J Prosthodont Res. 2023; **(**): ****–****

Journal of Prosthodontic Research

Original
articleReduced number of teeth with and without dental prostheses andlow frequency of laughter in older adults: Mediation by poor oral
function
Yudai Tamada a,b, Kenji Takeuchi a,b,c,*, Taro Kusama a,c, Masashige Saito d,e, Tetsuya Ohira f,g,
Kokoro Shirai h, Chikae Yamaguchi i, Katsunori Kondo j,k, Jun Aida l, Ken Osaka aa Department of International and Community Oral Health, Tohoku University Graduate School of Dentistry, Sendai, Japan, b Department of Preventive

Medicine, Nagoya University Graduate School of Medicine, Nagoya, Japan, c Division of Statistics and Data Science, Liaison Center for Innovative Dentistry,
Tohoku University Graduate School of Dentistry, Sendai, Japan, d Faculty of Social Welfare, Nihon Fukushi University, Mihama, Japan, e Center for Well-being
and Society, Nihon Fukushi University, Mihama, Japan, f Department of Epidemiology, Fukushima Medical University School of Medicine, Fukushima, Japan,
g Radiation Medical Science Center for Fukushima Health Management Survey, Fukushima Medical University, Fukushima, Japan, h Department of Social
Medicine, Public Health, Graduate School of Medicine, Osaka University, Suita, Japan, i Department of Nursing, Faculty of Nursing, Kinjo Gakuin University,
Nagoya, Japan, j Department of Social Preventive Medical Sciences, Center for Preventive Medical Sciences, Chiba University, Chiba, Japan, k Departmentof Gerontological Evaluation, Center for Gerontology and Social Science, Research Institute, National Center for Geriatrics and Gerontology, Obu, Japan, lDepartment of Oral Health Promotion, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Tokyo, Japan

Abstract
Purpose: Laughter is expected to have health-protective effects, but the potential link between tooth loss and laughter
remains unclear. Therefore, this study aimed to examine the association between tooth loss and a low frequency of laughter among older adults in Japan, to elucidate whether this association could be mitigated by dental prostheses, and toevaluate the magnitude of the association mediated by poor oral function.
Methods: We used cross-sectional data from 157,708 functionally independent participants aged ≥65 years (46.3% male)from the Japan Gerontological Evaluation Study. A modified Poisson regression model was applied to examine the association between the number of remaining teeth (≥20/10–19/0–9), dental prostheses use, and infrequent laughter (i.e.,laughing never or almost never). Causal mediation analysis was performed to assess whether the association was mediated by difficulties in eating hard foods, choking, or dry mouth.
Results: Among the participants, 9,129 reported infrequent laughter. Participants with ≤9 and 10–19 teeth who did notuse dental prostheses had a 1.29 and 1.14 times higher likelihood of infrequent laughter than those with ≥20 teeth, respectively. Furthermore, difficulty eating hard foods, choking, and dry mouth mediated 22.8%, 0.4%, and 4.3% of the association between fewer remaining teeth and infrequent laughter, respectively. Meanwhile, we did not find evidence for thedifferences in infrequent laughter between participants with ≤19 teeth using dental prostheses and those with ≥20 teeth.
Conclusions: Tooth loss among individuals without dental prostheses was associated with infrequent laughter, and thisassociation was mediated by poor oral function.
Keywords: Laughter, Denture, Oral status, Mediation, Japan
Received 13 March 2023, Accepted 13 September 2023, Available online 5 October 2023

1. Introduction
Laughter is believed to confer health benefits. Previous studies have shown that laughter strengthens the immune system[1],improves cardiovascular function[2], and buffers stress[3]. According
to recent large-scale studies, people with a lower daily frequency oflaughter have a higher risk of developing functional disabilities[4] ormortality[5], highlighting the importance of preventing a decrease

