International Journal ofEnvironmental Researchand Public Health
ArticleIs the Association between Green Tea Consumption
and the Number of Remaining Teeth Affected bySocial Networks?: A Cross-Sectional Study from theJapan Gerontological Evaluation Study Project
Manami Hoshi 1 , Jun Aida 1, * , Taro Kusama 1 , Takafumi Yamamoto 1 , Sakura Kiuchi 1 ,Tatsuo Yamamoto 2 , Toshiyuki Ojima 3 , Katsunori Kondo 4,5 and Ken Osaka 1
1

2
3
4
5

*

Department of International and Community Oral Health, Tohoku University Graduate School of Dentistry,
Sendai 980-8575, Japan; tohoku.hoshi@gmail.com (M.H.); chester930@gmail.com (T.K.);
yamamoto173998@gmail.com (T.Y.); sakurakawamura39@gmail.com (S.K.); osaka@m.tohoku.ac.jp (K.O.)
Department of Disaster Medicine and Dental Sociology, Graduate School of Dentistry, Kanagawa Dental
University, Yokosuka 238-8580, Japan; yamamoto.tatsuo@kdu.ac.jp
Department of Community Health and Preventive Medicine, Hamamatsu University School of Medicine,
Hamamatsu 431-3192, Japan; ojima@hama-med.ac.jp
Center of Preventive Medical Sciences, Chiba University, Chiba 260-0856, Japan; kkondo@chiba-u.jp
National Center for Geriatrics and Gerontology, Obu 474-8511, Japan
Correspondence: j-aida@umin.ac.jp; Tel.: +81-22-717-7639

Received: 9 March 2020; Accepted: 16 March 2020; Published: 20 March 2020


Abstract: Consumption of green tea without sugar, as well as social networks, are associated witha lower risk of tooth loss. There is a possibility of confounding both factors because tea is often
drunk with friends. Therefore, the present study aimed to examine whether green tea consumption
is beneficially associated with the number of remaining teeth, while considering social networks.
This cross-sectional study was based on the Japan Gerontological Evaluation Study (JAGES) in 2016.
Self-administered questionnaires containing questions about green tea consumption were mailed to34,567 community-dwelling residents aged ≥ 65 years. We used the number of remaining teeth as adependent variable, and green tea consumption and the number of friends met over the past month(social network size) as independent variables. Linear regression models with multiple imputation
were used. A total of 24,147 people responded (response rate = 69.9%), and 22,278 valid data were
included into our analysis. Participants’ mean age was 74.2 years (standard deviation = 6.3), and45.9% were men. Among the participants, 52.2% had ≥ 20 teeth, 34.2% drank 2–3 cups of green tea
per day, and 32.6% met ≥ 10 people over the past month. After adjusting for all potential confounders,both higher green tea consumption and a larger social network size were associated with moreremaining teeth (both p for trend < 0.001). The association of green tea was greater among thosewith smaller social networks (p for interaction < 0.05). The protective association of green tea wasremarkable among people with smaller social networks.
Keywords: green tea; tooth loss; oral health; social network; friends

1. Introduction
Tooth loss is the leading cause of disability-adjusted life years (DALYs) in oral diseases andconditions, accounting for 7.6 million DALYs [1]. Tooth loss deteriorates both physical and social
functions of oral health, such as mastication [2] and oral health-related quality of life functions,
including eating, speaking, smiling, sleeping, and communication [3]. In addition to these effects on

