- Study design
This study was conducted by recall of personal values in adolescence, using longitudinal data from the Japanese Study on Stratification, Health, Income, and Neighborhood (J-SHINE), which has been described in detail elsewhere (25). It was designed to investigate the complex associations between social factors and health. The ethics committee of the Graduate School of Medicine and Faculty of Medicine, The University of Tokyo approved this study. Consent for participation in the study and the agreement for publication was indicated by completing and returning the self-administered questionnaire. Additional written informed consent was obtained for participants who undertook physiological measurements and serum samplings.
- Study period and participants
J-SHINE data was collected at three points: between July 2010 and February 2011 (wave 1), July and December 2012 (wave 2), and October 2017 (wave 3). Target participants were randomly extracted from the Basic Resident Register of adults aged 25–50 years old from four municipalities in Japan (two in the Tokyo metropolitan area and two in neighboring prefectures). Wave 1 sample sizes were 4357 (valid response rate: 31.3% = valid data (n=4357) / originally selected sample (n=13920); cooperation rate: 51.8% = valid data (n=4357) / accessible sample (n=8408)). Wave 2 captured n=2961, 69.0% of wave 1 participants. Researchers invited all 2961, who answered the wave 2 survey, to undergo physical measurement and serum sampling using a self-administered finger-prick blood sampling kit (Demecal Kit; Leisure Inc., Tokyo, Japan). A subpopulation that underwent biomarker measurements was 1205. N=2787 answered the questionnaire on Wave 3 (response rate = 64.9%). We used the data from waves 2 and 3. People who agreed to participate in the study completed a self-administered questionnaire using a computer-aided personal instrument (CAPI). People who were unfamiliar with computers sat for a personal interview according to the CAPI. Eligible participants offered physical data and a serum sample in the wave 2 survey and completed a questionnaire about personal values in adolescence in wave 3. Those who had incomplete data for these variables and confounders (age and education) in 2012 were excluded. The flowchart of participant recruitment is shown in Figure 1.
(1) Exposure -The Personal Value in Adolescence-
Personal values in adolescence was developed based on Schwartz’s theory of 10 basic values (21, 26). Although a scale for measuring personal values in Schwartz’s theory was already developed, the scale has been used primarily for adults and not adolescents. The terms and constructs should be locally meaningful for adolescents (27). A research article that used this scale has already been published (24).
Personal values contain two components: value priorities and commitment to the values. Value priorities were measured by 11 items: economically succeeding, improving society, exploring interests, influencing society, actively challenging, cherishing familiar people, graduating from a famous school, and maintaining a stable life. These items are rated on a seven-point Likert scale (1=Not at all to 7=Very important) following the question, “When you were 15-16 years old, how important did you think the following values were in your life?” Test-retest reliability was acceptable (0.556 to 0.729) when examined in another dataset in a 1-month interval, except for the value of “Cherishing familiar people (0.372).” Cronbach’s alphas in the same sample ranged from 0.540 to 0.842. These 11 items can be possibly redefined into Schwartz’s four dimensions: Self-transcendence (universalism, benevolence); "Improving society" and "Cherishing familiar people". Conservation (conformity, tradition, security); "Not bothering others" and "Maintaining a stable life"; Openness to change (self-direction, stimulation, hedonism); "Having and keeping a belief", "Exploring what you were interested in" and "Actively challenging"; Self-enhancement (hedonism, achievement, power); "Being evaluated by others", "Economically succeeding", "Having influence on society" and "Graduating from a famous school".
Commitment to the values was measured by the Personal Values Questionnaire II (PVQ-II) (22). The Japanese PVQ-II consists of eight items (e.g., How committed are you to living this value?). Each item is rated on a five-point Likert scale. The internal consistency, concurrent, and structural validity have already been confirmed (28). In this study, we changed the items from the present to the past tense and instructed the participants to answer the items they had considered the most important when they were 15-16 years old. The sum of the PVQ-II scores was used for analysis; higher scores indicate greater commitment to the values. Test-retest reliability in 2 weeks in another dataset was 0.742, and Cronbach’s alpha was 0.851.
(2) Outcomes -Biomarkers and physical health index-
Primary outcomes in this study were important risk factors related to MetS and lifestyle-related disease. Venous samples were collected for low-density lipoprotein (LDL) cholesterol (mg/dL), high-density lipoprotein (HDL) cholesterol (mg/dL) and HbA1c (%, NGSP).
Body mass index (BMI, kg/m2) was calculated using the formula; weight (in kilograms) divided by height (in centimeters). Waist circumference (cm) was measured by a tape measure at the level of the umbilicus. Auscultatory systolic and diastolic blood pressure (mmHg) were assessed with a mercury column before and one minute after standing three times. We took three measurements and used the average.
(3) Sociodemographic characteristics
Sociodemographic variables included age and educational status (less than junior high school diploma, high school diploma, some college, more than university degree). These two covariates were used for adjusting in statistical analyses (partial correlation analysis and multiple linear regression analysis).
- Statistical Analysis
First, partial correlation coefficients between the personal values in adolescence and the health outcomes in adulthood were calculated, adjusting age, educational status, marital status, working status, household income, and all other variables of personal values and biomarkers related to MetS. Second, to examine overall associations between personal values and biomarkers of MetS, a standardized score of a proxy of MetS (Z-score) was calculated by summing up standardized scores of each Mets-related marker, higher scores indicating adverse conditions. We used the Z-score as a dependent variable of multiple linear regression analysis. Statistical significance was defined as p<0.05. All of the statistical analyses were performed using SPSS 26.0, Japanese version.
- Post-hoc statistical power calculation
After conducting the multiple regression analysis, we calculated the post-hoc statistical power (1-beta) for the significant effects. Among men, the smallest and significant effect size was 0.015 increase of R2. As a result, the estimated post-hoc power (1-beta) was 0.51 when the total sample size was 261, if the effect size f2 was 0.015, assuming that the alpha was less than 0.05 (two-tailed), using the G*Power 3 program (29, 30).