1. Target population and survey methods
The study was a cross-sectional design, with the target population all being employees (n = 8,215) in a light metal product manufacturing company in Toyama Prefecture, Japan. In January 2020, we conducted a survey on job characteristics, health behaviors, and HL using a self-administered questionnaire. The study was part of a longitudinal survey, with all the subjects providing written, informed consent after being provided with a document describing the entire study program. Information that could identify an individual during processing of the information was converted into a pseudonym by a person in charge at the business, with the researchers receiving de-identified data for analysis.
The work schedule (shift work or fixed daytime work) and working hours per week were self-reported. The shift schedule in the target factory included a rotating 3-shift schedule and a 2-shift schedule. Two-thirds of the 3-shift workers had a non-continuous shift system (5-day shifts, 5-night shifts, and 5-evening shifts), while one-third had a continuous shift system (3 or 4-day shifts, 3 or 4-night shifts, and 3 or 4-evening shifts).
For personal attributes, we asked about educational history (junior high school graduate, high school graduate, vocational school graduate, junior college / technical college graduate, university graduate, or graduate school graduate) and marital status (married, single, or divorced/widowed).
As for health behaviors, we asked about the frequency of weekly exercise (no weekly exercise or sports, light exercise at least once a week, intense exercise once or twice a week for 20 minutes or more, intense exercise 3 times or more a week for 20 minutes or more), smoking history (currently smoking, quit smoking, or no smoking history), frequency of general food intake per day or week, and the frequency of eating breakfast (almost every day, 4–5 days a week, 2–3 days a week, almost never). We also asked about eating vegetables separately for green and yellow vegetables and other vegetables, and the daily frequency of tooth brushing (less than twice a day, or more). The Japanese version of HLS-EU-47 [16] was used to evaluate HL, which consisted of 47 items, and had a possible score ranging from 0 to 50 points. The higher the score, the higher the HL.
We received survey responses from 6,962 people (response rate, 84.7%). Of these respondents, 5,915 (72.0%; 4,058 men and 1,857 women) had no errors in the analysis parameters. In the target company, women rarely engaged in 3-shift work. Therefore, we enrolled 2152 male manual workers aged between 18–64 years (961 fixed day workers, 1191 shift workers) in the final analysis (Fig. 1).
2. Analytical methods
The following conditions were defined as favorable habits: "Doing leisure time exercise more than once a week (doing exercise)", "currently not-smoking (non-smoking)", "having breakfast almost every day (having breakfast)", "brushing teeth at least twice a day (brushing teeth)”, and “eating green and yellow vegetables at least once a day (eating vegetables)”.
Prior to the multivariate logistic regression analysis, the percentages of favorable habits (exercise, smoking, breakfast intake, frequency of tooth brushing, green and yellow vegetable intake) were compared between the two groups according to each of the following factors: work schedule, working hours, educational attainment, marital status, and HL group. The HL score was divided into four quartiles according to the European Health Literacy Project 2009–2012 [17]: 0–25 points for inadequate, > 25 to 33 points for problematic, > 33 to 42 points for sufficient, and > 42 to 50 points for excellent. Other factors considered were educational attainment (lower or higher than high school education), marital status (married or single) and working hours (less or more than 48 hours/week).
The odds ratios (ORs) of favorable habits in shift workers relative to those in daytime workers were then calculated using multivariate logistic regression analysis following adjustment for age, HL, educational attainment, marital status, and working hours. The ORs of favorable habits in the HL groups were also calculated after adjusting for age, work schedule, educational attainment, marital status, and working hours.
Logistic regression structural equation models were used to analyze the mediating role of HL in the relationship between shift work and health behaviors. Figure 2 shows a schematic presentation of the framework used to examine the relationship between shift work and health behaviors. The upper part shows the total effect (c) of shift work on health behaviors adjusted for age, marital status, educational attainment, and working hours. The lower part of the framework shows the mediation model of the indirect effect of HL (a, b), and the direct effect of shift work (c’) on health behaviors. The 95% confidence intervals were calculated based on 5000 bootstrap resamples.
IBM SPSS v.27 was used for the analysis and PROCESS ver. 4.3 of SPSS for the mediation analysis. The level of statistical significance was set at p = 0.05.
3. Ethical considerations
The study was approved by the Kanazawa Medical University Medical Research Ethics Review Board (approval number I431). In addition, the study was approved by a committee composed of representatives of the target company.