DOI: https://doi.org/10.2186/jpr.JPR_D_23_00071*Corresponding author: Kenji Takeuchi, Department of International and Community
Oral Health, Tohoku University Graduate School of Dentistry, 4-1 Seiryo-machi, Aobaku, Sendai, Miyagi, 980-8575, Japan.
E-mail address: kenji.takeuchi.c4@tohoku.ac.jp
Copyright: © 2023 Japan Prosthodontic Society. All rights reserved.

in laughter frequency. People who laugh less frequently are moreprevalent in older than in younger populations[6]. Therefore, it isimportant to identify an effective approach to prevent the decreasein laughter frequency from the perspective of health maintenance inthe current aging society.
In daily settings, laughter is more likely to be evoked in conversations than in other situations (e.g., watching television or videos)[7]. According to the previous literature[8], people who lose theirteeth are more likely to have difficulties speaking, an essential
component of conversations, partially because tooth loss leads tofunctional restrictions. Hirosaki et al. reported that older adults withfewer remaining teeth tended to laugh less frequently[9]. Although
tooth loss is irreversible, using dental prostheses, such as dentures,
can compensate for oral function after tooth loss and has potential

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preventive effects on health and psychosocial deterioration due totooth loss[10–14]. However, it is unclear whether dental prosthesis
use has similar preventive effects on changes in the frequency oflaughter due to tooth loss.
The daily frequency of laughter is influenced by functional factors such as the ability of facial muscles. The neck and head organs,including facial muscles, used in laughter are partially similar to
those used in mastication or swallowing[15]; therefore, masticatory
and swallowing dysfunction may subsequently make laughing frequently functionally difficult. Considering that a variety of programs
have been implemented to improve older adults’ oral function[16],if it could be shown that poor oral function is associated with infrequent laughter, it would provide insights into the motivation topromote oral function intervention programs. However, there is a
paucity of evidence regarding whether oral function is associatedwith tooth loss and frequency of laughter.
Our group previously reported that the number of remaining
teeth was associated with positive and negative emotions[14,17].
Abbas et al. showed that a higher number of teeth is associatedwith a higher likelihood of happiness[14]. Kusama et al. showed thatfewer teeth were associated with a higher risk of depression[17].
However, laughter is not only a positive emotional expression, butalso has social meaning[18], such as improving social relationships byfacilitating communication with others, and the absence of laughter
is not a simple reflection of negative emotions. Despite its potential
uniqueness, only one study has examined the association between
tooth loss and laughter[9], but it did not investigate the effects ofdental prosthesis use on the association. Thus, we hypothesizedthat tooth loss would be associated with an increased likelihood of
infrequent laughter and that such a negative association would be
mitigated by dental prostheses. This study aimed to investigate theassociation between the number of remaining teeth and the use of
dental prostheses, and the frequency of laughter among older adults
in Japan and to evaluate whether this association is mediated bypoor oral function.

2. Methods
2.1. Study population and settingThis study used data from the Japan Gerontological Evaluation
Study (JAGES), established in 2010, to investigate the social determinants of health among older adults[19,20]. This study used crosssectional data from the 2016 wave conducted between Octoberand December 2016. In this wave, self-reported questionnaires weresent to 279,661 older adults, ≥ 65 years old, randomly recruited orcompletely enumerated from 39 municipalities in Japan. The questionnaire contained questions regarding sociodemographic, physical, and psychological characteristics. Among 196,438 respondents(response rate = 71.1%), 179,991 remained eligible after excluding
those who could not be successfully identified or did not provide
valid information on sex or age. After restricting the participants
to those whose activities of daily living were independent, 157,708
functionally independent participants (men, 72,944; women, 84,764)
were included in the analysis (Fig. 1).
2.2. Outcome assessment
The outcome of this study was a low frequency of laughter,
which was assessed based on the response to the following single-

