Oral health behaviours and metabolic syndrome: mediation effect of lifestyle habits

Abstract

Background

Recent studies have reported an association between toothbrushing frequency and systemic health. Plaque control, achieved through frequent brushing, is known to prevent dental caries and periodontal disease, potentially reducing the risk of systemic diseases. However, individuals with good oral health behaviours may also exhibit healthier lifestyle habits which could impact systemic health. In examining the relationship between oral health behaviours and systemic health, it is important to consider the influence of factors that mediate the relationship between them. This study conducted mediation analyses to evaluate the extent to which lifestyle habits and periodontal status mediate the relationship between oral health behaviours and systemic health.

Methods

Health examinations, including dental assessments, were performed on 15,579 individuals at the Aichi Health Promotion Foundation. Of these, 12,540 participants aged 40–74 years were included in the analysis. Mediation analyses were conducted using the number of metabolic syndrome (MS) components as the outcome variable, oral health behaviours (regular dental visits, use of interdental brushes or dental floss, and toothbrushing frequency) as exposure variables, lifestyle habits (smoking, drinking, exercise, and eating habits) and periodontal status as mediators, and age and sex as covariates.

Results

Regular dental visits showed significant total and indirect effects on MS, although no significant direct effect was observed. The use of interdental brushes or dental floss and toothbrushing frequency demonstrated significant total, direct and indirect effects on MS. Exercise habits, eating habits and periodontal status were significant mediators in the relationship between regular dental visits and MS. These mediators also significantly influenced the associations between both the use of interdental brushes or dental floss and toothbrushing frequency with MS.

Conclusion

The relationship between oral health behaviours and MS is mediated by lifestyle habits and periodontal status. Among the mediating variables tested in this study, the indirect effects of diet and exercise habits tended to be stronger than the indirect effect of periodontal disease. Oral health behaviours may influence systemic health through their impact on lifestyle habits.

Background

Oral diseases such as dental caries, periodontal disease, and tooth loss are the most common diseases, affecting nearly half of the world’s population. The prevalence of oral diseases is increasing due to population growth, the aging of the population, and increased tooth retention [1]. The most basic and important factor in preventing oral diseases is good oral health behaviours, including brushing one’s teeth. However, toothbrushing alone cannot completely remove plaque, which is a primary cause of periodontal disease and dental caries [2, 3]. Furthermore, more frequent toothbrushing does not necessarily lead to better oral hygiene. A study investigating the relationship between plaque removal frequency and periodontal status revealed that the periodontal condition of those who did not brush their teeth daily was poor, but there was no difference in the periodontal condition of those who brushed their teeth daily, even if the frequency of daily brushing increased [4].

Periodontal disease and tooth loss are risk factors for various chronic conditions, including metabolic syndrome (MS) [5–8]. Tooth loss is a risk factor for malnutrition and frailty [9, 10], periodontal disease is a risk factor for diabetes and cardiovascular disease [11, 12], and oral disease affects systemic health [11, 12]. Recent studies have reported an association between toothbrushing frequency and systemic health outcomes [13–16]. A follow-up study involving middle-aged Japanese individuals found that those who brushed their teeth three times a day had a significantly lower risk of developing MS compared to those who brushed only once a day or less [13]. Similarly, a potential link between toothbrushing frequency and non-alcoholic fatty liver disease has been suggested [16].

Good oral health behaviours may lead to good oral health and reduced risk of systemic disease. Even if an association between toothbrushing frequency and systemic health status is observed, it cannot be explained entirely by oral health status. Individuals who brush their teeth more frequently, or who use interdental brushes and dental floss, are likely to be more conscious of their overall health. Studies have shown that frequent toothbrushing is associated with lower smoking rates; indeed, smokers are less likely to visit the dentist regularly compared to non-smokers [17]. Moreover, individuals who brush their teeth more frequently tend to have healthier eating habits and are less likely to be obese [18]. Lifestyle has a strong influence on the development of the non-communicable diseases such as MS and fatty liver [19, 20], and the impact of periodontitis compared to lifestyle is considered small. Thus, the association between toothbrushing frequency and systemic health may be better explained by the presence of healthy lifestyle behaviours, such as regular exercise and a balanced diet, which positively influence systemic health.

In examining the relationship between oral health behaviours and systemic health, it is important to consider the existence and influence of factors that mediate the relationship between them. In this study, mediation analyses were conducted to evaluate the extent to which lifestyle factors and periodontal status mediate the relationship between MS as an indicator of systemic health and oral health behaviours, such as toothbrushing frequency, use of interdental brushes or dental floss, and regular dental visits.

