Journal of the National Institute of Public Health2012 Vol.61 No.4 p.366−373＜Original＞
The relationship between number of natural teeth andoral health behavior in adult Japanese people
Yoshimi NAKAYAMA１，２），Mitsuru MORI２）
１）
２）

Hokkaido Tomakomai Public Health Center
Department of Public Health, Sapporo Medical University School of Medicine

Abstract
Objectives: The Survey of Dental Diseases in Japan in 2005 has reported that most of 80 year old people have fewer than 20
teeth. The aim of the present study was to investigate the association of tooth loss with oral health behavior among Japanesepeople.
Methods: Subjects were 825 persons over 50 years in the eastern part of Hokkaido, Japan. The survey items were age, sex,
smoking status, oral health behavior such as frequency of tooth brushing, and so on. Furthermore, among the 825 subjects, weselected 750 subjects who had over 5 present teeth. The analysis of the relationship between number of natural teeth and toothbrushing, using an interdental brush or dental floss used those subjects. Odds ratios (ORs) and their 95% confidence intervals(CIs) of variables related to having teeth fewer than 20 were evaluated using the unconditional logistic regression model by sex.
Results: Habitual smoking was significantly associated with risk of having teeth fewer than 20 in the male subjects (OR=2.46,
95%CI: 1.05-5.78) as well as in the female subjects (OR=2.94, 95%CI: 1.09-7.94). Tooth brushing less than or equal to once a day(OR=2.57, 95%CI:1.38-4.80), nonuse of either an interdental brush (OR=2.14, 95%CI: 1.27-3.59) or dental floss (OR=3.56, 95%CI:
1.48-8.55) and no complaint of oral condition (OR=0.56, 95%CI:0.34-0.95) were significantly associated with risk of having teeth
fewer than 20 in the female subjects.
Conclusion: Frequent oral health behavior may reduce the risk of tooth loss.
keywords: tooth loss, oral health behavior, smoking, ageing(accepted for publication, 25th June 2012)

Ⅰ．Introduction
The 8020 movement was initiated in 1989 in Japan, withthe objective of helping individuals to retain 20 or moreteeth at 80 years of age. The Survey of Dental Diseases inJapan in 2005 has reported that the proportion of elderly
individuals with 20 or more teeth has increased year by
year, but that most 80 year old people still have fewer than
20 teeth. The proportion of 80 year old people having 20 ormore teeth was 24.1% [1]. Having 20 or more teeth at 80
years old contributed to enjoying a rich and varied diet.
Most studies reported the impact of dentition status on
nutritional conditions and dietary habits [2-8].
Several reports suggested that impaired mastication
ability by tooth loss might be a risk factor for cardiovasculardisease, gastrointestinal disease, and other healthproblems [2, 7]. Therefore, sustaining teeth is very
important. In previous studies of risk factors for tooth loss,
most surveys focused on the association of tooth loss
with socioeconomic status [4, 9, 10], osteoporosis [11-18],
smoking [4, 9, 19-24] and drinking alcohol [19,20]. However
the relationship of tooth loss to oral health behavior werestudied less well. Tamura [25] reported relationshipbetween oral health behavior and oral health status in
Japanese adult, but her analysis focused only on having afamily dentist and smoking status. Although Fukuda etal. [26] reported association between oral health habits andthe number of teeth present in Japanese aged 50-80 years at
dental clinics, survey items did not include frequency of
tooth brushing, using an interdental brush and having afamily dentist. Yoshino et al. [27] reported on the

Yoshimi Nakayama
2-2-21 Wakakusa town, Tomakomai-shi, Hokkaido 053-0021, Japan.
T e l: 0144-34-4168
Fax: 0144-34-4177
E-mail: nakayama.yoshimi@pref.hokkaido.lg.jp

366

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The relationship between number of natural teeth and oral health behavior in adult Japanese people

relationship between tooth loss and oral health behavior inJapanese aged 20-59 years by cohort study, but the numberof subject was small. Kondo et al. [28] and Ando et al. [29]reported on the relationship between tooth loss and oral
health behavior in adult Japanese by cohort study.
The aim of the present study was to investigate theassociation of number of natural teeth with oral healthbehavior among adult Japanese people.

