The subjects of the Yamagata study are participants of annual specific health checkup programs in seven cities in Yamagata Prefecture (Yamagata, Sakata, Kaminoyama, Tendo, Sagae, Higashine, and Yonezawa). Details of the Yamagata study have been described elsewhere . Briefly, the Yamagata Study was a community-based prospective cohort study that was a component of a molecular epidemiological study that utilized the regional characteristics of a 21st Century Center of Excellence (COE) program and the Global COE program in Japan. The target population of this study is the local population covered by national health insurance. National health insurance covers about 30% of the Japanese population, and the insured subjects are mainly agriculture, forestry and fisheries workers, self-employed, part-time workers, retirees, and unemployed. The number of potential subjects was 28,528 in this study. A total of 19,231 subjects aged 40 to 74 years provided consent to participate in the baseline survey of the Yamagata Study between 2009 to 2015. Of the 19,231 who filled out the questionnaire for the Yamagata study, 2,580 subjects who had missing answers in social support components and essential clinical information, including smoking, alcohol consumption, and medication, were excluded. The remaining 16,651 subjects (6,797 males and 9,854 females) were included in the final analysis of this study. The follow-up period was from 2009 to the end of 2015.
A self-administered questionnaire and reply envelope were distributed to the study participants at the specific health checkup site and returned by postal mail. When distributing the questionnaire, we explained the purpose of the survey, the voluntary nature of participation, and the protection of personal information, and the participants then gave written informed consent. This study was approved by the Ethics Review Committee of the Faculty of the Medical Department of Yamagata University (approval number 2018-464) and was conducted based on the Declaration of Helsinki. The baseline questionnaire gathered information on social support using the following five questions because a previous study showed that these questions and answers were associated with depression in the Japanese population : Do you have someone 1) whom you can consult when you are in trouble? 2) whom you can consult when your physical condition is not good? 3) who can help you in daily housework? 4) who can take you to a hospital when you don't feel well? and 5) who can take care of you when you are ill in bed? The answer to each question was binary (yes/no).
In addition, basic information on smoking, alcohol consumption, cohabitation, and education history were collected. In the Japanese education system, compulsory education in elementary and junior high school is 9 years, up to high school, it is 10-12 years, and in college or beyond, it is 13 years or more. All citizens in Japan are required to have compulsory education, so it is unlikely that they have no formal education. Therefore, in this study, education history was divided into three groups of 9 years or less, 10 to 12 years, and 13 years or more. For lifestyle-related diseases, information on the presence or absence of hypertension, diabetes, and dyslipidemia were collected. Hypertension was defined as a systolic blood pressure of 140 mmHg or more or diastolic blood pressure of 90 mmHg or more at health checkup site, or use of antihypertensive medication. Diabetes was defined as a fasting blood sugar level of 126 mg/dL or more, HbA1c (NGSP value) of 6.5% or more, or use of anti-diabetic medication. Dyslipidemia was defined as triglyceride of 150 mg/dL or more, HDL cholesterol less than 40 mg/dL, LDL cholesterol 140 mg/dL or more, or use of lipid-lowering medication. Obesity was defined as a body mass index (BMI) of 25 kg/m2 or more.
Classification of cause of death
Information on mortality was obtained from the death certificate. Cause of death was classified based on the International Statistical Classification of Diseases and Related Health Problems 10th Revision (ICD-10) code. Cardiovascular mortality was defined as the deaths due to the circulatory system (ICD-10 code I00-I99), such as acute myocardial infarction (I21), chronic ischemic heart disease (I25), cardiomyopathy (I42), heart failure (I50), subarachnoid hemorrhage (I60), intracerebral hemorrhage (I61) and cerebral infarction (I63). The cardiovascular disease was selected because previous studies had reported its association with social support [8, 10].
Factors related to the components of social support were evaluated by logistic regression analysis. Survival analysis to examine the association between social support components and all-cause and cardiovascular mortality was performed using the Kaplan-Meier method. Independent associations of social support components with mortality were examined by Cox proportional hazards with adjustment for possible confounding factors including age, gender, education period, smoking, alcohol consumption, obesity, hypertension, diabetes, and dyslipidemia. A p-value of less than 0.05 was considered statistically significant. All analyses were performed using the statistical software JMP 14.2 for Windows (SAS Institute Japan Ltd., Tokyo, Japan)