Methods

Study design and participants

This was a cross-sectional observational study. Between April 1, 2010, and March 31, 2011, a total of 15,579 individuals underwent health examinations, including dental examinations, at the Aichi Health Promotion Foundation. Among these, 12,540 individuals aged 40–74 years were included in this study.

Directed acyclic graph (DAG)

DAGs are used to illustrate assumptions about the relationships among variables. The assumptions take the form of edges going from one node to another. These edges are directed, as shown by a single arrowhead indicating their effect [21]. In this study, a DAG was used to identify a mediation model (Fig. 1). Table 1 provides the rationale for creating the directional pathways for the DAG shown in Fig. 1. The DAGitty online tool was used to check the DAG for consistency and validity. Lifestyle habits and periodontal status were considered mediators, and sex and age were considered covariates.

Fig. 1.

Directed acyclic graph

Table 1.

Description of rationale for pathways shown in the base DAG

Node from	Directionality of pathway	Node to	Rationale
Oral health behaviours	Towards mediator	Lifestyle habits  Drinking  Smoking  Exercise  Eating	People with poor oral health behaviour are more likely to have poor lifestyle habits. Those who brush their teeth less frequently and those who do not visit the dentist are more likely to have drinking and smoking habits [17, 22]. Those who visit the dentist regularly and brush their teeth frequently are more likely to have healthy exercise and eating habits [23, 24].
Oral health behaviours	Towards mediator	Periodontal status	Toothbrushing and the use of an interproximal device helps prevent periodontal disease [25]. Oral health instructions and professional plaque control can help prevent periodontal disease.
Oral health behaviours	Towards outcome	Metabolic syndrome	Toothbrushing and the use of an interproximal device helps prevent metabolic syndrome [26].
Lifestyle habits  Drinking  Smoking  Exercise  Eating	Towards outcome	Metabolic syndrome	Lifestyle factors such as drinking, smoking, exercise, and diet strongly influence the incidence and progression of metabolic syndrome [20].
Periodontal status	Towards outcome	Metabolic syndrome	Chronic inflammation caused by periodontitis may be associated with metabolic syndrome [6].

Exposure variables

Oral health behaviours were evaluated as exposure variables. These behaviours were assessed using a self-administered questionnaire that inquired about regular dental visits, daily toothbrushing frequency, and the use of interdental brushes or dental floss.

Outcome

The primary outcome was the total number of MS components met by each participant, as determined from the results of health examinations. The Japanese criteria for MS components were used [27], which included the following: waist circumference ≥ 85 cm for males and ≥ 90 cm for females; blood pressure ≥ 130/85 mmHg; triglyceride level ≥ 150 mg/dL or high-density lipoprotein cholesterol level < 40 mg/dL; and blood glucose level ≥ 110 mg/dL. Participants using medication for dyslipidaemia, hypertension, or diabetes mellitus, or with a history of such medications, were classified as meeting the corresponding criteria.

Mediators

Periodontal status and lifestyle habits, such as smoking, drinking, exercise, and eating behaviours, were examined as mediators. Lifestyle information was obtained via a self-administered questionnaire [28]. Responses were categorised as follows:

• Smoking: Current, past, or never.

• Drinking: Everyday, sometimes, or never.

• Exercise habits: Exercising at least twice a week (yes, no), engaging in walking-equivalent activity for at least 1 hour per day (yes, no), and walking at a fast pace (yes, no).

• Eating habits: Low intake of vegetables and seaweed (yes, no); frequent consumption of sweetened drinks (yes, no); preference for eating meat rather than fish (yes, no); eating snacks after dinner ≥ 3 times/week (yes, no); eating dinner just before bedtime ≥ 3 times/week (yes, no); and frequent consumption of fried foods (yes, no).

Periodontal status was assessed through dental examinations using the Community Periodontal Index (CPI) [29]. Each participant’s oral cavity was divided into sextants, and 10 representative teeth (11, 16, 17, 26, 27, 31, 36, 37, 46, and 47) were examined. Measurements were conducted at six sites per tooth (mesiobuccal, midbuccal, distobuccal, distolingual, midlingual, and mesiolingual) using a CPI probe (YDM, Tokyo, Japan). CPI codes were defined as follows:

• Code 0: Healthy.