Ⅱ．Methods
1．Subjects
The participants in this survey were 1034 subjects (457males, 577 females) over 20 years of age who had a dental
examination municipality conducted from June of 2010 toMarch of 2011, in Obihiro city and Honbetsu town of theTokachi area, located in the eastern part of Hokkaido, thenorthern island of Japan. The subjects in Obihiro city were
39,609 people aged 20 years or over who joined the national
health insurance. The subjects in Honbetsu town were 513
people aged 40 years or over who participated in a healthcheckup to preventive metabolic syndrome which wasstarted in 2008 by the government. The people who had adental examination in Obihiro city were 791 subjects (365males, 426 females), and 243 subjects (92 males, 151
females) in Honbetsu town. Among them, we selected 825
subjects (357 males, 468 females) aged over 50 years. Also,
209 subjects aged less than or equal to 49 years of age were
excluded because people who had few teeth were fewer.
The analysis of the relationship between number of natural
teeth and age, smoking status, having a family dentist,
regular dental check-ups, and complaints about oral
condition used 825 subjects.
Furthermore, among the 825 subjects, we selected 750
subjects (331 males, 419females) who had over 5 present
teeth. The analysis of the relationship between number ofnatural teeth and tooth brushing, using an interdental
brush or dental floss used those subjects. This is because
the few present teeth suffer under the influence of oral
health behavior such as tooth brushing.
This study was approved by the Ethical Committee of
Sapporo Medical University.
2．Method of Survey
For the survey, public health nurses in Honbetsu town ordental hygienists of a dental clinic in Obihiro cityinterviewed the subjects with a structured questionnaire on
the day of the dental check-up. The surveyed items wereage, sex, smoking status (current, former and non smoker),oral satisfactory degree, frequency of tooth brushing atpresent, using an interdental brush and dental floss atpresent, having regular dental check-ups, having a family

dentist.
The dental examination was conducted by a trained
dentist of a local dental clinic under sufficient artificial light,
with dental mirrors, and explorers. The contents of thedental examination were the diagnoses of dental caries(sound teeth, filled teeth, decayed teeth and missing teeth).
3．Analyses
We compared subjects who had fewer than 20 teeth withsubjects who had more than 20 teeth using an
unconditional logistic regression model by sex. We
analyzed data by sex, because sex was thought to be a
prominent confounding factor. The odds ratios (ORs) andtheir 95% confidence intervals (95%CIs) were estimatedwith regard to risk factor for tooth loss. Before themultivariate logistic regression analysis was conducted, we
confirmed that multicollinearity among the variables did notexist by the Spearman rank correlation test by sex. Tests ofstatistical significance were based on a two-sided P value,
and the α-error was set at the 5% level. The SAS system(ver.9.2) was employed for the analysis.

Ⅲ．Results
The average age among the 357 males was 69.5 years old(standard deviation, SD=9.7, range 50-96), and among the468 females, 68.2 years old (SD=8.4, range50-92).
Table 1 shows characteristics of the subjects by numberof teeth and sex. The number of subjects who brushed 2times a day mostly included both males and females, andsubjects who didn’t brush at all were a few. The proportionof nonsmoker in the male subjects was 58.7%, and in thefemale subjects it was 86.9%.
Table 2 shows tooth characteristics of the 825 subjects bysex. The average number of present teeth in the malesubjects was 18.1(SD=7.6), and in the female subjects it was
17.4(SD=7.9). DMFT (Average number of decayed teeth,
missing teeth or filled teeth) in the male subjects was
20.8(SD=6.3), and in the female subjects, 22.3(SD=5.3). For
both males and females in our study, DMFT and theaverage number of treated teeth was higher than results of
The Survey of Dental Diseases in Japan in 2005. The
proportion of people with an edentulous jaw in the malesubjects was 1.4%, and in the female subjects was 3.2%.
Table 3 shows a crude odds ratio with 95% confidence
intervals for number of teeth by sex. At an age of 70 years
or more (OR=2.11, 95%CI: 1.09-4.11) was significantly
associated with the risk of having teeth fewer than 20 in themale subjects. At an aged from 60 to 69 years (OR=2.36,
95%CI: 1.31-4.23), and at an age of 70 years or more
(OR=3.80, 95%CI: 2.12-6.81) , tooth brushing less than orequal to once a day (OR=2.67, 95%CI: 1.59-4.46) and no