Fig. 1. Flow diagram of the study participant selection

item question: “How often do you laugh out loud?”, with possible
options of “almost every day,” “1 to 5 days per week,” “1 to 3 days permonth,” and “never or almost never.” Considering that people wholaughed never or almost never have been reported to have a highrisk of cardiovascular disease[21] or functional disability[4], those
who answered “never or almost never” were defined as having a low
frequency of laughter.
2.3. Exposure assessment
The exposure in this study was a composite variable that indicated the dental status, defined by combining the following two
variables: number of remaining teeth and use of dental prostheses.
The number of remaining teeth was determined using the following
question: ‘How many natural teeth do you currently have?’, with possible options of “no natural teeth,” “1–4 natural teeth,” “5–9 natural
teeth,” “10–19 natural teeth,” and “≥ 20 natural teeth.” According toprevious validation studies[22,23], the self-reported number of teeth
is broadly consistent with the clinically examined number, especiallyin older populations. Additionally, the use of dental prostheses wasassessed using the following question: “Do you wear dentures orbridges (nonremovable dentures) or have dental implants?” with thepossible options of “no,” “dentures,” “dental bridges,” and “dental
implants.” Participants who selected “no” were defined as not using
dental prostheses, and those who selected at least one of the otherthree were defined as using dental prostheses. According to the definitions used in previous studies[24,25], dental status was classified
into five categories (≥ 20 teeth, 10–19 teeth using dental prostheses,
10–19 teeth without using dental prostheses, ≤ 9 teeth using dental
prostheses, or ≤ 9 teeth without using dental prostheses). According
to the methodology of previous mediation analysis studies[17,26,27],in addition to the result of the dental status–low frequency oflaughter association analysis that showed that the participants with≤ 19 teeth not using dental prostheses had a statistically higher

Y. Tamada, et al. / J Prosthodont Res. 2023; **(**): ****–****

3

Fig. 2. Diagram of the analytical model

prevalence of low frequency of laughter than those with ≥ 20 teeth,a binary variable representing fewer remaining teeth (≥ 20 teeth or ≤19 teeth without dental prostheses) was created for causal mediation
analysis.
2.4. Mediator assessment
The mediator of this study was poor oral function evaluatedusing the Kihon Checklist proposed by the Japanese Ministry of
Health, Labour, and Welfare[28,29]. Three aspects of poor oral
function (difficulty eating hard food, choking, and dry mouth) were
evaluated using the questionnaire. Each aspect was determined using the following question: “Do you have any difficulties eating hardfoods compared to 6 months ago?”; “Have you choked on your tea
or soup recently?”; and “Do you often experience a dry mouth?” Participants who answered “yes” to at least two of the three questions
were defined as those with poor oral function[30]. In addition to amodel that included poor oral function as a mediator, models that
separately included each aspect of oral function as a mediator wereconstructed to investigate the differences in mediating effects in all
three aspects in the analysis.
2.5. Other covariates
In this study, the following potential confounders were controlled: sex (men or women), age (65-69, 70-74, 75-79, 80-84, or ≥ 85
years), hypertension (diagnosed or not diagnosed), diabetes mellitus(diagnosed or not diagnosed), smoking habits (current, former, or
never), alcohol intake (current, former, or never), marital status(with or without a spouse), living alone (yes or no), social participation (active or not active), depressive symptoms (depressed or not
depressed), educational attainment (≤ 9 years or ≥ 10 years), andequivalent income (< 2.00, 2.00–3.99, or ≥ 4.00; 10,000$, 1 US$ = 100
JPY). Hypertension and diabetes mellitus were assessed using thefollowing question: “Have you ever been diagnosed with hypertension or diabetes mellitus?” Participants who answered “yes” to thisquestions were considered to have hypertension or diabetes mellitus. Participation in eight social activities was used to define social
participation (volunteer groups, sports groups or clubs, leisure activity groups, and senior citizen clubs). Participants who participated in
at least one social activity more than once per week were consideredsocially active. Depressive symptoms were evaluated using theJapanese version of the 15-item Geriatric Depression Scale (GDS-15).
Participants were divided into two groups according to their scores:
not depressed (0–4 points) and depressed (≥ 5 points)[31]. Equivalent
income was calculated by dividing the annual household income by
the square root of the number of people living together and wascategorized into three groups.
2.6. Statistical analysis
For demographic characteristics, summary statistics wereconstructed using the frequencies for categorical variables. To
calculate the adjusted prevalence ratios (PR) and their 95 percent
confidence intervals (95% CI) for the prevalence of low frequency oflaughter in relation to dental status, a modified Poisson regression
model (i.e., a Poisson regression with sandwich standard errors) wasused[32], because the prevalence of low frequency of laughter wascommon in some categories of dental status. According to previous
methodology studies[33,34], the odds ratio (OR) may exaggerate thetrue risk when the outcome is common; thus, a modified Poisson
regression model was used to avoid overestimation of the relative
risks. Next, causal mediation analyses were used to decompose thetotal effect (TE) of fewer remaining teeth on the prevalence of low
frequency of laughter into effects through mediators (i.e., naturalindirect effect [NIE]) and without mediators (i.e., natural direct effect[NDE])[17,35,36]. A causal diagram of the analytical model is shownin Figure 2. Mediators (i.e., poor oral function, difficulty eating
hard foods, choking, and dry mouth) were included in the models
separately. Logistic regression models were applied to estimate theadjusted OR for TE, NIE, and NDE, and their 95% CI were obtainedby bootstrapping with 1,000 replications. To evaluate the extent towhich poor oral function mediated the association between fewer
remaining teeth and the prevalence of low frequency of laughter, theproportion mediated (PM) on a risk-difference scale was estimated.
Multiple imputations using chained equations were conducted
to impute missing data on all variables, and 20 imputed datasets weregenerated according to recommendations in a previous study[37].
The estimates across the imputed datasets were combined using
Rubin’s rule[38]. As an auxiliary analysis, complete case analyses wereconducted to compare the estimates to those obtained from the

