Socioeconomic status and remaining teeth in Japan: results from the Toyama dementia survey

In Japan, the extension of a healthy life expectancy, healthcare, and disease prevention among elderly individuals are important issues. In humans, a decline in oral function can impact the entire body, leading to lower motor function and dementia [1]. In elderly individuals, a decline in the oral function may be attributed to conditions such as tooth loss, which is a consequence of other oral disorders, such as periodontal disease and caries. Accordingly, tooth loss has been described as a consequence of accumulation of dental diseases associated with aging.

Among elderly individuals, the proportion of those retaining ≥20 teeth at the age of 80 years has gradually increased from approximately 10% in the 1980s to approximately 50% in 2011. Although this increase in the remaining teeth among elderly individuals is encouraging, these teeth must be prevented from any harm due to periodontal disease. Over time, the percentage of individuals with a periodontal pocket with a depth of ≥4 mm has increased among individuals aged ≥65 years, despite declining trends among those aged < 64 years [2]. Furthermore, periodontal disease has been reported as the main cause of tooth loss among individuals aged ≥45 years [3]. Therefore, early prevention of periodontal disease is essential for preserving remaining teeth among elderly individuals.

In addition to the conditions related to oral hygiene (e.g., periodontal disease), lifestyle factors, such as eating habits, alcohol consumption, and smoking, as well as systemic diseases, such as diabetes, have been reported as risk factors for tooth loss [4]. Moreover, correlations of socioeconomic factors with periodontal disease and tooth loss have been reported [5‐7]. However, few studies have examined how socioeconomic status (SES) affects remaining teeth among elderly individuals in Japan [8]; furthermore, few studies have analyzed the association of remaining teeth with SES using an interaction term of educational and occupational histories [9]. Therefore, this study aimed to clarify the influence of SES on remaining teeth in Japanese elderly individuals. An understanding of this association could help identify strategies for avoiding the risk of tooth loss in elderly individuals.

The Toyama Dementia Survey was a random sample survey of elderly individuals aged ≥65 years living in the Toyama Prefecture, Japan. The survey screened elderly individuals for dementia and collected data on the use of appropriate dementia care. This cross-sectional survey was administered in 1985, 1990, 1996, 2001, and 2014. The present study was based on the results of the most recent survey, conducted in 2014 [10, 11].

Table 1 presents the demographic characteristics of the participants. The proportion of participants with no remaining teeth differed significantly depending on age; educational attainment level; occupation; smoking; history of diabetes, stroke, angina pectoris/cardiovascular disease, and osteoporosis.

Table 2 presents the effects of educational attainment level and occupation on the incidence of no remaining teeth based on an unadjusted analysis and an analysis adjusted for lifestyle variables and medical history. The proportion of individuals with no remaining teeth increased as SES decreased. Specifically, participants with an educational attainment level of ≤6 or 7–9 years were less likely than those with the level of ≥10 years to have remaining teeth, with age- and sex-adjusted ORs of 3.29 (95% CI: 1.90–5.71; model 1) and 1.57 (95% CI: 1.14–2.16; model 1), respectively. After adjusting the models for occupation, lifestyle factors, and medical history, the ORs for an educational attainment level of ≤6 years increased to 2.79–3.07 (models 3–5), whereas the ORs for the level of 7–9 years did not differ significantly from those for the level of ≥10 years.

Blue-collar workers were also less likely than white-collar workers to have remaining teeth, with an age- and sex-adjusted OR of 2.16 (95% CI 1.52–3.06; model 2). After adjusting the models for educational attainment level, lifestyle factors, and medical history as well, the ORs for a blue-collar job history increased to 1.89–1.93 (models 3–5). After adjusting for all potential confounders (model 5), a low educational attainment level, older age, current or former smoking habit, history of diabetes, and history of osteoporosis were found to be the factors that increase the risk of tooth loss. In contrast, current alcohol consumption was not found to increase this risk.

After adjusting for the interaction term of educational attainment level and occupational history in model 5, ORs of low educational attainment level and a blue-collar occupational history for remaining teeth hardly changed (3.08 and 1.79, respectively). Thus, the interaction term was not significant.

The present study found that a low SES, as indicated by a low educational attainment level (≤6 years) and blue-collar occupational history, was a risk factor for complete tooth loss in elderly individuals. The interaction term of a low educational attainment level and a blue-collar occupational history with tooth loss was not significant.

We also observed that an older age significantly increased the risk of complete tooth loss, which is attributable to the accumulation of dental diseases with age; in other words, elderly individuals are more likely to lose their teeth.

