1. Introduction

Subjective well-being (SWB) is synonymous with self-rated happiness: being satisfied with one’s life, having a positive effect, and having little negative effect.[1] Although it could be said that SWB is subjective, the link between SWB and general health has been surveyed.[1,2] Several researchers have revealed that a low level of SWB is not only a result of poor health but also a potential risk factor for disease.[3–7] Furthermore, a high level of SWB has the potential to lower mortality compared with low levels of SWB.[4,8–11] Currently, high levels of SWB seem to positively affect general health.

The risk factors for low levels of SWB have also been surveyed. Poor psychological and physical condition are among the most well-known risk factors for low levels of SWB.[12] Furthermore, socio-ecological factors, such as low income and low educational status[13,14]; lifestyle-associated factors, such as poor sleep quality,[15] unhealthy dietary habits,[14,16,17] insufficient exercise,[13] smoking and alcohol habits,[18] and marital status[19] are well-known risk factors for low levels of SWB. Furthermore, self-rated health, which is also a subjective indicator, has been shown to be positively correlated with SWB.[20] In addition to these risk factors for low levels of SWB, associations between oral health and SWB have also been reported. The presence of dental caries, low frequency of dental-care utilization, malocclusion, and a high score of a subjective oral health assessment using the 14-item oral health impact profile were also well-reported risk factors for low levels of SWB.[21–24] Furthermore, in 2022, one of the largest studies to survey the associations between the number of remaining teeth and dental prosthesis use with SWB in 178,090 older people in Japan was reported.[20] This study revealed that having a higher number of teeth and using a dental prosthesis were factors independently associated with high levels of SWB.[20] However, the target population of these studies were adolescents or relatively older people aged > 65 years. To our knowledge, there have been no studies on the association between SWB and oral health targeting people of a wide age range in the general population. Moreover, to the best of our knowledge, no study has surveyed the association between SWB and the combination of the number of remaining teeth and self-rated mastication. Ideally, using a prosthesis with fewer teeth means restoration of masticatory function is performed well even if the number of remaining teeth is low; however, most dental clinicians know that cases of prosthetic incompatibility do occur. Using prostheses does not always mean good mastication. Therefore, we conducted a cross-sectional study, using data from the Yamagata community-based cohort study, to comprehensively identify the risk factors for low levels of SWB, focusing on the associations between SWB and the combination of the number of remaining teeth and self-rated mastication. The present study hypothesized that an association between low levels of SWB and fewer remaining teeth with poor self-rated mastication. The additional hypothesis of the present study was that a significant association exists between low levels of SWB and fewer remaining teeth with good self-rated mastication, which may suggest an appropriate restoration of masticatory function using some form of prosthesis, would not be confirmed.

2. Materials and methods 2.1. Ethical approval

All procedures performed in this study involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and the Declaration of Helsinki (1964) and its later amendments or comparable ethical standards. The ethics committee of Yamagata University approved the study protocol (2022-106).

2.2. Study design and participants

This study was performed as part of an ongoing molecular epidemiological study utilizing the Regional Characteristics of the 21st Century Centers of Excellence Program in Japan. This study was community-based and included a baseline survey using a self-administered questionnaire. The general population aged between 40 and 79 years dwelling in the Yamagata prefecture, Japan, was the target population. A postal self-administered questionnaire survey was conducted from 2017 to 2021 among 8783 individuals from the Yamagata prefecture. The questionnaires were completed by 7447 individuals (2558 men, 4889 women; response rate: 84.8%); however, 601 were excluded due to incomplete data on the number of teeth, self-rated mastication, or SWB. A total of 6846 participants (2332 men, 4514 women) were included in the final statistical analysis.

2.3. Measurements

The self-reported questionnaire contained a survey on lifestyle, medical history, oral health, and dietary intake. SWB was assessed using a single-item question: “How happy are you?” and 4 possible answers were provided: very happy, happy, neutral, and unhappy. The number of remaining teeth and self-rated mastication were also assessed via single-item questions, respectively: “How many teeth do you have? (Restored teeth and post crown are counted but dental implants are not),” and “Can you bite tightly on both sides with your teeth or prostheses?” Regarding self-rated mastication, 3 possible answers were provided: can bite tightly on both sides, can bite tightly on 1 side, and can bite tightly on neither side. The answers to the questions on number of teeth and self-rated mastication were combined and the participants were classified into 4 groups: having ≥ 20 teeth and can bite tightly on both sides, having ≥ 20 teeth and cannot bite tightly on both sides, having < 20 teeth and can bite tightly on both sides, and having < 20 teeth and cannot bite tightly on both sides.