• Code 1: Bleeding on probing.

• Code 2: Calculus detected on probing.

• Code 3: Periodontal pockets 4–5 mm.

• Code 4: Periodontal pockets ≥ 6 mm.

• Code X: Sextants without target teeth.

The highest CPI code per participant, excluding sextants classified as Code X, was used as the individual CPI score. Twelve calibrated dentists (inter-examiner kappa ≥ 0.5) conducted the examinations under reflected light on dental chairs.

Covariates

Age and sex were included as covariates in the analysis.

Statistical analysis

Participants with missing data or those whose periodontal status was unknown (all sextants classified as Code X) were excluded. Toothbrushing frequency was categorised into three groups: ≤ 1 time/day, 2 times/day, and ≥ 3 times/day. Periodontal status was categorised as follows: healthy (CPI codes 0–2), moderate periodontitis (CPI code 3), and severe periodontitis (CPI code 4).

Exercise habits were quantified based on “yes” responses to the three exercise-related questions. Similarly, eating habits were evaluated based on “yes” responses to the six dietary-related questions. Table 2 shows the operationalisation of each variable. The association between the number of MS components and each variable was assessed using chi-squared tests for categorical variables and analysis of variance (ANOVA) for continuous variables.

Table 2.

Operationalisation of the variables

	Definition	Assessment	Response options
Outcome
Metabolic syndrome	The total number of MS components met	Waist circumference, blood pressure, triglyceride level or high-density lipoprotein cholesterol level, and blood glucose level	Five-point scale from 0 to 4.
Exposure variable
Regular dental visits		Do you visit the dentist regularly?	Two-step evaluation (Yes / No).
Toothbrushing frequency	Daily tooth brushing frequency	How many times do you brush your teeth daily?	Three-point scale (≤ 1 time/day, 2 time/day, or ≥ 3 time/day).
Use of interdental brush or dental floss		Do you use dental floss or interdental brushes?	Two-step evaluation (Yes / No).
Mediators
Drinking	Current drinking habit	Do you drink alcohol?	Three-point scale (every day, sometimes, or never).
Smoking	Smoking habit	Do you smoke cigarettes (including electronic cigarettes)?	Three-point scale (current, past, or never).
Exercise	Number of “yes” responses to three questions about exercise habits	Walking or equivalent physical activity for at least one hour per day in daily life. Lightly sweaty exercise for at least 30 minutes a day, at least twice a week, for at least one year. Walking speed is faster than that of the same sex at the same age.	Four-point scale from 0 to 3.
Eating	Number of “yes” responses to six questions about eating habits	Vegetables and seaweed are not eaten very often. Drink juice daily. Eats more meat (beef and pork) than fish. Snacking after dinner at least 3 times a week. Eating dinner within 2 hours before bedtime at least 3 times a week. Eats fried foods frequently.	Seven-point scale from 0 to 6.
Periodontal status	Community Periodontal Index	Code 1: healthy, code 2: calculus detected on probing, code 3: periodontal pockets 4-5 mm, and code 4: periodontal pockets ≥ 6 mm.	Three-point scale (code 0-2, code 3, and code 4).

Parallel mediation analyses were performed with the number of MS components as the outcome, oral health behaviours as exposure variables, lifestyle habits and periodontal status as mediators, and age and sex as covariates. Model 4 from the PROCESS macro for SPSS was used for mediation analysis. The 95% confidence interval (CI) of the mediating effect was estimated using 5,000 bootstrap samples. Each oral health behaviour was analysed individually. Statistical analyses were conducted using SPSS version 28.0 (IBM Corp., Armonk, NY, USA) and the Hayes PROCESS macro version 4.2. A p-value < 0.05 was considered statistically significant. This study was reported according to A Guideline for Reporting Mediation Analyses.

Results

Of the 15,579 people who underwent health examinations during the study period, 12,540 participants aged 40–74 years were eligible for the study. After excluding 71 participants whose CPI code was X, the final analysis included 12,469 individuals. Regarding the number of MS components, the number of participants with 0, 1, 2, 3, and 4 components was 2,201 (17.7%), 3,875 (31.7%), 3,104 (24.9%), 2,359 (18.9%), and 930 (7.5%), respectively. For the analysis involving toothbrushing frequency, 10,048 participants were included, excluding those with missing data regarding toothbrushing frequency.