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Yoshimi NAKAYAMA，Mitsuru MORI

usage of an interdental brush (OR=2.03, 95%CI: 1.37-3.02)
or dental floss (OR=2.76, 95%CI: 1.39-5.49) weresignificantly associated with the risk of having teeth fewer
than 20 in the female subjects. Current smoking (OR=2.11,
95%CI: 1.01-4.40) was significantly associated with the riskof having teeth fewer than 20 in the female subjects.
However oral complaint, no regular dental check-ups andhaving no family dentist were not associated with the risk of
having teeth fewer than 20.

To adjust for potential confounding factors, a multivariate
unconditional logistic regression model was used by sex.
As shown in Tables 4, for the multivariate analysis of themale subjects, we compulsively included age, complaint oforal condition, frequency of tooth brushing, using aninterdental brush and dental floss, regular dental checkups, having a family dentist and smoking status in themodel. Current smoking was significantly associated withthe risk of having teeth fewer than 20 in the male subjects

Table 1 Characteristics of the subjects by number of teeth by sex
males

femalesnumber of teeth≦19

number of teeth

≧20

≦19

≧20

n

n (%)

n (%)

n

n (%)

n (%)

Age(year): 50-5947

17(36.2)30(63.8)70

21(30)

49(70)

60-69

132

58(43.9)74(56.1)

193

97(50.3)

96(49.7)

70-79

139

67(48.2)

72(51.8)

163

94(57.7)

69(42.3)80＋

39

30(76.9)9(23.1)42

33(78.6)9(21.4)Complaint of oral condition: No

125

54(43.2)

71(56.8)

169

95(56.2)

74(43.8)

Yes

214

108(50.5)

106(49.5)259

128(49.4)

131(50.6)

Frequency of tooth brushing: Nothing

17

10(58.8)7(41.2)4

4(100)

0(0)Sometimes

15

6(40.0)9(60.0)179(52.9)8(47.1)Once a day

101

51(50.5)50(49.5)

59

40(67.8)

19(32.2)

2 times a day

122

51(41.8)

71(58.2)

181

76(42.0)105(58.0)≧3 times a day

63

23(36.5)40(63.5)

135

58(43.0)77(57.0)

Using an interdental brush: Ever

69

25(36.2)

44(63.8)

177

65(36.7)

112(63.3)

Never

262

121(46.2)

141(53.8)

242

131(54.1)

111(45.9)

19

5(26.3)14(73.7)

46

12(26.1)

34(73.9)

Using a dental floss: Ever

312

141(45.2)

171(54.8)373

184(49.3)

189(50.7)

Regular dental check-ups: Yes

Never

179

89(49.7)90(50.3)

217

114(52.5)

103(47.5)

No

156

71(45.5)85(54.5)

216

119(55.1)

97(44.9)

Having own family dentist: Yes

262

127(48.5)

135(51.5)

339

176(51.9)

163(48.1)

61

29(47.5)32(52.5)

72

40(55.6)32(44.4)

Smoking status: Nonsmoker

No

201

94(46.8)

107(53.2)373

197(52.8)

176(47.2)