Int. J. Environ. Res. Public Health 2020, 17, 2052; doi:10.3390/ijerph17062052

www.mdpi.com/journal/ijerph

Int. J. Environ. Res. Public Health 2020, 17, 2052

2 of 10oral function, tooth loss potentially exacerbates general health status. Previous studies have shownthat the number of remaining teeth predicts increases in all-cause mortality, cardiovascular diseases,
coronary heart disease [4], and the onset of dementia [5].
Green tea consumption is reported to associate with a lower risk of tooth loss [6,7]. Green tea
contains a high quantity of fluoride [8] and polyphenols such as catechins [9], and in most East Asian
countries, is drunk often without sugar. Previous studies have shown that it can prevent both dental
caries and periodontal disease [10–12]. In addition to its beneficial effects on various health outcomes,
such as all-cause mortality, cardiovascular disease, cancers, non-alcoholic fatty liver disease, metabolic
syndrome, diabetes, and other health outcomes [13–20], green tea consumption is also considered tohave an effect on oral health. In spite of abundant benefits from the basic sciences, there are relatively
fewer clinical studies on the merit of green tea on oral health [21].
However, previous studies on green tea and health have not considered the effect of the social
aspect of green tea consumption. Japan has a traditional tea ceremony culture with over 1000 yearsof history. In addition to the afternoon tea-drinking culture in England, tea drinking has spreadwidely into the general population. Because green tea was traditionally drunk with others in thetea ceremony, even now it is often drunk when people meet with others. Therefore, it is possiblethat the health effects of green tea are confounded by the effect of social relationships [22,23]. Social
relationships have beneficial effects on health [24]. In the dental field, social networks are reported toassociate with a lower risk of tooth loss [25]. Possible mechanisms between social networks and oral
health are stress-buffering and the diffusion of new information and health behaviors through social
networks [26,27]. In fact, studies have suggested that smoking behavior and food intake were diffusedthroughout social networks [28–30]. However, no study has examined the association between green
tea consumption and the number of remaining teeth with consideration of social networks. Therefore,
the present study aimed to examine whether green tea consumption is beneficially associated with thenumber of remaining teeth, considering social networks.
2. Materials and Methods
2.1. Ethics Approval
Ethical approval for this study was obtained from the Ethics Committee of the National Center
for Geriatrics and Gerontology (approval number: 992) and Chiba University (approval number: 2493).
Participants were informed that participation in the present study was voluntary, and that returning
the questionnaire with responses was taken as consent to participate.
2.2. Study Design, Setting, and ParticipantsThis cross-sectional study investigated whether the association between green tea consumption
and the number of remaining teeth was affected by social networks. We analyzed cross-sectional
data from the Japan Gerontological Evaluation Study (JAGES), particularly from the 2016 JAGES
survey. JAGES is one of the few population-based gerontological surveys in Japan that focuses onsocial determinants of health and the social environment [31].
Between October and December 2016 (Takahama town was included in the survey only fromJanuary 2017 on), self-administered questionnaires were mailed to 279,661 functionally independentcommunity-dwelling residents aged ≥65 years living in 39 municipalities in Japan. The questionnaires
containing questions about green tea consumption were mailed to 34,567 residents.
2.3. Dependent Variable
We used the number of remaining teeth as a dependent variable. Participants were asked
the following question: “How many remaining teeth do you have, including teeth covered bycrowns? The total number of remaining teeth on an adult, including wisdom teeth, should be 32.”
Respondents chose one of the following options: 0, 1–4, 5–9, 10–19, or ≥ 20 teeth. Following a previous

Int. J. Environ. Res. Public Health 2020, 17, 2052

3 of 10study, we changed these categories into a continuous variable: a score of 0 (complete edentulism),
2.5 (1–4 teeth), 7 (5–9 teeth), 14.5 (10–19 teeth), or 26 (≥20 teeth) [32]. We used this continuous variable
as the dependent variable in linear regression analysis.
2.4. Independent Variables
We used social network size and green tea consumption as independent variables. We assessedsocial network size with the following question: “How many friends/acquaintances have you seen overthe past month? Count the same person as one, no matter how many times you have seen him/her.”
Respondents chose one of the following options: 0, 1–2, 3–5, 6–9, or ≥10 people. We assessed green tea
consumption by asking about the number of cups of green tea drunk in a day, categorized as follows:<1, 1, 2–3, or ≥4 cups/day. We used these categorical variables as independent variables in linear
regression analysis. We did not include the variables on dental diseases, because they were consideredas the mediators between green tea consumption and the number of remaining teeth. If mediators
are adjusted, it causes a biased estimation of the association of the main exposure, which is describedin the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) statement
as follows: “Inappropriate decisions may introduce bias, for example, by including variables that are in thecausal pathway between exposure and disease (unless the aim is to assess how much of the effect is carried by theintermediary variable)” [33].
2.5. Covariates
We considered the following variables as potential confounders: sex (male/female), age (65–69,
70–74, 75–79, 80–84, or ≥85 years old), smoking status (never, former, or current), years of education(≤9, 10–12, or ≥13 years), history of diabetes mellitus (yes/no), equivalent annual income (USD < 10,000,
10,000–19,999, 20,000–29,999, 30,000–39,999, or ≥40,000; JPY 100 = USD 1), frequency of brushing teeth(≥twice/day or ≤once/day), eating snacks every day (yes/no), and living alone (yes/no).
2.6. Statistical Analyses
We applied linear regression models to more easily interpret the results of the interaction. In model
1, social network size, green tea consumption, sex, and age were included. All other variables wereadded in model 2. In model 3, an interaction term was added to examine the interaction effect betweensocial network size and green tea consumption on the number of remaining teeth. We assumed“missing at random (MAR)” for missing data, and applied multiple imputation by chained equations(MICE) [34]. All analyses were conducted using the Stata 15 software (Stata Corp, College Station,
TX, USA).
3. Results
Of the target 34,567 population, 24,147 people responded (response rate = 69.9%). For our study,
we analyzed 22,278 valid data. Participants’ mean age was 74.2 years (standard deviation = 6.3), and45.9% were men.
Table 1 shows the descriptive statistics of the respondents. Respondents who had larger social
networks and drank more cups of green tea tended to have a higher number of remaining teeth.