Table 3 shows the characteristics of the study participants according to the number of met criteria for MS components. Use of interdental brushes or dental floss, toothbrushing frequency, drinking habit, smoking habit, periodontal status, exercise habit, eating habit, sex, and age were significantly associated with the number of met criteria for MS components.

Table 3.

Characteristics of participants according to the number of metabolic syndrome component

	Number of metabolic syndrome components
	0 (n =2,201)	1 (n = 3,875)	2 (n = 3,104)	3 (n = 2,359)	4 (n = 930)	p-value
Exposure variable
Regular dental visit	n (%)
No	1,564 (71.1)	2,727 (70.4)	2,231 (71.9)	1,737 (73.6)	658 (70.8)	0.079
Yes	637 (28.9)	1,148 (29.6)	873 (28.1)	622 (26.4)	272 (29.2)
Use of interdental brush or dental floss
No	1,660 (75.4)	2,883 (74.4)	2,418 (77.9)	1,904 (80.7)	764 (82.2)	< 0.001
Yes	541 (24.6)	992 (25.6)	686 (22.1)	455 (19.3)	166 (17.8)
Toothbrushing frequencya
< 1 time/day	256 (14.3)	685 (21.8)	682 (27.5)	597 (31.7)	260 (34.8)	< 0.001
2 time/day	1,111 (62.1)	1,874 (59.5)	1,408 (56.8)	1,082 (57.4)	407 (54.4)
> 3 time/day	423 (23.6)	590 (18.7)	387 (15.6)	205 (10.9)	81 (10.8)
Mediator variable
Drinking habit
Never	842 (38.3)	1,456 (37.6)	990 (31.9)	701 (29.7)	254 (27.3)	< 0.001
Sometimes	730 (33.2)	1,203 (31.0)	907 (29.2)	704 (29.8)	292 (31.4)
Everyday	629 (28.6)	1,216 (31.4)	1,207 (38.9)	954 (40.4)	384 (41.3)
Smoking habit
Never	1,204 (54.7)	1,818 (46.9)	1,108 (35.7)	693 (29.4)	241 (25.9)	< 0.001
Past	481 (21.9)	1,009 (26.0)	1,070 (34.5)	929 (39.4)	428 (46.0)
Current	516 (23.4)	1,048 (27.0)	926 (29.8)	737 (31.2)	261 (28.1)
Periodontal status
Healthy	1,278 (58.1)	1,905 (49.2)	1,223 (39.4)	847 (35.9)	327 (35.2)	< 0.001
Moderate periodontitis	713 (32.4)	1,451 (37.4)	1,374 (44.3)	1,074 (45.5)	422 (45.4)
Severe periodontitis	210 (9.5)	519 (13.4)	507 (16.3)	438 (18.6)	181 (19.5)
			Mean (SD)
Exercise habit	0.7 (0.8)	0.7 (0.8)	0.7 (0.8)	0.6 (0.8)	0.6 (0.7)	0.002
Eating habit	1.1 (1.1)	1.1 (1.1)	1.2 (1.1)	1.3 (1.1)	1.3 (1.1)	< 0.001
Covariates
Sex			n (%)
Men	1,184 (53.8)	2,546 (65.7)	2,590 (83.4)	2,129 (90.3)	877 (94.3)	< 0.001
Women	1,017 (46.2)	1,329 (34.3)	514 (16.6)	230 (9.7)	53 (5.7)
	Mean (SD)
Age	48.3 (7.2)	51.7 (8.1)	53.6 (8.4)	54.5 (8.3)	55.1 (8.0)	< 0.001

Abbreviation: SD Standard deviation

^an = 10,048

Tables 4 and 5 show the results of mediation analyses in which the number of met criteria for MS components was the outcome; regular dental visits, use of interdental brushes or dental floss, and toothbrushing frequency were the exposure variables; drinking, smoking, exercise, eating habits, and periodontal status were the mediators; and age and sex were the covariates.

Table 4.

Mediation analysis of each oral health behaviour on the number of metabolic syndrome component

	Outcome variable: number of metabolic syndrome components
	Coefficient	SE	95% CI
Regular dental visita
Total effect	-0.051	0.022	-0.094, -0.008
Direct effect	-0.021	0.022	-0.064, 0.022
Indirect effect	-0.030	0.004	-0.037, -0.023
Use of interdental brush or dental flossa
Total effect	-0.094	0.024	-0.140, -0.048
Direct effect	-0.067	0.024	-0.113, -0.021
Indirect effect	-0.027	0.004	-0.034, -0.020
Toothbrushing frequencyb
Total effect	-0.173	0.018	-0.208, -0.139
Direct effect	-0.146	0.018	-0.181, -0.111
Indirect effect	-0.027	0.004	-0.035, -0.019

Abbreviations: SE Standard error, CI Confidence interval

^an = 12,469

^bn = 10,048

Table 5.