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imputed datasets. All statistical analyses were performed using Stata(version 17.0; Stata Corp, College Station, TX, USA), and two-sided Pvalues < 0.05 were considered statistically significant in all cases. This
study followed the Strengthening the Reporting of Observational
Studies in Epidemiology (STROBE) guidelines.
2.7. Ethical considerations
The JAGES protocol and informed consent procedure were approved by the Ethics Committees of the National Center for Geriatrics
and Gerontology (No. 992) and Chiba University (No. 2493). The
JAGES participants were informed that participation in the study was
voluntary, and that completing and returning the questionnaire wasinterpreted as informed consent.

3. Results
Table 1 shows the characteristics of the study population according to their dental status. Of the participants (age: 73.8 ± 6.0
years [mean ± standard deviation], 46.3% male), 55.3% had ≥ 20
teeth, 17.6% had 10–19 teeth using dental prostheses, 3.7% had
10–19 teeth without using dental prostheses, 21.7% had ≤ 9 teeth
using dental prostheses, and 1.6% had ≤ 9 teeth without using dental
prostheses. In addition, 11.2% of participants with ≥ 20 teeth had
poor oral function. On the contrary, among participants with ≤ 19
teeth who did not use dental prostheses, 22.2% had poor oral function. The characteristics of the study population before imputation
are presented in Table S1.
Table 2 presents the results of the modified Poisson regression
analysis of the association between dental status and the prevalenceof low frequency of laughter. Among the participants, 9,129 (5.8%)
reported low frequency of laughter. Participants with 10–19 or ≤9 teeth without dental prostheses had 1.14 (95% CI, 1.04–1.26) or
1.29 (95% CI, 1.16–1.42) times higher prevalence of low frequencyof laughter than those with ≥ 20 teeth, respectively. In contrast,
no significant differences were observed in the prevalence of low
frequency of laughter between participants with ≤ 19 teeth using
dental prostheses and those with ≥ 20 teeth. In addition, among theparticipants with 10–19 teeth, those using dental prostheses had
1.17 (95% CI, 1.06–1.29) times higher prevalence of low frequency oflaughter than those not using dental prostheses. Similar results wereobtained in the complete case analysis (Table S2).
Table 3 shows the results of the causal mediation analyses
among participants with ≤ 19 teeth without using dental prostheses
and those with ≥ 20 teeth. Although the effect size was modest,
the estimated TE of fewer remaining teeth in the prevalence of low
frequency of laughter was significantly mediated by poor oral function (TE: OR, 1.22 [95% CI, 1.11–1.33]; NIE: OR, 1.02 [95% CI, 1.01–1.04];
PM, 11.8%). In addition, all three aspects of oral function (difficultyin eating hard foods, choking, or dry mouth) showed statisticallysignificant mediating effects; however, while the effect size of having
difficulty eating hard foods was moderate, those with experiencesof choking or dry mouth were modest. Specifically, difficulties ineating hard foods mediated 22.8% of the association between fewer
remaining teeth and a low frequency of laughter. On the other hand,
having experienced choking and dry mouth mediated 0.4% and4.3% of the association, respectively. Similar results were observed in
the complete case analysis (Table S3).