We further observed that a current and former smoking status were strong risk factors for tooth loss, consistent with the results of previous studies [15, 16]. Tobacco smoke contains approximately 5300 chemicals, of which at least 250 are harmful. These chemicals enter the blood circulation and can cause cancer, arteriosclerosis, tissue inflammation, and other disorders. Notably, the oral cavity is the organ first exposed to tobacco smoke. Smoking enhances the pathogenicity of oral cavity bacteria and simultaneously impairs immune responses and wound healing, leading to periodontal disease and caries. Furthermore, the chemicals contained in tobacco also suppress gingival bleeding and harden the gingiva. This can conceal the symptoms of periodontal disease and simultaneously reduce the peripheral blood flow, reducing the capacity for healing in the oral cavity. Thus, smoking is directly related to tooth loss. The Health Promotion Law prevents the risk of passive smoking exposure [17]. However, prior to the enactment of this law in 2002, smoking was more frequent and common in the workplace. This background may have led to the oral health deterioration observed in participants in this study. Conversely, this study found that a current drinking habit reduced the risk of complete tooth loss. Although previous studies have noted that excessive drinking is associated with periodontal disease [18, 19], the mechanism linking drinking to periodontal disease remains to be elucidated. Further studies of the association between alcohol consumption and tooth loss are warranted.

The finding that a significant association exists between diabetes and the absence of remaining teeth is consistent with the results of previous studies [20, 21]. The hyperglycemia associated with diabetes causes blood vascular damage and increases susceptibility to infection, including periodontal disease, which involves the entry of infectious bacteria into the gingiva and blood vessels. Furthermore, periodontal disease causes gingival inflammation, and the resulting secretion of inflammatory substances into blood circulation interferes with the ability of insulin to reduce blood glucose levels. Thus, diabetes and periodontal disease exert reciprocal negative effects. The results of the present study suggest that diabetes prevention and control can help prevent tooth loss. The study also revealed a significant association between osteoporosis and remaining teeth. Periodontal disease is likely to worsen in participants with osteoporosis because they have poor bone mass in oral cavity [22], which is shown by the results of this study. The prevalence of osteoporosis is particularly high in postmenopausal women. Thus, it is important to think about countermeasures by paying attention to osteoporosis when attempting prevention of tooth loss in people aged ≥65 years.

The present study further identified a blue-collar occupational history as a risk factor for tooth loss, consistent with a previous report that workers in skilled trades, manufacturing, sales, transportation, and communications face a higher risk of periodontal disease than those in professional and technical fields [23]. Similarly, transportation drivers have been reported to have a greater risk of tooth loss than white-collar workers [24]. Blue-collar workers tended to remain on work sites and to work in three-shift cycles, which may lead to irregular meal times and, consequently, irregular tooth brushing habits. Thus, the lifestyle related to blue-collar employment is more likely to lead to tooth loss.

Additionally, participants with an educational attainment level of ≤6 years had an approximately 2–3 fold greater risk of tooth loss than those with a level of ≥10, even after adjusting for age, sex, occupation, lifestyle, and medical history. Educational attainment level may affect lifestyle habits, as children of more highly educated parents have been reported to brush their teeth more frequently per day and to have fewer dental caries [25, 26], suggesting that highly educated families have better dental hygiene habits. The parental educational history may also influence the formation of a child’s teeth. Tooth development begins during early pregnancy and requires a balanced inoculation of protein, calcium, and various vitamins. A previous study has reported ingestion of a nutritionally balanced diet only among individuals with a high SES [27]. Therefore, a low household SES may affect the nutritional intake of the mother and the fetal tooth development, leading to less healthy teeth later in life. Furthermore, previous reports have suggested that individuals with a low educational attainment level do not tend to participate in medical checkups [28], whereas those with a low SES tend to be less able to maintain health [29]. Therefore, we speculated that individuals with a low SES are less likely to take countermeasures to address the risks of caries and periodontal disease, regardless of their access to relevant information.

Education is followed by an appropriate occupation; thus, a low educational attainment level has a higher influence on tooth loss than the occupational history [28, 30]. The oral cavity is the only internal organ that can be examined visually and treated directly. Although individuals may not notice the signs of early tooth decay and periodontal disease, dental plaque can be removed easily using a toothbrush or dental floss. Although an individual’s SES may affect the dental hygiene level and tooth strength, countermeasures that broaden one’s knowledge about oral care and enable the acquisition of good dental lifestyle habits (e.g., tooth brushing) may overcome the effects of a low SES. Furthermore, a social system that enables access to periodic dental examinations throughout a person’s lifetime and an environment supporting proper oral care are important.

However, this study has several limitations. First, the responses were collected retrospectively rather than ad hoc, which might have affected the accuracy of the data. Second, we may have selected participants who were relatively more cooperative and led relatively healthier lifestyles, which could have introduced selection bias. Third, the study design may have led to a risk of survival bias, as we were unable to collect data from individuals who died during the survey period. Fourth, we did not examine local factors, such as oral hygiene and dietary habits, and therefore, we may have overestimated the association between socioeconomic factors and tooth loss. Fifth, we did not collect the data on remaining teeth number, and it may have led to an insufficient analysis. Despite these limitations, our study meaningfully confirms that both socioeconomic factors and systemic illnesses related to tooth loss have an effect on the maintenance of remaining teeth.

The present study found that a low educational attainment level (≤6 years) and a blue-collar occupational history are independently associated with remaining teeth in an elderly Japanese population. Furthermore, in this population, educational attainment level had a relatively more important influence on the risk of tooth loss compared with the occupational history.