Indirect effect of each health behaviour on the number of metabolic syndrome components

	Mediator	Coefficient	SE	95% CI
Regular dental visita	Total	−0.030	0.004	−0.037, −0.023
	Drinking habit	0.000	0.0002	−0.0004, 0.001
	Smoking habit	−0.0002	0.001	−0.003, 0.002
	Exercise habit	−0.014	0.003	−0.019, −0.009
	Eating habit	−0.012	0.002	−0.017, −0.008
	Periodontal status	−0.004	0.001	−0.007, −0.002
Use of interdental brush or dental flossa	Total	−0.027	0.004	−0.034, −0.020
	Drinking habit	−0.0001	0.0003	−0.001, 0.001
	Smoking habit	−0.0001	0.001	−0.003, 0.003
	Exercise habit	−0.013	0.002	−0.018, −0.009
	Eating habit	−0.011	0.002	−0.015, −0.007
	Periodontal status	−0.003	0.001	−0.005, −0.0004
Toothbrushing frequencyb	Total	−0.027	0.004	−0.035, −0.019
	Drinking habit	0.000	0.0002	−0.0004, 0.0004
	Smoking habit	0.0004	0.003	−0.005, 0.005
	Exercise habit	−0.009	0.002	−0.013, −0.006
	Eating habit	−0.015	0.003	−0.020, −0.010
	Periodontal status	−0.003	0.001	−0.006, −0.002

Abbreviations: SE Standard error, CI Confidence interval

^an = 12,469

^bn = 10,048

Table 4 shows the total, direct and indirect effects of each oral health behaviour on MS. Regular dental visits had significant total and indirect effects on MS, with coefficients of − 0.051 (95% CI: − 0.094, − 0.008) and − 0.030 (95% CI: − 0.037, − 0.023), respectively. There was no significant direct effect of regular dental visits on MS. Use of interdental brushes or dental floss had significant total, direct, and indirect effects, with coefficients of − 0.094 (95% CI: − 0.140, − 0.048), − 0.067 (95% CI: − 0.113, − 0.021) and − 0.027 (95% CI: − 0.034, − 0.020), respectively. Toothbrushing frequency also had significant total, direct and indirect effects, with coefficients of − 0.173 (95% CI: − 0.208, − 0.139), − 0.146 (95% CI: − 0.181, − 0.111) and − 0.027 (95% CI: − 0.035, − 0.019), respectively.

Table 5 shows the indirect effects of each mediator on the association between each oral health behaviour and MS. Exercise habits, eating habits and periodontal status had significant indirect effects on the association between regular dental visits and MS, with coefficients of − 0.014 (95% CI: − 0.019, − 0.009), − 0.012 (95% CI: − 0.017, − 0.008) and − 0.004 (95% CI: − 0.007, − 0.002), respectively. Exercise habits, eating habits and periodontal status also had significant indirect effects on the association between the use of interdental brushes or dental floss and MS, with coefficients of − 0.013 (95% CI: − 0.018, − 0.009), − 0.011 (95% CI: − 0.015, − 0.007) and − 0.003 (95% CI: − 0.005, − 0.0004), respectively. Similarly, exercise habits, eating habits and periodontal status had significant indirect effects on the association between toothbrushing frequency and MS, with coefficients of − 0.009 (95% CI: − 0.013, − 0.006), − 0.015 (95% CI: − 0.020, − 0.010) and − 0.003 (95% CI: − 0.006, − 0.002), respectively.

Discussion

We found that lifestyle habits and periodontal status mediate the relationship between oral health behaviours and MS. The indirect effects through periodontal status were smaller than those observed through exercise and eating habits.

A strength of this study was its analysis of a large number of participants. The inclusion of varied and detailed questions about exercise and eating habits in the questionnaire allowed the results to reflect the participants’ lifestyle habits comprehensively. Additionally, this study provides valuable insights, as few prior studies have explored the lifestyle-mediated influence in the relationship between oral health behaviours and MS.

