Background
The concern over mental health as an essential factor of the global burden of disease in public health has risen [
1,
2]. Mental health is closely related to aspects of physical health such as cardiovascular disease [
3] and diabetes [
4] worldwide. In order to develop effective public health and clinical interventions for mental health, it is necessary to better understand the mechanisms that underlie interventions between mental health and other health conditions [
5]. The association between mental health and oral health as a part of physical health has been examined [
6,
7]. For example, patients with mental illnesses had a higher proportion of oral problems such as dental caries, periodontal disease, and edentulousness compared to a general population [
8‐
10].
Kisely et al. [
8,
9] suggested that oral health interventions for patients with mental illnesses should include advice on lifestyle habits and oral hygiene, the management of iatrogenic dry mouth, and early dental referral. It is important to develop and evaluate interventions that will increase the utilization of dental care (particularly preventive dental services) among people with mental illness in order to improve their oral health and reduce the dental expenditures among this vulnerable population [
11].
Oral health-related quality of life (OHQoL), which is closely related to oral health status [
12‐
16], is important for the management of oral health [
17,
18]. Several studies reported the oral health status in patients with mental illnesses in psychiatric clinics [
8,
9,
12,
13,
19], but few studies have investigated the oral health status of individuals in a general population who have or may have poor mental health. It has been proposed that public health interventions for psychological distress (which is not classified as mental illness) among individuals in general populations are important [
20,
21].
Psychological distress was shown to be related to negative impacts on the number of teeth and oral health-related quality of life (OHQoL) in an older general population using self-reports [
22]. A population-based cross-sectional study demonstrated that depression is associated with poor dental health evaluated by questionnaires [
23]. The World Health Organization (WHO) recommended the approach of providing a program for the continuous improvement of oral health as an essential factor of general health that is related to quality of life [
24]. The Japan Dental Association stated that good oral health is related to the extension of healthy life expectancy in Japan’s super-aging society [
25]. Therefore, a clarification of the relationship between psychological distress and oral health status or OHQoL is a considerable task in public health. We hypothesized that psychological distress is closely related with poor oral health status as well as low OHQoL even in a general population. In the present study, we evaluated the participants’ oral health status by using questionnaires as well as dental examinations to clarify the precise associations between mental health status and oral health status/OHQoL in a general population.
We have conducted a community-based prospective cohort study (the Nagasaki Islands Study) that involves the results of an annual medical health check-up in Goto City, Nagasaki Prefecture, in the western part of Japan [
26,
27]. The municipal government of Goto City has been promoting medical examinations of community-dwelling adults for screening and treating non-communicable diseases under Japan’s Ministry of Health, Labor and Welfare. As an additional investigation in the Nagasaki Islands Study, we evaluated the participants’ psychological distress and OHQoL by questionnaires and performed dental examinations.
The purpose of the present study was to clarify the association of psychological distress and oral health status with OHQoL in Japanese community-dwelling individuals, using data from the Nagasaki Islands Study.
Discussion
This cross-sectional study aimed to clarify the association of psychological distress and oral health status with OHQoL in Japanese community-dwelling individuals by using data from the Nagasaki Islands Study. The results of our analyses demonstrated that psychological distress as defined by a K6 score ≥ 5 points and OHQoL defined by the GOHAI were strongly related in this community-dwelling population.
We observed that high psychological distress reflected by the K6 score was associated with female gender, the lack of habitual exercise, no social participation with neighbors, falling within the prior year, and low OHQoL as shown by the GOHAI. In previous studies, female gender, falling, and poor social participation were also related to psychological distress [
37‐
40]. The new knowledge gained in the present study is that low OHQoL was closely related to psychological distress in a community setting.
Poor mental health status has been reported to be associated with both poor oral health status and low OHQoL [
12,
13]. A population with mental illness was reported to be likely to have more risk factors related to poor oral health (e.g., lack of motivation to engage in oral hygiene behaviors and adverse effects of medications [mainly xerostomia]) compared to the general population [
10]. Our present findings revealed that the lack of social participation with neighbors was strongly related to the high psychological distress indexed by the K6. This result is compatible with a study that showed a positive association between social participation and mental and physical health [
39].
