Association of physical function with masticatory ability and masticatory habits: a cohort study

Abstract

Background

Few studies have evaluated masticatory ability and habits in relation to physical function. This study aimed to investigate the association of physical function with both masticatory ability and masticatory habits.

Methods

In this cohort study, we followed up with 146 community-dwelling older adults aged 65–84 years for 1 year. Physical function domain scores on the Kihon Checklist were used to assess physical function. Masticatory ability was examined using objective measurements and self-administered questionnaires. Data on masticatory habits were obtained using self-administered questionnaires. The Mann–Whitney U test was used to analyze the association between masticatory ability and masticatory habits as exposures; logistic regression analysis was used to analyze the effect of exposure on the outcome.

Results

A relationship was found between objective and subjective masticatory ability; however, no relationship was found between objective masticatory ability and masticatory habits. Furthermore, subjective masticatory ability and masticatory habits appeared to influence physical function 1 year later (odds ratio [OR]: 6.00, 95% confidence interval [CI]: 1.44–25.05; OR: 6.49, 95% CI: 2.45–17.22).

Conclusion

Masticatory ability and habits may be associated with a decline in physical function after 1 year in community-dwelling older adults. To maintain the physical function of these individuals, early intervention that addresses not only masticatory ability but also masticatory habits is necessary.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12903-024-05051-6.

Background

The number of older adults is increasing worldwide, and the percentage of the total population over 65 years old is expected to rise from 9.3% in 2020 to 17.8% in 2060 [1]. The aging of the population will lead to an increase in illness and disability, and the number of those in need of long-term care will gradually increase. Thus, efforts to extend healthy life expectancies are becoming increasingly important.

Maintaining physical function is essential for older adults to live independently. Physical function can be measured by physical performance tests or self-report tests, which have been shown to be associated with frailty [2], hospitalization [3, 4], and an increased risk of death [3–8]. Therefore, it is important to evaluate physical function appropriately and use the data for interventions in older adults.

Previous studies have shown that masticatory ability is positively related to physical function [9]. Moreover, it has been evaluated both objectively and subjectively, and several studies have been conducted to examine the relationship with physical function [10–16]. Furthermore, the continuation of proper masticatory habits, such as “chewing food well,” may help manage good eating behaviors and help maintain and improve physical function. However, few studies have investigated the relationship between masticatory habits and physical function in older adults.

We hypothesized that the decline in physical function in older adults may be related not only to masticatory ability but also to masticatory habits. The purpose of this study was to clarify whether masticatory ability and habits influence the decline in physical function and to conduct a 1-year prospective study of community-dwelling older adults aged 65 years and older.

Methods

Study design

In June 2018, of the 10,868 residents aged 65 years and older living in Higashiura Town, Aichi Prefecture, 10,016 people, excluding those requiring nursing care or support, were selected and sent information for a group health examination. Of the 1,210 people who applied to participate, 993 were included in the group health examination conducted in Higashiura Town, Aichi Prefecture, between September and October 2018. Of these, 552 underwent an oral examination at a dental clinic. The number of teeth was recorded at the dental clinic, and all other information was obtained during the group health examination. The target population comprised 160 people who participated in a group health checkup in 2019. Of the 160 people who also participated in the follow-up examination in 2019, those who answered poorly to three or more of the five questions assessing physical function in the baseline survey were excluded. Finally, data from 146 individuals were included in this study. All participants provided written informed consent prior to participation in the study. This study was conducted in accordance with the Declaration of Helsinki and approved by the Ethics Committee of the Aichi Dental Association (1) and the Ethics Committee for Epidemiology of Hiroshima University (E2021-2704).