However, several limitations should be acknowledged. First, lifestyle habits were assessed through self-reported questionnaires, which are subjective and may not accurately represent the actual amount of exercise performed or food consumed. The questionnaire items used in this study are widely used in health examinations in Japan but have not been validated [28]. Second, the study was conducted in a specific region of Japan, limiting the generalisability of the findings. Third, data on socioeconomic factors, such as income and education—which are known to influence lifestyle and health status [30, 31]—were unavailable. Furthermore, the study population was biased toward men, as most participants were health examination attendees, predominantly employees undergoing annual workplace screenings. The associations observed in this study between oral health behaviours, lifestyle habits and MS may reflect confounding rather than true mediation. Although mediation analyses were performed using oral health behaviours as exposure variables and lifestyle habits as mediators to evaluate their effects on MS, the cross-sectional design of this study prevented establishing a temporal relationship or causation between oral health behaviours and MS.

Several studies have investigated the relationship between toothbrushing frequency and MS. A 3-year follow-up study reported that individuals brushing their teeth ≥ 3 times per day had a significantly lower risk of MS compared to those brushing ≤ 1 time per day [13]. Another follow-up study found that more frequent toothbrushing was associated with a lower risk of MS, with significant direct and indirect effects observed in mediation analyses using periodontal status as a mediator [15]. Consistent with these findings, this study demonstrated significant associations between toothbrushing frequency and MS, including both direct effects and indirect effects mediated by periodontal status, exercise habits, and eating habits.

This study extended the scope of previous research by including regular dental visits and the use of interdental brushes or dental floss as exposure variables in the mediation analysis. Few studies have examined the association of interdental cleaning devices with MS, and no previous studies have evaluated the relationship between regular dental visits and MS. In one study examining the relationship between oral health behaviours and MS components, individuals who did not use dental floss daily had higher risks of abdominal obesity, hypertension and hyperglycaemia [26, 32]. Similarly, this study found that individuals who used interdental brushes or dental floss and those who regularly visited dentists exhibited fewer MS components than those who did not. Mediation analyses revealed significant direct and indirect effects of interdental cleaning device use on MS, mediated by periodontal status, exercise habits, and eating habits. Regular dental visits, however, exhibited only indirect effects.

It is likely that individuals with good oral health behaviours are generally more health-conscious, leading to better lifestyle habits, such as regular exercise, healthier eating, and avoidance of smoking and excessive drinking [23]. For instance, those who regularly visit dentists and use interdental cleaning devices tend to have better eating habits, while frequent toothbrushing is associated with exercise habits and non-smoking behaviour. Consequently, oral health behaviours may influence MS via their association with lifestyle habits such as exercise and eating habits.

Given the close relationship between oral health behaviours and lifestyle habits, these behaviours may not only affect MS but also other lifestyle-related diseases. Although this study focused on MS as the outcome, previous studies have reported associations between toothbrushing frequency and non-alcoholic fatty liver disease, as well as with decreased renal function [16, 33]. Future research should explore further the relationship between oral health behaviours and other systemic diseases.

In this study, toothbrushing frequency and the use of interdental brushes or dental floss had both significant direct and indirect effects in relation to MS. Individuals who brush their teeth more frequently may have a more relaxed lifestyle, allowing them to manage their time and mental well-being effectively. Consequently, they might experience less stress in their daily lives. Since excessive stress and fatigue are recognised risk factors for lifestyle-related diseases, toothbrushing frequency and the use of interdental cleaning devices might exert a direct effect on MS. However, this study did not assess participants’ stress levels, leaving the relevance of this potential mechanism unclear.

Health literacy, self-efficacy, and adherence are associated with maintaining a healthy lifestyle. Those with high self-efficacy and health literacy are more likely to have a healthy lifestyle [34, 35]. Self-efficacy promotes adherence to lifestyle guidance [36], which in turn reduces mortality due to MS [37]. Similarly, those with good oral health are more likely to be health literate and more concerned about oral health. Those with higher self-efficacy have better gingival status [38]. Socioeconomic determinants such as education, occupation, and income are thought to influence psychological aspects, resulting in differences in lifestyle habits. Socioeconomic disparities also influence health outcomes [30, 31]. Individuals with higher education, better social status, and greater wealth tend to exhibit higher health awareness, adopt healthier lifestyle habits and, consequently, have a lower risk of disease compared to those with fewer resources [30]. Socioeconomic factors also affect oral health behaviours and lifestyle habits, including drinking, smoking, exercise and eating [31], potentially acting as confounding variables in the association between oral health behaviours and MS. Unfortunately, this study lacked data on socioeconomic factors and indices of phycological status, such as health literacy, self-efficacy, and adherence, and could not adjust for these variables in the analysis. This limitation may partly explain the significant direct effects of toothbrushing frequency and interdental cleaning device use on MS. Future research should incorporate socioeconomic factors into the analysis to understand more precisely their influence.