Int. J. Environ. Res. Public Health 2020, 17, 2052

4 of 10

Table 1. Descriptive statistics by the number of remaining teeth (N = 22,278).
Characteristics

SubitemFrequency

Total

No. of Remaining Teeth

N

0
(%)

1–4
(%)

5–9
(%)

10–19
(%)

≥20
(%)

22,278

8.2

6.1

9.8

20.9

52.2

Social network size(no. friends met/month)

0
1–2
3–5
6–9
≥10

1865
4097
5437
2937
7269

12.2
10.3
7.9
7.3
6.2

8.6
7.5
6.5
5.2
4.6

11.1
11.0
10.3
9.7
7.8

21.9
21.8
22.2
21.7
19.2

43.7
46.5
50.6
53.3
60.3

Green tea consumption(cups/day)

<1
1
2–3
≥4

4027
3257
7621
6596

8.6
7.7
7.9
8.0

7.0
5.5
6.2
5.6

10.1
8.6
9.8
9.6

21.6
22.2
20.9
19.8

50.3
53.7
52.5
54.1

Sex

Male
Female10,222
12,056

9.6
7.1

7.1
5.4

10.2
9.4

21.1
20.6

50.0
54.1

Age (years)

65–69
70–74
75–79
80–84
≥85

6568
5922
5065
3169
1554

3.4
6.1
8.7
13.9
23.7

3.7
5.2
6.4
9.7
12.3

6.8
9.4
11.1
12.2
14.4

21.1
21.6
20.6
21.1
17.6

63.7
55.8
50.5
37.7
25.0

Education (years)

≤9
10–12
≥ 13

7406
8845
5659

12.8
6.3
4.9

8.1
5.5
4.4

12.5
8.8
7.5

21.3
21.5
19.4

41.1
55.8
62.2

Equivalent income(USD)<10,000
10,000–19,999
20,000–29,999
30,000–39,999≥40,000

2278
6185
4161
2592
1931

12.0
8.1
6.4
5.3
5.3

8.0
6.2
4.8
4.2
3.6

14.1
9.9
7.8
6.6
7.4

21.8
21.5
20.9
19.8
17.8

40.0
52.1
58.7
62.7
64.7

Smoking status

Never
FormerCurrent

13,150
6345
2382

6.7
10.2
11.2

5.0
7.2
8.7

8.6
10.7
13.6

19.8
21.6
25.2

56.9
48.4
38.7

DiabetesNoYes

18,560
2782

7.9
10.8

6.1
6.8

9.5
12.0

20.7
22.5

53.7
45.8

Brushing teeth(times/day)≥2≤1

15,749
5663

4.5
16.3

4.9
9.3

9.1
11.6

21.2
20.4

57.9
39.5

Eating snacks every day

No
Yes

7463
13,748

6.9
8.5

5.5
6.3

9.7
9.6

19.7
21.6

56.3
51.2

Living aloneNoYes

17,249
2462

7.7
8.1

5.6
6.8

9.0
12.0

20.8
21.0

54.5
49.6

Non-responses are excluded.

Figure 1 shows the association between social network size and green tea consumption(non-responses are excluded). Respondents with larger social networks tended to drink morecups of green tea.

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Figure 1. Social network size and green tea consumption.
Figure 1. Social network size and green tea consumption.