Additional covariates, other than the number of teeth and self-rated mastication, were derived from a similar previous study which investigated the risk factors for low levels of SWB. That study had the largest sample size in Japan and was reported in 2022.[20] The additional covariates were sex, age, marital status, educational status, self-rated health status, household income, and feelings of depression. Regarding marital status, 6 possible answers were provided: married, never married, divorced, separated, widowed, and others. Three possible answers were provided regarding educational status: university graduate or above, high school/junior college/professional training college/dropped out of university, and junior high school graduate. Self-rated health was assessed using the single-item question: “On the whole, how would you rate your general health in this last month?” with 5 possible answers; namely, excellent, very good, good, fair, and poor. Six possible answers were provided to describe the household income: > 15 million yen, 12 to 15 million yen, 9 to 12 million yen, 6 to 9 million yen, 3 to 6 million yen, and < 3 million yen. To assess mental health, we asked about the frequency of feelings of depression in the last week, and 4 possible answers were provided: never, occasionally, often, and always.

2.4. Statistical analyses

The distribution of characteristics was analyzed using the chi-squared test and the Kruskal–Wallis test for quantitative and qualitative variables. To determine the association between SWB and several variables, an ordinal logistic regression model was used. First, crude odds ratios and 95% confidence intervals for the risk of low levels of SWB (categorized as very happy, happy, neutral, and unhappy) were calculated. Second, adjusted odds ratios and 95% confidence intervals were calculated by the multivariate ordinal logistic regression analysis using the significant variables from the univariate analysis (P < .05). Statistical significance was set at P < .05. All statistical analyses were performed using SPSS version 25.0 (IBM Corp., Armonk, NY).

3. Results

Table 1 shows the distribution of the characteristics of the participants according to the groups based on SWB (very happy, happy, neutral, and unhappy). A significant distribution between each group was found for the following parameters: the combination of the number of remaining teeth and self-rated mastication, sex, marital status, self-rated health, household income, and feelings of depression.

Table 1 - Participant characteristics. Variable SWB P value† Very happy Happy Neutral Unhappy Number of remaining teeth and self-rated mastication, N (%)  ≥20 and can bite tightly bilaterally 844 (72.7) 2978 (68.1) 756 (61.6) 51 (60.0) <.001 *  ≥20 and cannot bite tightly bilaterally 96 (8.3) 425 (9.7) 149 (12.1) 15 (17.6)  <20 and can bite tightly bilaterally 167 (14.4) 682 (15.6) 205 (16.7) 13 (15.3)  <20 and cannot bite tightly bilaterally 54 (4.7) 288 (6.6) 117 (9.5) 6 (7.1) Sex, N (%)  Male 353 (30.0) 1474 (33.7) 466 (38.0) 39 (45.9) <.001 *  Female 808 (70.0) 2899 (66.3) 761 (62.0) 46 (54.1) Age, median (lowest, highest), yr 69 (40, 75) 68 (46,76) 68 (42,76) 67 (46,75) .250 Marital status, N (%)  Married 1001 (87.0) 3680 (84.9) 877 (72.2) 39 (46.4) <.001 *  Never married 24 (2.1) 138 (3.2) 114 (9.4) 21 (25.0)  Divorced 33 (2.9) 133 (3.1) 66 (5.4) 9 (10.7)  Separated 3 (0.3) 18 (0.4) 13 (1.1) 2 (2.4)  Widowed 83 (7.2) 354 (8.2) 140 (11.5) 13 (15.5)  Other 7 (0.6) 12 (0.3) 4 (0.3) 0 (0.0) Educational status, N (%)  University graduate or above 132 (11.5) 458 (10.6) 105 (8.7) 9 (10.6) .200  High school/junior college/professional training college/dropped out of university 915 (79.5) 3474 (80.1) 972 (80.3) 70 (82.4)  Junior high school graduate. 104 (9.0) 407 (9.4) 133 (11.0) 6 (7.1) Self-rated health, N (%)  Excellent 187 (16.2) 183 (4.2) 19 (1.6) 2 (2.4) <.001 *  Very good 264 (22.9) 900 (20.8) 181 (14.9) 8 (9.6)  Good 588 (51.0) 2654 (61.3) 672 (55.4) 29 (34.9)  Fair 94 (8.2) 550 (12.7) 314 (25.9) 33 (39.8)  Poor 19 (1.6) 44 (1.0) 28 (2.3) 11 (13.3) Household income, N (%)  >1500 million yen 30 (2.8) 53 (1.3) 8 (0.7) 0 (0.0) <.001 *  1200–1500 million yen 24 (2.2) 79 (1.9) 7 (0.6) 0 (0.0)  900–1200 million yen 72 (6.6) 185 (4.5) 33 (2.9) 2 (2.5)  600–900 million yen 162 (14.9) 512 (12.6) 86 (7.6) 4 (5.0)  300–600 million yen 432 (39.8) 1599 (39.3) 380 (33.4) 18 (22.5)  <300 million yen 366 (33.7) 1642 (40.3) 625 (54.9) 56 (70.0) Feelings of depression, N (%)  Never 914 (80.5) 2793 (65.8) 445 (37.5) 9 (10.8) <.001 *  Occasionally 201 (17.7) 1304 (30.7) 550 (46.3) 25 (30.1)  Often 19 (1.7) 111 (2.6) 144 (12.1) 24 (28.9)  Always 1 (0.1) 35 (0.8) 48 (4.0) 25 (30.1)

SWB = subjective well-being.