Physical function

Physical function was assessed using the “Kihon Check List” (KCL) [17]. The KCL is a self-administered questionnaire with 25 “yes/no” questions, which was originally developed by the Ministry of Health, Labor and Welfare for the purpose of preventing nursing care and for the early selection of high-risk older adults who may need nursing care in the future. In recent years, it has also been used worldwide to assess frailty [17–19]. The questionnaire encompasses seven domains: activities of daily living, physical function, nutrition, oral function, social isolation, cognitive function, and depression. In this study, we focused on the “physical function” domain, which includes questions such as: “Do you normally climb stairs without using handrail or wall for support?”, “Do you normally stand up from a chair without any aids?”, “Do you normally walk continuously for 15 minutes?”, “Have you experienced a fall in the past year?”, and “Do you have a fear of falling while walking?” These were answered by questionnaire. For each question, one point was added to the answer that was considered problematic; the higher the score, the more defective the condition was rated. Total scores were calculated (score 0–5) for 2018 and 2019. Participants whose scores remained unchanged across both years were classified as “maintaining physical function.” Conversely, individuals exhibiting a decrease in scores in 2019 were categorized as “improving physical function,” whereas those with increased scores were identified as “deteriorating physical function”.

Masticatory ability and masticatory habits

These variables were measured at baseline. Masticatory ability was assessed objectively and subjectively. To perform the objective measurements, all dentists, dental hygienists, and office staff performing the examinations participated in preliminary discussions and training [20]. For objective evaluation, the masticatory function was assessed using a GULCO SENSOR GS-II (GC, Japan). This device measured the glucose concentration in the spit filtrate following a 20-s chewing of glucose-containing gummies. Individuals with glucose levels below 100 mg/dL were classified as having impaired masticatory function. Similarly, a DENTALPRESCALE II (GC, Japan) was utilized to evaluate the occlusal force; those with a force less than 500 N were considered to have a low occlusal force. The assessment of remaining teeth in the oral cavity, excluding missing teeth, remaining root teeth, or implants, was conducted in dental clinics by dentists or dental hygienists, with a threshold set at 20 teeth.

The subjective evaluations included two questions to assess masticatory function and occlusal status. One question in the oral function domain of the KCL was used to assess masticatory function [17]: “Do you have any difficulties eating tough foods compared to 6 months ago?” If the answer was “yes,” they were defined as the “poor group.” For the evaluation of occlusal status, the evaluation by the Oral Health Examination, Assessment, and Health Instruction Program for Adults, developed by the Japan Dental Association, was modified to allow for yes/no responses: “Can you bite tightly on both sides of the back teeth with your teeth or dentures?” [21–24] If the answer was “no,” they were included in the “poor group”.

Masticatory habits were assessed by asking, “Do you chew food slowly and well?” This query addresses the frequency and pace of chewing. Participants responded to these questions using a yes/no format as per a questionnaire designed by the Japan Dental Association [21–23]. If participants answered “no,” they were categorized as part of the “poor group”.

Subjective assessments of masticatory ability and habits were gathered through participant-completed questionnaires.

Covariates

Data including age, sex, use of dentures, and medical history (hypertension, diabetes mellitus, heart disease, stroke, and cancer) at baseline were obtained using a questionnaire. If a patient had a history of at least one of the five diseases, this was considered applicable. Body mass index (BMI) was calculated by dividing the weight (kg) by the square of height (m²).

Statistical analysis

Categorical variables are presented as number (percentage), and continuous variables are presented as median (interquartile range). Participants were divided into a “good group” and a “poor group” based on the answers obtained regarding their subjective masticatory ability and masticatory habits. Non-normally distributed objective masticatory ability measurements were compared between groups using the Mann–Whitney U test. Masticatory ability and habits variables stratified by age and sex were analyzed using Fisher’s exact test.