Table 2 shows the results of linear regression analysis with multiple imputation. After adjustingTable
2 shows
resultsof linearmore
regression
with multiple
imputation.
After adjusting
for allcovariates
inthe
model
2, meeting
than 3analysis
friends/month
was significantly
associatedwith aforallcovariates
in
model
2,
meetingmore
than
3
friends/monthwassignificantly
associatedwithahigher number of remaining teeth (model 2). Drinking more than 2 cups of green tea/day was also
highernumberof remaining(model2). Drinking
morethanOverall,
2 cups ofgreen
tea/day
was also
significantly
associatedwith ateeth
highernumberof remaining
teeth.
largersocialnetworksize
significantly
associatedwith a higher
numberof remaining
teeth.with
Overall,largersocial network
size
and higher green
tea consumption
weresignificantly
associateda highernumberof remaining
andtea consumption
were significantly
a highernumberbetweenof remaining
teethhigher(both pgreen
for trend< 0.001). In model
3, there was aassociatedsignificantwith
negativeinteractionsocial
teeth
(both
p for
trend
< 0.001).
In model 3,(pthere
was a significant
network
size
and
green
tea consumption
for interaction< 0.05).negative interaction between social
network size and green tea consumption (p for interaction < 0.05).
Table 2. Association of each factor with the number of remaining teeth by linear regression analysis
with
imputation(N factor= 22,278).
Tablemultiple
2. Associationof eachwith the number of remaining teeth by linear regression analysis
with multiple imputation (N = 22,278).
Characteristics

Characteristics

SubitemSubitemSocial network size

1–2

Social network size

1–2

(no. friends met/month) (ref. = 0)

3–5
6–9

(no. friends met/month) (ref. = 0)

3–5

≥10
6–9
Green tea consumption (cups/day)1≥10(ref. = <1)Green tea consumption (cups/day)
(ref.
= <1)
Sex
(ref. = male)Age (years) (ref. = 65–69)

2–3

1

≥4

2–3
Female
70–74

≥4

75–79

Sex (ref. = male)

Female80–84

Age (years) (ref. = 65–69)
Smoking status (ref. = never)

70–74
≥85
Former
75–79
Current

80–84≥ 85
Smoking status (ref. = never)FormerCurrent

Model 1Model 2Model 3β (95%CI)1Model

β (95%CI) 2
Model

β (95%CI) 3
Model

β (95%
0.24 CI)(−0.28 0.24
to 0.76)1.08(–0.28 to 0.76)(0.57 to 1.60) ***
1.08
1.57
(1.02
******(0.57toto2.12)
1.60)
2.61
1.57
(2.12 to 3.11) ***(1.020.72
to 2.12) ***(0.28 to2.61
1.16) **
(2.120.81
to 3.11) ***(0.45 to0.72
1.18) ***
1.16
(0.28
to
1.16) **
(0.78 to 1.53) ***
0.81
0.89
(0.64
******(0.45toto1.15)
1.18)
−1.81
1.16
(−2.14 to −1.49) ***(0.78−3.13
to 1.53) ***(−3.47 to 0.89
−2.79) ***(0.64−5.82
to 1.15) ***(−6.22 to −5.42) ***
–1.81
−9.18
(–2.14to–1.49)(−9.71 to −8.66)******

β0.19(95% CI)(−0.32 to
0.70)
0.19
0.76
(–0.32 to 0.70)(0.26 to 1.26) **
0.76
0.95
(0.41
to to
1.49)
** **
(0.26
1.26)
1.69
0.95
(1.21 to 2.18) ***(0.41
to 1.49) **
0.35
1.69
(−0.08 to
0.77)(1.210.48
to 2.18) ***(0.13 to 0.83)
0.35 **
0.67(–0.08
to 0.77)(0.31 to 1.04) ***
0.48
−1.14
(−1.47
to −0.81)
*****
(0.13
to 0.83)−1.53
0.67(−1.85 to −1.22) ***(0.31
to 1.04) ***−2.72–1.14 ***(−3.06 to −2.38)−5.16(–1.47
to –0.81) ***(−5.56 to −4.76) ***
–1.53
−8.17
(–1.85to–1.22)(−8.69 to −7.66)*** ***–2.72−2.45(−2.79to to−2.10)*** ***(–3.06–2.38)−4.33–5.16(−4.78 to −3.88) ***β 0.38(95% CI)(−0.14 to
0.90)
0.38
1.18
(–0.14 to 0.90)(0.55 to 1.80) ***
1.18
1.62
(0.81
to to
2.43)
*** ***
(0.55
1.80)
2.61
1.62
(1.62 to 3.59) ***(0.810.62
to 2.43) ***(0.12 to2.61
1.12) *
(1.621.05
to 3.59) ***(0.40 to 0.621.70) **
1.56
(0.12
to 1.12) *(0.64 to 2.48) **
1.05
−1.14
(−1.47
to −0.81)
*****
(0.40
to 1.70)−1.53
1.56(−1.85 to −1.22) ***(0.64
to 2.48) **−2.72(−3.06 to –1.14−2.38) ***−5.16(–1.47
to –0.81) ***(−5.57 to −4.76) ***
–1.53
−8.18
(–1.85–1.22)(−8.70to to−7.67)*** ***–2.72−2.45(−2.80to to−2.11)*** ***(–3.06–2.38)−4.33–5.16(−4.78 to −3.87) ***–3.13(–3.47 to –2.79) ***–5.82(–6.22 to –5.42) ***–9.18(–9.71 to –8.66) ***(–5.56 to –4.76) ***–8.17(–8.69 to –7.66) ***–2.45(–2.79 to –2.10) ***–4.33(–4.78 to –3.88) ***(–5.57 to –4.76) ***–8.18(–8.70 to –7.67) ***–2.45(–2.80 to –2.11) ***–4.33(–4.78 to –3.87) ***Int. J. Environ. Res. Public Health 2019, 17, xInt. J. Environ. Res. Public Health 2020, 17, 2052
Education (years) (ref. = ≤9)