*Statistically significant (P<.05).

†P value based on the chi-squared test or Kruskal–Wallis Test.

Table 2 shows the significant variables in the univariate and multivariate ordinal logistic regression analysis of the risk of poor SWB. All the parameters, namely, the combinations of teeth numbers and self-rated mastication, sex, age, marital status, educational status, self-rated health, household income, and feelings of depression were found to be significant. Statistical significance was observed for all parameters, except for age, in the multivariate logistic regression analysis. Specifically, individuals who had < 20 teeth and could not bite tightly on both sides (i.e., had poor mastication ability) had a 1.300-fold risk for low levels of SWB compared with individuals who had ≥ 20 teeth and could bite tightly on both sides (i.e., had good mastication ability). Women were at lower risk of low levels of SWB than men. Compared with married individuals, individuals who had never married had a 3.732-fold risk of poor self-rated happiness. Divorced, separated, and widowed status were also risk factors for low levels of SWB. Moderate educational status, meaning graduation from high school, junior college, or professional training college, or dropping out of university, were risk factors for a low level of SWB compared with high educational status, meaning a university graduate degree or above. Compared with individuals who rated their health as “excellent,” those who rated it as “very good,” “good,” “fair,” or “poor” had a significant risk of low levels of SWB. A household income of < 3 million yen or 3 to 6 million yen was a significant risk factor for low levels of SWB, compared with an income > 15 million yen. The frequencies of feelings of depression in the previous week rated as “occasionally,” “often,” and “always” were risk factors compared with a frequency of “never.”

Table 2 - Crude and adjusted odds ratios and 95% confidence intervals of the variables associated with poor SWB. Poor SWB (very happy, happy, neutral, and unhappy) Crude OR (95% CI) P value Adjusted OR (95% CI)† P value Teeth numbers and sefl-rated mastication  ≥20 and can bite tightly bilaterally 1 1  ≥20 and cannot bite tightly bilaterally 1.451 (1.232–1.706) <.001 * 1.181 (0.986–1.412) .071   < 20 and can bite tightly bilaterally 1.208 (1.054–1.383) .006 * 1.114 (0.952–1.303) .176   < 20 and cannot bite tightly bilaterally 1.687 (1.392–2.044) <.001 * 1.300 (1.043–1.621) .020 * Sex  Male 1 1  Female 0.799 (0.721–0.884) <.001 * 0.628 (0.557–0.707) <.001 * Age 0.993 (0.986–1.000) .049 * 1.002 (0.994–1.011) .583 Marital status  Married 1 1  Never married 4.092 (3.258–5.145) <.001 * 3.732 (2.878–4.845) <.001 *  Divorced 2.002 (1.545–2.593) <.001 * 1.530 (1.145–2.044) .004 *  Separation 3.469 (1.837–6.547) <.001 * 2.627 (1.327–5.197) .006 *  Widowed 1.610 (1.354–1.912) <.001 * 1.557 (1.276–1.900) <.001 *  Other 0.651 (0.284–1.487) .308 0.573 (0.236–1.392) .219 Educational status  University graduate or above 1 1  High school/junior college/professional training college/dropped out of university 1.178 (1.004–1.381) .044 * 1.332 (1.120–1.586) .001 *  Junior high school graduate. 1.302 (1.048–1.616) .017 * 1.185 (0.925–1.520) .179 Self-rated health  Excellent 1 1  Very good 3.615 (2.886–4.527) <.001 * 3.529 (2.759–4.513) <.001 *  Good 4.860 (3.947–5.983) <.001 * 4.879 (3.881–6.135) <.001 *  Fair 11.078 (8.732–14.055) <.001 * 6.482 (4.973–8.457) <.001 *  Poor 10.751 (6.959–16.610) <.001 * 6.580 (4.043–10.708) <.001 * Household income (million yen)  >15 1 . 1  12–15 1.384 (0.795–2.411) .251 1.344 (0.740–2.442) .332  9–12 1.481 (0.927–2.370) .100 1.314 (0.795–2.171) .287  6–9 1.664 (1.079–2.563) .021 * 1.553 (0.976–2.469) .063  3–6 2.177 (1.436–3.300) <.001 * 1.898 (1.213–2.971) .005 *  <3 3.456 (2.280–5.238) <.001 * 2.754 (1.756–4.319) <.001 * Feelings of depression  Never 1 1 .  Occasionally 2.889 (2.578–3.238) <.001 *