Logistic regression analyses were conducted to evaluate the association between the deterioration of physical function (dependent variable) and masticatory ability and habits (independent variables). Crude odds ratios (ORs), adjusted OR, and 95% confidence intervals (CIs) were calculated. Crude and adjusted analyses were performed separately for each independent variable, specifically masticatory ability and habits. The adjusted OR was computed after adjusting for age, sex, BMI, medical history, and use of dentures. It has been reported that cognitive function and depressive symptoms can affect subjective evaluation [25]. Furthermore, malnutrition is closely related to the decline of physical function [26]. Although we did not conduct a questionnaire survey to evaluate these factors in all participants, the KCL questionnaire includes domains that evaluate cognitive function, depression, and nutrition. Logistic regression analysis was also conducted, in which at least one of the three cognitive function domains in the KCL was defined as “cognitive decline,” [27, 28] and at least two of the five depression domains were defined as “depression” [27, 28] and added as adjustment factors. The nutrition domain in the KCL consists of BMI and weight loss; therefore, we also conducted analyses that included items regarding weight loss as an adjustment factor. SPSS ver. 28.0 (IBM Corp., Armonk, NY, USA) was used for statistical analysis. The level of statistical significance was set at p < 0.05.

Results

Characteristics of the participants

Basic characteristics, stratified by sex at baseline, are presented in Table 1. The subjects’ median age was 73 years; 77 (52.7%) identified themselves as men and 69 (47.3%) as women, boasting median ages of 73 and 72 years, respectively, in 2018. Thirty patients (20.5%) exhibited deteriorating physical function scores over 1 year. The median number of teeth was 26, with 87.7% (n = 128) of participants retaining 20 or more teeth. Denture usage was reported by 54 individuals. Masticatory ability and habits, categorized by age and sex, are detailed in Additional File 1. Women demonstrated a significantly higher percentage of individuals with 20 or more teeth compared to men (p = 0.025). Moreover, a greater percentage of individuals under 74 years of age exhibited superior objectively measured masticatory function than those aged 75 and above (p = 0.028).

Table 1.

Population characteristics at baseline

Characteristic	Overall (n = 146)	Men (n = 77)	Women (n = 69)
Age (years)†	73.0 (69.0–77.0)	73.0 (70.0–77.0)	72.0 (68.0–76.0)
BMI (kg/m2)†	22.5 (20.7–24.2)	22.7 (21.1–24.2)	22.1 (20.3–24.2)
Physical function, n (%)
Improvement or unchanged	116 (79.5)	61 (79.2)	55 (79.7)
Deterioration	30 (20.5)	16 (20.8)	14 (20.3)
Systemic disease, n (%)
Hypertension	64 (43.8)	41 (53.2)	41 (59.4)
Diabetes mellitus	14 (9.6)	67 (87.0)	65 (94.2)
Heart disease	13 (8.9)	70 (90.9)	63 (91.3)
Stroke	2 (1.4)	76 (98.7)	68 (98.6)
Cancer	21 (14.4)	62 (80.5)	63 (91.3)
Dental information
Number of teeth†	26.0 (24.0–27.0)	26.0 (23.0–27.0)	26.0 (25.0–27.0)
Denture use, n (%)	54 (37.0)	32 (41.6)	22 (31.9)

Abbreviations: BMI, body mass index

^† Median (interquartile range)

Relationship between masticatory ability and masticatory habits

Table 2 shows the relationship between masticatory abilities and habits. The group with good subjective masticatory function had significantly more teeth and higher occlusal force than did the group with poor subjective masticatory function (p < 0.05). Moreover, the group with good subjective occlusal status had a significantly higher number of teeth, masticatory function, and occlusal force than did the group with poor subjective occlusal status (p < 0.05). However, no significant differences were observed between the good and poor masticatory habits groups in any of the objective measurements.

Table 2.

Comparison of objective masticatory evaluations according to subjective evaluation and masticatory habits

	Overall (n = 146)	Subjective masticatory function			Subjective occlusal status			Masticatory habits†
		Good (n = 126)	Poor (n = 20)	p value	Good (n = 136)	Poor (n = 10)	p value	Good (n = 90)	Poor (n = 54)	p value
Number of teeth (n)	26.0	26.0 (25.0–28.0)	23.0 (16.0–25.0)	< 0.001*	26.0 (24.0–28.0)	23.5 (15.8–26.0)	0.014*	26.0 (23.0–27.0)	26.0 (24.0–28.0)	0.448
Masticatory function (mg/dL)	192.5	196.5 (152.0-231.3)	157.5 (114.3-205.8)	0.067	195.0 (150.5-231.8)	147.0 (111.8-189.5)	0.042*	188.0 (148.8-220.3)	199.5 (148.0-236.8)	0.341
Occlusal force (N)	845.0	877.3 (638.2-1223.4)	640.5 (494.3-861.2)	0.011*	860.8 (626.5-1214.4)	566.0 (383.3-733.3)	0.005*	843.0 (627.2-1158.7)	860.8 (583.9-1273.5)	0.822

