect depressive disorders, including suicidal ideation, in the general population [10]. The Korean version of PHQ-9, which is also considered reliable and valid, was included in the KNHNES VI–2 survey [11]. In this study, we defined a participant as having suicidal ideation if they provided a positive response (i.e., “yes”) to the ninth item on the PHQ-9, which asked “Have you ever been thought that you would be better off dead, or of hurting yourself in recent two weeks?”
Diseases that served as the causes for visits to primary care providers were identified via diagnostic information and had been recorded by physicians according to the Tenth International Classification of Diseases (ICD-10). The following codes and classifications were applied: A00–B99, Certain infectious and parasitic diseases; C00–C97, Neoplasms; E00–E90, Endocrine, nutritional and metabolic diseases; F00–F99, Mental and behavioral disorders; G00–G99, Diseases of the nervous system; H00–H59, Diseases of the eye and adnexa; H60–H95, Diseases of the ear and mastoid process; I00–I99, Diseases of the circulatory system; J00–J99, Diseases of the respiratory system; K00–K93, Diseases of the digestive system; L00–L99, Diseases of the skin and subcutaneous tissue; M00–M99, Diseases of the musculoskeletal system and connective tissue; N00–N99, Diseases of the genitourinary system; O00–O99, Pregnancy, childbirth and the puerperium; R00–R99, Symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified; S00–T98, Injury, poisoning and certain other consequences of external causes; and Z00–Z99, Factors influencing health status and contact with health services. We also identified self-reported underlying diseases, including hypertension, diabetes mellitus, dyslipidemia, stroke, cardiovascular disease (angina or myocardial infarction), depression, and cancer diagnosed by a medical doctor.
The potential confounders included age, gender, residence, household income, education, marital status, body mass index (BMI), current smoking, alcohol consumption, and physical activity. Residence was classified as urban or rural. Household income was classified into quartiles. Education status was classified into two categories: high school or lower and college or higher. Occupations were classified as follows: manager, professional or office worker; service or sales; manual worker and unemployed (e.g., housewife or student). Cohabitation status was classified as either living alone or living together. BMI was derived from the measured weight and height and reported in units of kg/m2. Obesity was defined as a BMI of ≥25 kg/m2 according to the current Asian obesity guidelines [12]. A current smoker was defined as someone who had smoked ≥ 100 cigarettes (5 packs) throughout his or her lifetime and continued to smoke at the time of the survey. A heavy drinker was defined as a person who consumed at least one bottle of soju (Korean distilled spirits) during a 1-week period. Regular physical activity was defined as participation in vigorous activity (e.g., running or mountaineering) for ≥75 min/week or moderate intensity activity (e.g., light swimming, badminton or walking) for ≥150 min/week [13]. Strength exercise was defined as the performance of exercise at least twice a week.
2.3. Statistical analysis
The survey procedures in SAS 9.2 (SAS Institute Inc., Cary, NC, USA) were applied to complex sampling designs. The characteristics of the study population are reported as means ± standard errors (SEs) for continuous variables and as weighted percentages (%) with SEs for categorical variables. We categorized participants into two groups according to suicidal ideation, using the ninth item of the PHQ-9. Differences in ICD-10 codes among the two groups were analyzed using a weighted t-test and chi-square test. Multiple logistic regression analyses were used to examine the potential associations of suicidal ideation with specific diseases after controlling for age, household income, education, occupation, marital status, smoking, alcohol habit and physical activity. Statistical significance was set at a two-sided p value <0.05.