4. Discussion
In the present study of functionally independent Japanese older
adults, we found that participants with ≤ 19 teeth and who did notuse dental prostheses were associated with a higher likelihood of
low frequency of laughter compared to those with ≥ 20 teeth, suggesting the possibility that prosthodontic treatments can prevent a
decrease in laughing frequency after tooth loss. Poor oral function
also mediated this association. Difficulties in eating hard foods mediated 22.8% of this association. On the other hand, we did not findevidence of differences in the prevalence of low frequency of laughter between participants with ≤ 19 teeth using dental prostheses andthose with ≥ 20 teeth.
The findings of this study are broadly in line with those of theexisting literature. According to a previous study[9], there is a clear
inverse association between the number of remaining teeth andprevalence of low frequency of laughter. In this study, despite thepotential preventive effects of using dental prostheses against adecrease in laughter frequency, their use was not considered in theanalyses. Although dental prosthesis nonusers with ≤ 19 teeth had
a significantly higher prevalence of low frequency of laughter thanthose with ≥ 20 teeth, the frequency of laughter among dental prosthesis users with ≤ 19 teeth was slightly different from those with ≥20 teeth. In addition, this is the first study to elucidate the mediating
effects of poor oral function on the association between a low number of teeth and high prevalence of low frequency of laughing. In thiscontext, we believe that the findings of this study will enhance ourunderstanding of the association between oral status and frequencyof laughter in daily life.
Our study observed that a lower number of teeth was associated with a higher likelihood of low frequency of laughter, and thisassociation was predominantly mediated by difficulty in eating hardfoods. Given that decreased occlusal forces indicate the presenceof masticatory dysfunction[39], the masticatory muscles could havedeclined in participants with difficulty eating hard foods; therefore,
it could be functionally difficult for them to laugh because the masticatory muscles play critical roles in facial expressions[40,41]. On thecontrary, in our study, there was no evidence of differences in theprevalence of low laughing frequency between participants with ≥20 teeth and those with ≤ 19 teeth. As is known, the main objectiveof prosthetic treatment is to restore occlusal function after toothloss[42], which was empirically supported by our results that, especially among participants with ≤ 9 teeth, dental prostheses users had
a 20% lower prevalence of difficulty eating hard foods than nonusers(see Table 1). Taken together, the use of dental prostheses prevented
the decline in occlusal and masticatory functions after tooth loss;
and consequently, the frequency of laughter was maintained.
Other explanations for a lower number of teeth that directly
leads to a higher likelihood of a low frequency of laughter may bepossible. First, people who lose their teeth, especially when they do
not use dental prostheses, may hesitate to laugh in front of others
for aesthetic reasons[43], probably resulting in a low frequency oflaughter. On the flip side, it is possible that dental prostheses restored smile aesthetics after tooth loss and reduced hesitation, so
the prevalence of low frequency of laughter was slightly different
between participants with ≤ 19 teeth using dental prostheses andthose with ≥ 20 teeth. Second, tooth loss leads to difficulty in eating
food, which, in turn, may reduce opportunities to eat together[25]. In
this context, people who lost their teeth missed the opportunity toY. Tamada, et al. / J Prosthodont Res. 2023; **(**): ****–****