Note: Mann–Whitney U test median (interquartile range)

† n = 144

*p < 0.05

Association of physical function with masticatory ability and masticatory habits

Table 3 presents the results of intergroup comparison and logistic regression analyses of physical function, masticatory ability, and masticatory habits. Logistic regression analysis revealed that “poor subjective occlusal status” and “poor masticatory habits” were linked to physical function deterioration in both crude and adjusted OR. However, no significant associations emerged for objective evaluation items. A table presents the logistic regression analysis outcomes, adding cognitive function, depression, and nutrition domains from the KCL as adjustment factors alongside subjective masticatory ability and habits from the self-administered questionnaire [Additional File 2]. Subjective occlusal status and masticatory habits continued to significantly impact physical function decline after these adjustments.

Table 3.

Association of physical function with the objective and subjective masticatory evaluation and masticatory habits

		Intergroup comparison of physical function on masticatory ability and habits		Logistic regression analysis for deterioration of physical function
		Improvement/ Unchanged	Deterioration	Crude OR (95% CI)	p value	Adjusted OR (95% CI)†	p value
Objective masticatory ability
Masticatory function
	≥ 100 mg/dL	110 (80.3) §	27 (19.7) §	1		1
	< 100 mg/dL	6 (66.7) §	3 (33.3) §	2.04 (0.48–8.67)	0.336	2.09 (0.42–10.35)	0.364
Occlusal force
	≥ 500 N	100 (80.0) §	25 (20.0) §	1		1
	< 500 N	16 (76.2) §	5 (23.8) §	1.25 (0.42–3.74)	0.690	1.41 (0.44–4.53)	0.563
Number of teeth
	≥ 20	103 (80.5) §	25 (19.5) §	1		1
	< 20	13 (72.2) §	5 (27.8) §	1.59 (0.52–4.86)	0.421	2.93 (0.66–12.93)	0.156
Subjective masticatory ability
Subjective masticatory function
	Good	101 (80.2) §	25 (19.8) §	1		1
	Poor	15 (75.0) §	5 (25.0) §	1.35 (0.45–4.06)	0.597	2.49 (0.68–9.14)	0.169
Subjective occlusal status
	Good	111 (81.6) §	25 (18.4) §	1		1
	Poor	5 (50.0) §	5 (50.0) §	4.44 (1.19–16.51)	0.026*	6.00 (1.44–25.05)	0.014*
Masticatory habits
Masticatory habits‡	Good	80 (88.9) §	10 (11.1) §	1		1
	Poor	35 (64.8) §	19 (35.2) §	4.34 (1.83–10.29)	< 0.001*	6.49 (2.45–17.22)	< 0.001*

Abbreviations: OR, odds ratio; CI, confidence interval; BMI, body mass index

^† Adjusted for age, sex, BMI, systemic disease, dentures

^‡n = 144

^§ n (%)

*p < 0.05

Discussion

In this study, we longitudinally examined the relationship between physical function, assessed using the KCL, and masticatory ability and habits in community-dwelling older adults aged 65 and older. The results showed that poor subjective occlusal status and poor masticatory habits were associated with a decline in physical function 1 year later.