Numerous studies have reported associations between periodontitis and MS. An 8-year follow-up study found that individuals with poor periodontal health had a higher risk of developing MS [39]. Similarly, this study observed a significant association between periodontal status and MS. Chronic inflammation caused by periodontitis affects systemic health beyond the oral cavity [40], making periodontitis a recognised risk factor for MS. It is plausible that individuals with good oral health behaviours are less likely to develop MS due to their better periodontal health. However, in this study, the indirect effects of periodontal status on MS were smaller than those mediated by lifestyle habits, such as exercise and dietary practices.

Individuals with good oral health behaviours were also less likely to engage in drinking and smoking. Although drinking and smoking were significantly associated with MS in bivariate analyses, they did not demonstrate significant indirect effects as mediators in the relationship between oral health behaviours and MS. Smoking is a well-established risk factor for various lifestyle-related diseases [41–43]; however, it is also associated with weight loss, which may weaken its association with MS [44]. Furthermore, appropriate alcohol consumption is generally not harmful to health [45]. In this study, the lack of detailed data on the amount of alcohol consumed may have influenced the results, which did not show a significant association between drinking and MS.

Many studies have assessed the relationship between frequency of toothbrushing and systemic health but those studies considered lifestyle habits confounding factors. The originality of this study lies in the fact that we examined lifestyle habits as mediators in a mechanism by which oral health behaviours affect systemic health. However, the influence of lifestyle habits and oral health behaviours on oral and systemic health status is very complex. It is difficult to determine whether lifestyle habits are confounding or mediating factors. The mediating effect of lifestyle habits in this study is only one possible mechanism. Another interesting result is that the frequency of toothbrushing and the use of interdental cleaning devices had a direct effect on MS. It is possible that toothbrushing itself may serve as a light form of exercise and/or prevent snacking. However, it is unlikely that brushing the teeth more often or using an interdental brush will reduce the risk of MS. For those with poor oral health behaviour, it is important to provide health guidance that also leads to lifestyle improvement. Another reason for the direct effects observed in this study may be the influence of confounding factors that were not considered, such as socioeconomic factors. Possible methods that could be used in future studies include Bayesian estimation using prior distributions of lifestyle and socioeconomic determinants, and approaches such as structural analysis of covariance.

Conclusions

The relationship between oral health behaviours and MS is mediated by lifestyle habits and periodontal status. Among the mediating variables tested in this study, the indirect effects of diet and exercise habits tended to be stronger than the indirect effect of periodontal disease. Oral health behaviours may influence systemic health through their impact on lifestyle habits. For those with poor oral health behaviour, it is important to provide health guidance that also leads to lifestyle improvement.

Abbreviations

MS

Metabolic syndrome

DAG

Directed acyclic graph

CPI

Community Periodontal Index

CI

Confidence interval

Authors’ contributions

MS contributed to conception, design, analysis, interpretation, and draft and critically revised the manuscript. YS contributed to conception, design, interpretation, and draft and critically revised the manuscript. SY contributed to conception, design, data acquisition, and critically revised the manuscript. TK contributed to conception, design, data acquisition, and critically revised the manuscript. All authors gave their final approval and agree to be accountable for all aspects of the work.

Funding

This work was supported by JSPS KAKENHI Grant Number JP22K10353 and JP23K16239.

Data availability

The data that support the findings of this study are available from the Aichi Health Promotion Foundation, but restrictions apply to the availability of these data, which were used under license for the current study, and so are not publicly available. Data are however available from the authors upon reasonable request and with permission of the Aichi Health Promotion Foundation.

Declarations

Ethics approval and consent to participate

Written informed consent to use health examination data for research was obtained from all participants. The Institutional Review Board (IRB) of Aichi Gakuin University School of Dentistry approved the study design, data-collection methods, and the procedure for obtaining informed consent (approval number 618). The study was conducted strictly in accordance with the relevant provisions of the World Medical Association Declaration of Helsinki. This study was reported according to the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines for reporting observational studies.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.