Ex-smoker

62

29(46.8)33(53.2)

19

9(47.4)10(52.6)

Current smoker

79

45(57.0)34(43.0)3726(70.3)

11(29.7)

Table 2 Tooth characteristics of 825 subjects by sex
males

females58(16.3%)

103(22.0%)

10-19 teeth

114(31.9%)

142(30.3%)≧20 teeth

185(51.8%)

223(47.7%)

Average number of present teeth(SD)18.1(7.6)

17.4(7.9)

Average number of decayed teeth(SD)1.2(2.5)

0.9(2.1)

Average number of treated teeth(SD)9.7(5.5)

10.8(6.0)

Number of present teeth: 0-9 teeth

Average number of intact teeth(SD)7.2(6.3)

5.7(5.3)

DMFT(SD)

20.8(6.3)

22.3(5.3)

SD: Standard Deviation
DMFT: average number of decayed teeth, missing teeth or treated teeth

368

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The relationship between number of natural teeth and oral health behavior in adult Japanese people(OR=2.46, 95%CI: 1.05-5.78).
As shown in Tables 5, for the multivariate analysis of thefemale subjects, we compulsively included age, complaint oforal condition, frequency of tooth brushing, using aninterdental brush and dental floss, regular dental checkups, having a family dentist and smoking status in themodel. At an age from 60 to 69 years (OR=3.38, 95%CI: 1.587.23), and at an age of 70 years or more (OR=3.91, 95%CI:

Table 3 Crude odds ratios(ORs), and 95% confidence intervals(CIs) of having less than 20 existing teeth with univariate logistic
regression analysis by sex
males
Variables

OR(95%CI)

femalesp-value

Age: 50-59 years

1.00(reference)

60-69 years

1.38(0.70-2.75)

0.355

2.11(1.09-4.11)

0.027

70＋ years

1.00(reference)Yes

1.34(0.86-2.09)

Frequency of tooth brushing: ≧2 times a day

1.52(0.97-2.39)

Using an interdental brush: Ever

1.00(reference)Never

1.51(0.87-2.61)

Using a dental floss: Ever
Never

2.31(0.81-6.57)

No

0.85(0.55-1.30)
1.00(reference)

3.80(2.12-6.81)<0.0010.76(0.52-1.12)

0.170

1.00(reference)
0.067

2.67(1.59-4.46)<0.0011.00(reference)
0.140

2.03(1.37-3.02)<0.0011.00(reference)
0.117

1.00(reference)

Having own family dentist: Yes

0.004

1.00(reference)
0.197

1.00(reference)

Regular dental check-ups: Yes

2.36(1.31-4.23)
p for trend<0.001

1.00(reference)≦Once a day

p-value1.00(reference)

p for trend=0.001
Complaint of oral condition: No

OR(95%CI)

2.76(1.39-5.49)

0.004

1.00(reference)
0.442

1.11(0.76-1.62)

0.594

1.00(reference)No0.96(0.55-1.68)

Smoking status: Nonsmoker

1.00(reference)

0.896

1.16(0.69-1.93)

0.575

Ex-smoker

1.00(0.57-1.77)

0.999

0.80(0.32-2.02)

0.643

Current smoker

1.51(0.89-2.55)

0.126

2.11(1.01-4.40)

0.046

1.00(reference)

Table 4 Multivariate odd ratios(ORs), and 95% confidence intervals(CIs) of having lessthan 20 existing teeth with multivariate logistic regression analysis in males
VariableOR(95%CI)

p-value

Age: 50-59 years

1.00(reference)

60-69 years

0.94(0.42-2.11)

0.879

70＋ years

1.69(0.76-3.75)

0.195

Complaint of oral condition: No
Yes
Frequency of tooth brushing: ≧2 times a day≦Once a day
Using an interdental brush: Ever
Never

1.00(reference)
1.47(0.87-2.48)
1.26(0.75-2.11)
1.53(0.82-2.86)
1.00(reference)Never