In this study, subjective occlusal status was evaluated using the question, “Can you bite firmly on both sides of the back teeth with your teeth or dentures?” and its relationship with physical function was examined. In older adults, good occlusion of the back teeth is important not only for nutritional intake but also for walking ability and muscle strength [29–32]. Kimura et al. reported that the occlusal support provided by premolars and molars is related to the perception of clenching these back teeth and that this support is significantly associated with life satisfaction and physical function, as measured by the one-legged balance and Timed Up & Go tests in older adults [33]. Although the present study did not provide a diagnostic examination of occlusal support, it is suggested that participants who responded that they could not bite down tightly with their left and right back teeth may have had poor occlusal support in the molar region, which affected their balance and walking function, leading to a subjective decline in physical function. However, no significant correlation was observed for objective evaluation items. Cross-sectional studies primarily confirm the relationship between objective masticatory ability and physical function [34]. However, a prospective study by Iwasaki et al. [35] reported a link between physical aspects of frailty and masticatory ability. The results of this study may differ from those of Iwasaki et al. owing to the younger average age of the subjects and the shorter follow-up period.

We believe that dietary diversity is associated with subjective masticatory ability, masticatory habits, and physical function. According to J. Kwon et al., a decrease in subjective masticatory ability is thought to increase the risk of reduced dietary diversity [36]. Moreover, maintaining a high level of dietary diversity long-term is reported to sustain physical function [37]. Consequently, individuals with high subjective masticatory ability can achieve balanced nutritional intake by enhancing dietary diversity. Additionally, a varied diet often includes foods that promote the habit of “chewing well,” such as meats and fibrous vegetables. Dietary proteins might help maintain muscle function in older adults [38], and the consumption of antioxidants (vitamins, etc.) from fruits and vegetables could bolster skeletal muscle strength [39], thereby preserving physical function. Conversely, in this study, even when nutritional items were included as adjustment factors, subjective masticatory ability and masticatory habits remained linked to physical function. The nutrition used for adjustment included BMI and weight loss, but the actual nutrients ingested were not assessed. As indicated earlier, subjective masticatory ability and masticatory habits may influence physical function via dietary diversity. Thus, future research should explore the connection with subjects’ food intake and nutrients.

Analysis of the relationship between objective masticatory ability, subjective masticatory ability, and masticatory habits showed that the subjectively poor occlusal status group had significantly fewer teeth and lower occlusal force and masticatory function than did the good group. Further, the poor subjective masticatory function group had significantly fewer teeth and lower occlusal force than did the good subjective masticatory function group. However, when comparing the masticatory habits between the groups, no significant differences were observed in any of the objective evaluation items. Hatayama et al. reported no significant difference in the number of chews between groups with and without oral hypofunction in 98 older subjects [40], which is similar to the findings of the present study. Notably, Hatayama et al. evaluated the number of chews using instrumental measurements. Conversely, in this study, masticatory habits, i.e., the number of chews, were assessed through a self-administered questionnaire asking, “Do you chew your food slowly and well?” Previous research has reported that self-reports of eating quickly are associated with the number of chews [41]. Given that the questions regarding masticatory habits in this study encompass the element of speed, “slowly”, they are expected to correlate with objective assessments of the number of chews. The number of chews is believed to vary depending on the patient’s health condition [42], age [43], and environmental factors such as dietary habits [44, 45]. Health conditions, including oral function, and age evolve over time. It is speculated that the number of chews unconsciously increases to compensate for the decline in function owing to these changes. On the other hand, it has been reported that individuals with poor masticatory ability do not chew longer than those with good masticatory ability but tend to swallow whole [46]. As outlined, findings on factors that evolve over time and their impact on the number of chews are inconsistent and warrant further investigation. Conversely, the living environment that influences chewing includes the conscious act of consuming foods (such as proteins and dietary fibers) that necessitate thorough chewing. The current questionnaire on masticatory habits is more aligned with conscious behavior rather than the unconscious increase in the number of chews. Therefore, it is suggested that no differences were observed in objective masticatory ability. Moreover, this study suggests that chewing on a daily basis until a patient is self-perceived as “chewing well” is effective in maintaining physical functions. Given that evidence on subjectively assessed masticatory habits is still lacking, further investigations are desirable.

