Association between the longest job and oral health: Japan Gerontological Evaluation Study project cross-sectional study

Data from a cross-sectional study, collected as part of the Japan Gerontological Evaluation Study (JAGES) Project, an on-going Japanese prospective cohort study, were used for this study. JAGES aims to conduct empirical studies from gerontological and social epidemiological perspectives. The sample was restricted to those who did not already have a physical or cognitive disability, defined by not receiving public long-term care insurance benefits, at baseline. From July 2010 to January 2012, a mail survey, exceptionally collected by visit in one prefecture, was conducted in a random sample of 169,215 community-dwelling individuals aged 65 years or over residing in 31 municipalities in 12 prefectures in Japan.

The questionnaire on oral health status and oral health behavior was sent to a random sample of 46,009 subjects, of which 27,732 (60.3%) responded. All subjects in one municipality were excluded because the questions used in the present study were not included in the questionnaire. After excluding 4,541 subjects who did not answer about the longest job held, a total of 23,191 subjects (11,310 males and 11,881 females) aged 65 or older in 30 municipalities were included in the present study.

Number of teeth, denture/bridge use and subjective oral health status were evaluated for oral health status, based on a self-administered questionnaire [14]. Respondents were asked to indicate the number of teeth as 20 or more, 10-19, 1-9 or 0. Denture/bridge use was ascertained by asking, “Do you use a denture or bridge?” with possible answers dichotomized into yes and no. Subjective oral health status was ascertained by asking, “How do you rate your oral health (teeth, gum and denture)?” with possible 4 grade answers dichotomized into good and poor.

Oral health behavior was evaluated based on dental visit for treatment and use of an interdental brush or a dental floss. Dental visit for treatment was ascertained by asking, “Did you visit a dentist for treatment (including adjustment of a denture) within 6 months?” with possible answers dichotomized into yes and no. Use of an interdental brush or a dental floss was ascertained by asking, “Do you use an interdental brush or dental floss?” with possible answers dichotomized into yes and no.

Information on the longest job held was used as an explanatory variable and ascertained by asking, “What was the job that you had done for most of your working life?” with possible answers of professional/technical, administrative, clerical, sales/service, skilled/labor, and agriculture/forestry/fishery workers, others and no occupation.

Data on socio-demographics (sex, age, educational attainment, equivalent income) were obtained using a self-administered questionnaire and used as covariates. To adjust household income for household size, equivalent income was calculated by dividing the household income by the square root of the number of household members, and placed into one of seven categories (less than US$5,000, US$5,000-9,999, US$10,000-14,999, US$15,000-19,999, US$20,000-29,999, US$30,000-39,999, and US$4,000,000 or higher) (US$1 = 100 Japanese yen) [15].

Data on the number of dentists working in hospitals or clinics were obtained from the Survey of Physicians, Dentists and Pharmacists conducted by the Ministry of Health, Labour and Welfare, Japan in 2010. Data on population in 2010 and area of inhabitable land of each municipality (city, town or village) were obtained from the National Population Census Survey conducted by the Ministry of Internal Affairs and Communications, Japan. The number of dentists working in hospitals or clinics per 100,000 people (density of dentists) and population density were calculated for each municipality. The density of dentists was categorized into four groups (lowest, low middle, high middle, or highest) based on 25th, 50th, and 75th percentiles. Population density was categorized into four groups (metropolitan, urban, semi-urban, or rural-agricultural).

The percentage of respondents with poor oral health status and poor oral health behavior were calculated for each longest job in each sex. Two-level (first level: individual, second level: municipality) multilevel Poisson regression models with random intercepts and fixed slopes were used separately for males and females to calculate multilevel prevalence ratios (PRs) for the longest job after adjusting for age, educational attainment and equivalent income as individual-level variables and densities of dentists and population as municipality-level variables, with each variable of oral health status or oral health behavior as the outcome. In each model, subjects lacking a outcome variable were excluded from the analysis. However, covariates that included missing data were recorded by reassigning missing values to separate missing categories in order to maximize the number of subjects included in the statistical analysis and thereby maximize statistical power: data lacking information of educational attainment and equivalent income were included as “others or data missing” (the category of missing was combined with that of others) and “data missing” categories, respectively. Moreover, subjects with 20 or more teeth and those with 9 or less teeth were excluded in the models in which denture/bridge use and use of interdental brush/dental floss were used as outcome variables, respectively, because some subjects with 20 or more teeth and those with 9 or less teeth might not need to use denture/bridge and interdental brush/dental floss, respectively. All statistical analyses were performed using IBM SPSS Statistics 21 (International Business Machines Co., New York, NY, USA) and MLwiN 2.28 (Centre for Multilevel Modelling, University of Bristol, Bristol, UK).

A detailed explanation of the objectives of the JAGES was sent by mail along with the self-administered questionnaire. People who were willing to participate in the study voluntarily completed and mailed back the questionnaire. The JAGES protocol and its informed consent procedure were reviewed and approved by the Ethics Committee on Research of Human Subjects at Nihon Fukushi University.