2.26(0.69-7.39)

0.182
0.177

1.00(reference)No0.65(0.38-1.10)

Having own family dentist: Yes

1.00(reference)

No

0.386

1.00(reference)

Using a dental floss: Ever
Regular dental check-ups: Yes

0.154

1.00(reference)

1.09(0.58-2.08)

0.113
0.781

Smoking status: Nonsmoker

1.00(reference)

Ex-smoker

1.17(0.42-3.25)

0.771

Current smoker

2.46(1.05-5.78)

0.038

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Yoshimi NAKAYAMA，Mitsuru MORI
Table 5 Multivariate odd ratios(ORs), and 95% confidence intervals(CIs) of having lessthan 20 existing teeth with multivariate logistic regression analysis in females
VariableOR(95%CI)
1.00(reference)

60-69 years

3.38(1.58-7.23)

0.002

70＋ years

3.91(1.80-8.49)<0.001

Complaint of oral condition: No
Yes
Frequency of tooth brushing: ≧2 times a day≦Once a day
Using an interdental brush: Ever
Never
Using a dental floss: Ever

1.00(reference)
0.56(0.34-0.95)

0.030

1.00(reference)
2.57(1.38-4.80)

0.003

1.00(reference)
2.14(1.27-3.59)

0.004

1.00(reference)Never

3.56(1.48-8.55)

Regular dental check-ups: Yes

1.00(reference)No0.73(0.44-1.22)

Having own family dentist: Yes

1.00(reference)No

1.07(0.54-2.15)

0.005
0.233
0.841

Smoking status: Nonsmoker

1.00(reference)

Ex-smoker

1.20(0.39-3.68)

0.754

Current smoker

2.94(1.09-7.94)

0.034

1.80-8.49), no complaint of oral condition(OR=0.56,
95%CI:0.34-0.95), tooth brushing less than or equal to oncea day (OR=2.57, 95%CI: 1.38-4.80), no usage of an interdetalbrush (OR=2.14, 95%CI: 1.27-3.59) or dental floss (OR=3.56,
95%CI: 1.48-8.55) were significantly associated with the riskof having teeth fewer than 20. Current smoking was
significantly associated with the risk of having teeth fewer
than 20 in the female subjects (OR=2.94, 95%CI: 1.09-7.94).

Ⅳ．Discussion
In this study, a lower tooth brushing, the non-use of aninterdental brush or dental floss was significantly
associated with few natural teeth in females.
Aida et al. [30] reported that about of 85% permanent
teeth extracted were extracted due to caries and its sequela,
and periodontal disease in Japan. Therefore, dental caries
and periodontal disease are strong contributors to toothloss. Tseveenjav et al. [31] reported that people who brush
teeth less than 2 times a day were more likely to havedecayed teeth compared to those who brush teeth twice ormore daily among adults aged 33 to 64 years. Pakpourl
et al. [32] reported a significant relation between a high
DMFT (average number of decayed teeth, missing teeth orfilled teeth) and a lower frequency of tooth brushing anddental flossing among adolescents aged 14 to 18 years.
Ojima et al. [33] reported that the frequency of daily
brushing was significantly correlated with the prevalence of370