The evaluation of physical function in this study was conducted by calculating the total score from five items that indicated physical function decline in the KCL and checking the change in the score over a 1-year period. The KCL was originally developed to identify the risk of persons requiring nursing care or support in Japan’s long-term care insurance system, but it has been reported that the KCL also assesses frailty [18] and predicts the incidence of dependency and mortality [47]. Previous studies have evaluated subjects on the total score of 25 questions [17, 48] or on the total score for each domain of KCL at one point in time [49]. However, only a few studies have examined changes in KCL scores over time [50, 51]. Assessment at one point in time is valid for determining those at severe risk but changes over time should also be assessed as they are useful for early prevention of progression for those at low risk. Therefore, this study evaluated the changes in physical function domain scores on the KCL. This evaluation method is considered effective because anyone can easily answer “yes/no” to each question, and changes in physical function over time can be checked by determining if there is an increase or decrease from the previous score. Therefore, to the best of our knowledge, using this method may allow for successful early intervention for low-risk patients.

As a result of the evaluation using this method, 20.5% of the subjects experienced a decline in physical function over a 1-year period. This decline was found to be related to subjective masticatory ability, which encompasses subjective occlusal status and masticatory habits. Although the present study was conducted over 1 year, future research should explore how the rate of physical function decline, as defined herein, evolves over multiple years. Moreover, it is crucial to investigate if interventions could lessen the incidence of diminished physical function. The average score for assessing physical function rose from 0.47 to 0.61 across the year. Moreover, the highest observed score increased to 4 in 2019 from 2 in 2018, indicating a rise in the challenges associated with physical activities, including falls or concerns about falling, among community-dwelling older adults. These outcomes imply that even independently living older individuals may face a reduction in physical function within a brief timeframe, underscoring the potential significance of early oral function-based interventions.

This study has some limitations. First, although calculations were performed using multiple potentially influential confounders, the lack of information on cognitive function, depression, and nutrition using a dedicated questionnaire limited our ability to control for these effects. However, for cognitive function, depression, and nutrition, the results of the KCL were used as adjustment factors to verify the effects. Second, a low percentage (14.7%) of the study participants completed the follow-up surveys. A comparison between the 146 individuals who participated in the follow-up survey and the 824 who only participated in the 2018 baseline survey, excluding 23 participants owing to missing data, revealed a significantly higher percentage of individuals in the analysis group receiving at least one dental checkup annually (p < 0.017, chi-squared test) (Table, Additional File 3). Therefore, it is assumed that the subjects included in the analysis had a high awareness of disease prevention, and the external validity of the results may be low. Third, owing to the small sample size, a detailed analysis by sex was not possible. Fourth, in this study, masticatory habits were evaluated through responses to a questionnaire. The reliability and validity of this question have not been confirmed, and participants may have interpreted the question differently. Therefore, validating the psychometric properties of this questionnaire item remains a task for future research. Owing to these limitations, it is necessary to conduct long-term follow-ups and large-scale studies and validate the questionnaire’s reliability in the future.

Conclusions

It has been suggested that masticatory ability and habits are associated with a decline in physical function after one year in community-dwelling older adults. To maintain the physical function of these individuals, early intervention that addresses not only masticatory ability but also masticatory habits is necessary.

Electronic supplementary material

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Abbreviations

BMI

Body mass index

CI

Confidence interval

KCL

Kihon Check List

OR

Odds ratio

Author contributions

MT contributed to the analysis and interpretation of data and also drafted the manuscript. RN, HF, MK, YK, and SY contributed important intellectual content during manuscript drafting or revision. NU contributed to the design, investigation, and organization of the study. MN supervised the study. All authors were involved in drafting and critically revising the manuscript and have approved the final manuscript.

Funding

The authors did not receive support from any organization for the submitted work.

Data availability

The datasets generated and/or analysed during the current study are not publicly available due to privacy or ethical restrictions but are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

This study was approved by the Ethics Committee of the Aichi Dental Association (1) and the Ethics Committee for Epidemiology of Hiroshima University (E2021-2704). All participants provided written informed consent.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.