5

Table 1. Baseline characteristics of the study population according to the dental status (n = 157,708)
Dental Status
Total(n = 157,708)≥ 20 teeth(n = 87,233)

10–19 teeth
using dental
prostheses(n = 27,831)

10–19 teeth
without using
dental prostheses(n = 5,859)≤ 9 teeth using
dental prostheses(n = 34,273)≤ 9 teeth without using dental
prostheses(n = 2,512)

n

%

n

%

n

%

n

%

n

%

n

%

84,76453.7

48,84756.0

15,329

55.1

2,602

44.4

17,145

50.0

841

33.5

Sex
Women
Age, years
65–69

49,42831.3

32,572

37.3

7,630

27.4

2,285

39.0

6,087

17.8

854

34.0

70–74

43,33127.5

25,316

29.0

7,799

28.0

1,655

28.3

7,887

23.0

674

26.8

75–79

35,80722.7

18,450

21.2

6,869

24.7

1,108

18.9

8,895

26.0

484

19.3

80–84

20,329

12.9

8,465

9.7

3,948

14.2

564

9.6

7,042

20.5

311

12.4

≥ 85

8,813

5.6

2,430

2.8

1,585

5.7

248

4.2

4,362

12.7

188

7.5

69,04643.8

36,62342.0

12,750

45.8

2,741

46.8

15,903

46.4

1,029

41.0

20,53713.0

10,010

11.5

3,828

13.8

959

16.4

5,336

15.6

404

16.1

Hypertension
Diagnosed
Diabetes mellitus
Diagnosed
Smoking habits
Current

16,964

10.8

6,958

8.0

3,195

11.5

1,024

17.5

5,071

14.8

717

28.5

Former

45,56228.9

23,728

27.2

8,305

29.8

1,708

29.1

11,070

32.3

751

29.9

Never

95,18260.4

56,54764.8

16,332

58.7

3,127

53.4

18,132

52.9

1,044

41.6

Current

61,31638.9

35,64040.9

10,766

38.7

2,442

41.7

11,551

33.7

918

36.5

Former

16,346

10.4

7,890

9.0

2,924

10.5

698

11.9

4,421

12.9

413

16.4

Never

80,04650.8

43,70350.1

14,140

50.8

2,720

46.4

18,302

53.4

1,181

47.0

115,076

73.0

66,422

76.1

20,045

72.0

4,276

73.0

22,769

66.4

1,564

62.3

20,75713.2

10,740

12.3

3,789

13.6

809

13.8

4,945

14.4

473

18.8

52,79533.533,239

38.1

8,923

32.1

1,407

24.0

8,801

25.7

425

16.9

32,696

20.7

15,382

17.6

6,082

21.9

1,593

27.2

8,665

25.3

974

38.8

107,809

68.4

65,16874.7

18,495

66.5

3,802

64.9

19,058

55.6

1,286

51.2

Alcohol intake

Marital status
With a spouse
Living alone
Yes
Social participation
ActiveDepressive symptoms
Depressed
Educational attainment≥ 10 years
Equivalent income (10,000$; 1$ = 100JPY)< 2.0078,21649.6

38,53944.2

14,509

52.1

3,381

57.7

20,055

58.5

1,732

69.0

2.00–3.99

62,121

39.4

37,62543.1

10,595

38.1

1,999

34.1

11,291

32.9

611

24.3

≥ 4.0017,371

11.0

11,069

12.7

2,727

9.8

480

8.2

2,926

8.5

168

6.7

24,854

15.8

9,755

11.2

5,692

20.5

1,100

18.8

7,553

22.0

755

30.1

42,43926.913,322

15.3

10,915

39.2

2,130

36.4

14,543

42.4

1,529

60.9

27,84017.7

14,219

16.3

5,256

18.9

983

16.8

6,856

20.0

527

21.0

Yes
31,44819.9
Note: Each value is mean of 20 imputed datasets.