p-value

Age: 50-59 years

a more severe form of periodontitis among adults aged ≧40years. Oshikohji et al. [34] reported that a lower frequencyof tooth brushing was significantly associated with theproportion of teeth with a probing depth≧4mm(periodontitis) among 35-44 years old. Therefore,
depending on the lower frequency of tooth brushing,nonuse of an interdental brush and dental floss, oralcavities become poor hygiene and cause dental caries andperiodontal disease. Finally these oral problems lead totooth loss.
Yoshino et al. [27] and Kondo et al.[28]reported lower
tooth brushing, the non-use an interdental brush or dental
floss were not significantly associated with tooth loss bycohort study. Ando et al.[29]reported that no habit of using
an interdental brush or dental floss were significantly
associated with tooth loss by cohort study. Yanagisawa
et al. [22] showed similar findings to our results, but
subjects were only men and their analysis was unadjusted
for confounders. Gillbert et al.[35]reported the associationof tooth loss with tooth brushing and flossing, but the agesof the subjects were over 65 years old and the analysis was
unadjusted for age and sex. We conducted a multivariate
analysis of people over 50 years old by sex.
The Healthy Nippon 21 program began in 2000 in
Japan [36]. The program did not contain frequency of toothbrushing. In the future, this program should include
increasing the proportion of people who brush their teeth
over once a day to prevent tooth loss. Additionally, in

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The relationship between number of natural teeth and oral health behavior in adult Japanese people

particular, for elderly females, municipality had better
recommend the brushing of teeth over once a day, using aninterdental brush and dental floss to prevent from toothloss. People in the Tokachi area of Hokkaido had moredental problems compared to people nationwide. To
encourage tooth brushing more than once a day, using aninterdental brush and dental floss, an oral health strategy
concerning school-age-chirdren is important. We would liketo communicate with the school board of municipality about
this result.
In our study, current smoking was significantly
associated with few natural teeth both males and females.
Most studies [4, 9, 19-24] reported that smoking was
associated with tooth loss. Klein et al. [20] reported thatcurrent smokers and past smokers were more likely tohave lost teeth than nonsmokers adjusted for age and sex.
Krall et al. [21] indicated that individuals who continued tosmoke cigarettes had a 2.4-fold (men) to 3.5-fold risk
(women) of tooth loss compared with non-smokers by alongitudinal analysis. Yanagisawa et al. [22] reported thatcurrent smokers were more likely to have more than eight
missing teeth compared to non-smokers adjusted for age
and oral health behavior, the adjusted ORs of having morethan eight missing teeth were significantly higher incurrent smokers who smoked 21 or more cigarettes per
day compared with those who had never smoked among
Japanese men. Hanioka et al. [19] reported that current
smoking was significantly associated with tooth loss both inmales and females among Japanese people. Although our
results were not significant in males, the odds ratios (ORs)of current male smokers was 1.51 relative to nonsmokers.
Current smokers might be more likely to have fewer than
20 teeth compared to non-smokers.
There are some limitations to our study. First, our results
clearly fail to identify the cause of tooth loss because of thecross-sectional study. Further study such as a cohort study
is necessary to establish the relationship between tooth
loss and oral health behavior. Secondly, our study did notinvestigate socioeconomic status. Several studies [4, 9, 10]reported on the impact of socioeconomic status on toothloss. Thirdly, self- selection bias might exist in our study
because relatively healthy subjects participated in a dental
examination of municipality. Additionally, the subject region
was only Hokkaido prefecture in Japan. However, theaverage number of teeth and average number of decayedteeth in the subjects were similar to the data of the surveyof dental disease in 2005 by the Ministry of Health, Labourand Welfare of Japan [1]. However, our study subjects had
more DMFT (average number of decayed teeth, missing
teeth or filled teeth), but a smaller proportion of subjects
with a edentulous jaw than the data of the survey of dental

disease in 2005.

Ⅴ．Conclusion
In this study, people who brush teeth less than or equal
to once a day were more likely to have few teeth comparedto those who brush teeth twice or more daily in thefemales. Females people who did not use an interdental
brush and a dental floss were more likely to have few teeth
compared to those who use that.
Both males and females, current smokers were more
likely to have few teeth compared to nonsmokers.
This study suggests that there is a significant
relationship between few natural teeth, oral health behavior
such as frequency of tooth brushing, using an interdental
brush or dental floss and smoking status. Good oral healthbehavior and nonsmoking are important to prevent toothloss. We need to highlight that these results are available
for health promotion planning for the municipality in theeastern region of Hokkaido Prefecture.

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