Moreover, the mPPD and the mCAL values in the present study tended to be higher in the high-K6 group compared to the low-K6 group, although the differences were not significant. We have speculated that high psychological distress may be associated with a reduction of daily activities such as both social participation and oral hygiene behaviors in community settings. Previous studies showed that oral health problems including periodontitis may play a role in the development or worsening of depressive symptoms [
41,
42]. Considering our present findings, it appears that clinicians should pay attention to their patients’ psychogenic background in the management of oral health care and OHQoL for the optimal oral health care of individuals with high psychological distress, even in the general population.
The results of our analyses demonstrated that the factors contributing to poor OHQoL as shown by the GOHAI were female gender, high psychological distress reflected by the K6, the total number of teeth, the number of dental caries, and having visited a dental clinic within the previous 6 months. Unsurprisingly, dental conditions are closely correlated with the OHQoL scores [
12‐
14,
16]. In the present study, notably, psychological distress as revealed by the K6 was a substantial contributing factor to OHQoL. The previous reports proposed that the patient’s psychogenic background should be considered as a factor of patient’s subjective symptoms, such as medically unexplained toothache, atypical odontalgia, or self-reported xerostomia, that may be associated with reduced OHQoL in the management of oral care [
43,
44]. The results of the present study suggested that people with high psychological distress may perceive subjectively OHQoL as poor in a general population. On the other hand, it has been reported that compared to individuals without psychological distress, those with poor mental health are likely to use oral health services less often [
19]. In the present study, the percentage of participants who had visited a dental clinic within the past 6 months tended to be lower in the high-K6 group compared to the low-K6 group (although the difference was not significant), and this suggests that individuals with high psychological distress may not be likely to visit a dental clinic even if they have poor OHQoL. Although the participants in the present study had mild both psychological distress and oral health problems, high psychological distress strongly related to poor OHQoL even in community-dwelling people.
The World Health Organization (WHO) recommended the approach of providing a program for the continuous improvement of oral health as an essential factor of general health that is related to quality of life [
24]. The Japan Dental Association [
25] stated that good oral health is related to the extension of healthy life expectancy in Japan’s super-aging society. People who make regular dental visits are at a low risk of tooth loss [
45]. A randomized controlled study demonstrated that an oral health promotion program for individuals with mental illness reduced their plaque index values [
46]. We suggest that it is important to encourage individuals with high psychological distress (even those in general populations) to have regular dental health visits not only for the treatment of caries or periodontal disease but also for the maintenance of their oral health to prevent the deterioration of their OHQoL.
Another interesting finding of the present study is that compared to the men, the women had higher psychological distress and poorer OHQoL. During their lifetime, women worldwide are about twice as likely as men to develop depression [
47]. In global and long-term trends, the relationship between female gender and the risk of mental health may be explained by a previously reported gender gap [
37]. In addition, although women have reported more complaints about dental pain and chewing problems and showed greater concern about their oral health’s social and psychological impacts than men, these gender differences vary among distinct populations [
48]. A consideration of the gender gap may thus be important when seeking to maintain the appropriate psychological distress levels and oral health management in general populations.
There are some potential study limitations that warrant discussion. First, because it was a cross-sectional study, a causal relationship was not shown. Longitudinal studies are necessary to clarify causality in the future. Second, the mean age of the participants was somewhat elderly at 65.9 years, and it is possible that our findings cannot be extrapolated to younger populations. Because it has been reported that the proportion of high psychological distress is higher in not only older populations but also younger populations [
20], additional research is necessary to clarify the association between psychological distress and oral health status in younger populations.
Third, the ratio of individuals undergoing oral health examinations in their annual health check-ups was < 50% of the participants. Our results may not reflect the status of the overall health check-up population. Fourth, because the K6 is not designed for the diagnosis of mental illnesses and we did not ask the participants about their history of using psychotropic drugs, it was difficult to identify whether or not the participants had mental illnesses. Fifth, some selection biases might have been present because we did not obtain the status of psychological distress and oral health in individuals who did not undergo the health check-ups. The frequency of complications such as stroke, ischemic heart disease, and diabetes mellitus that could affect psychological distress was low in the present study, but comparable to another Japanese cohort study [
49]. Lastly, we could not include all coefficient factors, such as education level [
50] and economic status [
51], which were reported as important factors for preventive care in public health.
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