15,951

18.3

6,143

22.1

1,271

21.7

7,403

21.6

680

27.1

Oral function
Poor
Difficulty in eating hard foods
Yes
Choking
Yes
Dry mouth

laugh with others, and this may have decreased their daily frequencyof laughter.
The strengths of this study include its relatively large sample size

and adjustment for various key sociodemographic factors and underlying health conditions, which were expected to address potential confounding biases. However, this study had several limitations.
First, the possibility of reverse causality owing to the cross-sectional

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6

Table 2. Association between the dental status and the prevalence of low frequency of laughter (n = 157,708)n

Low frequencyof laughter, n (%)

Crude
PR (95% CI)

AdjustedPR (95% CI)*

AdjustedPR (95% CI)*

Dental status≤ 9 teeth without using dental prostheses

2,512

354 (14.1)

2.97 (2.68–3.29)

1.29 (1.16–1.42)

1.31 (1.18–1.46)≤ 9 teeth using dental prostheses

34,2732,590 (7.6)

1.59 (1.52–1.67)

1.05 (0.99–1.10)

1.07 (1.003–1.13)

10–19 teeth without using dental prostheses

5,859

481 (8.2)

1.73 (1.58–1.90)

1.14 (1.04–1.26)

1.17 (1.06–1.29)

10–19 teeth using dental prostheses

27,831

1,566 (5.6)

1.19 (1.12–1.26)

0.98 (0.93–1.04)

1.00 (reference)≥ 20 teeth
87,233
4,138 (4.7)
1.00 (reference)
1.00 (reference)
1.02 (0.96–1.08)
Abbreviations: PR: prevalence ratio, CI: confidence interval. * Adjusted for sex, age, hypertension, diabetes mellitus, smoking habits, alcohol intake, marital
status, living alone, social participation, depressive symptoms, educational attainment, and equivalent income.

Table 3. Effect decomposition of total effect of fewer remaining teetha on the prevalence of low frequency of laughter (n = 95,604b)
TE

NDE

NIE

PMd

OR (95% CIe)

OR (95% CIe)

OR (95% CIe)%

Oral function: poor

1.22 (1.11–1.33)

1.19 (1.08–1.31)

1.02 (1.01–1.04)

11.8

Difficulty eating hard foods: yes

1.23 (1.11–1.34)

1.17 (1.05–1.30)

1.04 (1.00–1.08)

22.8

Choking: yes

1.22 (1.11–1.33)

1.22 (1.11–1.33)

1.00 (1.00–1.00)

0.4Mediatorsc

Dry mouth: yes
1.21 (1.10–1.32)
1.20 (1.09–1.31)
1.01 (1.00–1.01)
4.3
Abbreviations: OR: odds ratio, CI: confidence interval, TE: total effect, NDE: natural direct effect, NIE: natural indirect effect, PM: proportion-mediated. All
models were adjusted for sex, age, hypertension, diabetes mellitus, smoking habits, alcohol intake, marital status, living alone, social participation, depressive
symptoms, educational attainment, and equivalent income. a Tooth loss was classified into two categories (> 20 teeth/≤ 19 teeth without using dental prostheses). b People who had ≤ 19 teeth using dental prostheses were excluded. c Each mediator was separately included. d Proportion mediated were calculated
on a risk difference scale. e Estimated by bootstrap with 1000 replications.

design of this study cannot be ruled out. For instance, according to thebroaden-and-build theory[44], it may be plausible that having morelaughter experiences as an expression of positive emotions could
improve health-seeking behaviors such as increasing the frequencyof regular dental visits or tooth brushing, which may contribute topreventing tooth loss. Second, the number of teeth was assessedusing a self-reported questionnaire. In previous studies[22,23], thevalidity of self-reported number of teeth was confirmed; however,the possibility of misclassification of the number of teeth remains.
Third, although this study assessed the three aspects of oral function
as mediators of the association between fewer remaining teeth anda low frequency of laughter, the effect sizes were modest to moderate, which may suggest the presence of other essential mediating
factors, such as smile aesthetics. Denture loss has been reported to
be associated with embarrassment to laugh in front of others[43],suggesting that smile aesthetic impairments reduce the frequencyof laughter. Further studies are needed to explore the major mediating effects on the link between dental status and laughter. Fourth,the participants may not be representative of the Japanese national
older adult population because of the voluntary nature of their participation. Therefore, our findings should be interpreted with caution, especially when generalizing them to other populations[45].

