A cross-sectional study was conducted based on data from the KNHANES, 2007-2012; the KNHANES was conducted by the Korea Centers for Disease Control and Prevention. Specifically, a sample survey targeting the entire non-institutionalized civilian population was conducted with a complex, stratified, multistage, cluster-sampling design.¹⁰⁾ Subjects included in the KNHANES, 2007-2012 were 50,405 individuals aged ≥1 year. Of these individuals, 32,222 were 19 to 69 years. Subjects with no anthropometric data (n = 1,980) and pregnant women (n = 214) were excluded; this resulted in 30,028 subjects remaining (12,925 men and 17,103 women). Among the remaining subjects, 1,338 were underweight; however, 216 were excluded because they failed to complete all of the questionnaires. Therefore, 1,122 (148 men and 974 women) were included in the analysis. This study was approved by institutional review board of Samsung Medical Center (IRB file no.: 2014-07-012).

All anthropometric measurements and surveys were conducted in mobile examination centers by trained health professionals. The self-administered surveys included questions about health behaviors such as smoking, drinking, physical activity, and weight control. Data on socioeconomic status and history of chronic disease or cancer were obtained through face-to-face interviews.

Anthropometric measurements were taken while participants wore light clothing and no shoes. Height was measured to the nearest 0.1 cm (range, 6 to 230 cm) using a SECA-225 apparatus (SECA Deutschland, Hamburg, Germany). Weight was measured to the nearest 0.1 kg (≤200 kg) using a GL-6000-20 apparatus (CASKOREA, Seoul, Korea). BMI was calculated by dividing weight (kg) by the square of height (m²); an individual was considered underweight if there BMI was <18.5 kg/m².

The study population was divided into a weight loss attempt group and a non-attempt group; this was based on the questionnaires (e.g., weight loss attempt vs. maintain, gain, and no attempt during the past one year). Body self-perception was answered via five items (very thin, somewhat thin, average, somewhat obese, and very obese); these were classified into thin, average, or obese. Physical activity was calculated by the Korean version of the International Physical Activity Questionnaire (IPAQ) short form;¹³⁾ it was calculated as a continuous score (unit: metabolic equivalents [MET]-min/wk) and was classified into three categories. High physical activity was any one of the following 2 criteria (vigorousintensity activity on at least 3 days and accumulating at least 1,500 MET-min/wk or 7 or more days of any combination of walking, moderate-intensity or vigorous intensity activities achieving a minimum of at least 3,000 MET-min/wk). Moderate physical activity was any one of the following 3 criteria (3 or more days of vigorous activity of at least 20 minutes per day, 5 or more days of moderate-intensity activity and/or walking of at least 30 minutes per day, 5 or more days of any combination of walking, moderate-intensity or vigorous-intensity activities achieving a minimum of at least 600 MET-min/wk. Low physical activity was the lowest level not to meet above two criteria (IPAQ: http://www.ipaq.ki.se). Smoking was classified into current and noncurrent smokers. A calculated AUDIT (alcohol use disorders identification test) score was used to define alcohol use disorder; this was defined as a score of more than 16 points.¹⁴⁾ Marital status was classified into married, unmarried and other (divorced, separated, and widowed). Education level was classified into high (more than high school graduate) and low (less than middle school). Income was categorized by quartiles; subjects were classified as high or low. Occupation was classified by the Korean Standard Classification of Occupations revised in 2007; specifically, individuals were categorized as non-manual workers, manual workers, and other (students, housewives, and unemployed; soldiers were excluded). Chronic disease was diagnosed by a doctor and was defined as hypertension, dyslipidemia, diabetes, stroke, ischemic heart disease, arthritis, asthma, liver cirrhosis, and renal failure. Doctor-diagnosed cancers of the stomach, liver, colon, breast, cervix, lung, thyroid, and other organs were also reported. Individuals who had been previously diagnosed by a doctor as having any kind of cancer were considered as having a history of cancer.

Indicators related to mental health were defined as doctor-diagnosed depression, depressive symptoms lasting more than two consecutive weeks, suicidal ideation during the past year, any suicidal attempts during the past year, and the degree of stress awareness.

Complex samples analysis was used to analyze the KNHANES raw data; IBM SPSS ver. 21.0 (IBM Co., Armonk, NY, USA) was used. Data were expressed as mean ± standard error or percentage ± standard error. The unweighted number of each variable was not shown. Chi-square tests and Student t-tests were used to analyze every component by weight loss attempt. Logistic regression analysis was used to control for the significant factors known to influence suicidal ideation or depression. There was no cancer diagnosis in the weight loss attempt group; thus, it was subsequently excluded from the logistic regression model. Depressive symptoms are known to be a weighty factor in similar studies; therefore, depressive symptoms were added to model 2 for sensitivity analysis. Differences were considered to be statistically significant when the P-value was < 0.05. The prevalence of suicidal ideation in adults was about 15%¹⁰⁾ and the assumption of the control group was about 30%. Therefore, if alpha was 0.05, beta was 0.8, and the proportion between two groups was 0.1, adequate sample sizes were 667 for the non-attempt group and 67 for the attempt group.