5. Conclusions
This study demonstrated that having fewer teeth and not using
dental prostheses were associated with a high likelihood of low frequency of laughter among older Japanese adults. Furthermore, poor
oral function, especially difficulty in eating hard foods, mediated thisassociation. Meanwhile, the use of dental prostheses was observed
to mitigate the association between a lower number of teeth and ahigher likelihood of low frequency of laughter. Our results suggestthat facilitating the use of dental prostheses and preventing a decline in oral function may contribute to preventing a decrease in thefrequency of laughter due to tooth loss.

Appendix
Supplementary materials can be found in the online version of
the journal.

Acknowledgments
This study used data from JAGES (the Japan Gerontological
Evaluation Study). This study was supported by Grant-in-Aid for
Scientific Research (15H01972, 15H04781, 15H05059, 15K03417,
15K03982, 15K16181, 15K17232, 15K18174, 15K19241, 15K21266,
15KT0007, 15KT0097, 16H05556, 16K09122, 16K00913, 16K02025,
16K12964, 16K13443, 16K16295, 16K16595, 16K16633, 16K17256,
16K17281, 16K19247, 16K19267, 16K21461, 16K21465, 16KT0014,
16KK0059, 17K04305, 17K34567, 17K04306, 25253052, 25713027,
26285138, 26460828, 26780328, 18H03018, 18H04071, 18H03047,
18H00953, 18H00955, 18KK0057, 19H03901, 19H03915, 19H03860,
19K04785, 19K10641, 19K11657, 19K19818, 19K19455, 19K24060,
19K20909, 20H00557, 20H03954, 21KK0168, 22H03299, 22H03352,
23K18370) from JSPS (Japan Society for the Promotion of Science);
Health Labour Sciences Research Grants (H26-Choju-Ippan-006,
H27-Ninchisyou-Ippan-001, H28-Choju-Ippan-002, H28-NinchisyouIppan-002,
H29-Chikyukibo-Ippan-001,
H30-JyunkankinadoIppan-004, 18H04071, 19FA1012, 19FA2001, 21GA1001) from the
Ministry of Health, Labour, and Welfare, Japan; the Research andDevelopment Grants for Longevity Science from Japan Agency
for Medical Research and Development (AMED) (JP17dk0110027,
JP18dk0110027, JP18ls0110002, JP18le0110009, JP20dk0110034,
JP20dk0110037), the Research Funding for Longevity Sciences from
National Center for Geriatrics and Gerontology (24-17, 24-23, 29-42,
30-30, 30-22, 21-20); Open Innovation Platform with Enterprises, Research Institute, and Academia (OPERA,JPMJOP1831) from the JapanY. Tamada, et al. / J Prosthodont Res. 2023; **(**): ****–****

Science and Technology (JST); a grant from the Japan Foundation For
Aging And Health (J09KF00804); a grant from Innovative Research
Program on Suicide Countermeasures (1-4); a grant from Sasakawa
Sports Foundation; a grant from Japan Health Promotion & Fitness
Foundation; a grant from Chiba Foundation for Health Promotion& Disease Prevention; the 8020 Research Grant for fiscal 2019 fromthe 8020 Promotion Foundation (adopted number:19-2-06); a grantfrom Niimi University (1915010); and grants from Meiji Yasuda LifeFoundation of Health and Welfare. YT was supported by the Nagoya
University CIBoG WISE Program from MEXT. The funders had no rolein the study design, data analysis, data interpretation, manuscript
preparation, or decision to submit the manuscript for publication.
The views and opinions expressed in this article are those of the authors and do not necessarily reflect the official policies or positions of
the respective funding organizations.

Conflict of interest statement
The authors declare no conflict of interest.

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