6 of 10
6 of 10

10–12
≥ 13

Characteristics

Diabetes (ref. = no)

Table 2. Cont.

Subitem

Yes

Education(years)(USD)(ref. = ≤9)
Equivalent
income10–12
10,000–19,999(ref. = <10,000)

20,000–29,999≥13Diabetes (ref. = no)Equivalent income (USD)(ref. = <10,000)

Brushing teeth (ref. = ≥twice/day)Yes

30,000–39,999

10,000–19,999≥20,000–29,999
40,00030,000–39,999≤ once/day≥40,000

Eating snacks every day (ref. = no)Brushing teeth (ref. = ≥twice/day)

Living alone (ref. = no)

Eating snacks every day (ref. = no)Interaction between green tea
Living alone (ref. = no)consumption and social network
size Interaction between green tea

consumption and social network size

Yes
≤once/day

Yes

Yes
Yes

Model 1β (95% CI)

1.67(1.37 to 1.96) ***2.45(2.12 to 2.79) ***Model 2–0.96βCI) ***(–1.32(95%to –0.61)
1.67
1.39(1.37 to(1.00
to 1.96)1.77)***
***
2.45
1.98
(2.12 to 2.79) ***(1.54 −0.96
to 2.43) ***(−1.32 to−0.61) ***
2.35
1.39
(1.86 to2.83) ***
(1.00 to 1.77) ***
2.60
1.98
(2.09
to 2.43)3.11)******(1.54 to2.35
–3.50
(1.86 to 2.83) ***(–3.80
to –3.20) ***
2.60
–1.07
(2.09 to3.11) ***(–1.34 −3.50
to –0.81) ***(−3.80 to−3.20) ***
–0.27
−1.07
(–0.65 to 0.12)(−1.34 to −0.81) ***−0.27(−0.65 to 0.12)

1.66(1.37 to 1.96) ***2.46(2.12 to 2.79) ***Model 3–0.96βCI) ***(–1.32 (95%
to –0.61)
1.66
1.39(1.37 toto 1.96)(1.011.78)***
***
2.46
1.98
(2.12 to 2.79) ***(1.54 to2.43) ***
−0.96(−1.32 to−0.61) ***
2.35
1.39
(1.86 to2.83) ***
(1.01 to 1.78) ***
2.60
1.98
(2.08
3.11)******(1.54 toto 2.43)
2.35–3.50(1.86 to 2.83) ***(–3.79to –3.20) ***2.60(2.08 –1.07
to 3.11) ***(–1.34 to–0.80) ***−3.50(−3.79 –0.26
to −3.20) ***−1.07(–0.65 to 0.12)(−1.34 to −0.80) ***
−0.26
–0.09
(−0.65 to 0.12)(–0.17 −0.09
to –0.002) *(−0.17 to −0.002) *CI: confidence interval; ref.: reference. Model 1: Included social network size, green tea consumption,
CI: confidence interval; ref.: reference. Model 1: Included social network size, green tea consumption, sex, and age;sex,
and2:age;
Model
2: Model1 + years
smoking
status, history
years ofeducation,
historyof diabetes
mellitus,
Model
Model
1 + smoking
status,of education,of diabetes
mellitus,
equivalent
income,frequencyof brushingteeth, eating
snacksofevery
day, andlivingalone;snacks
Modelevery
3: Modelinteraction
betweengreen3:tea
equivalent
income,frequency
brushing
teeth,
eatingday,2 +
andliving alone;Modelconsumption and social network size. * p < 0.05, ** p < 0.01, *** p < 0.001.
Model 2 + interaction between green tea consumption and social network size. * p < 0.05, ** p < 0.01,*** p < 0.001.

Figure 2 shows the interaction effect between social network size and green tea consumption.
Figure 2 shows the interaction effect between social network size and green tea consumption.
There was no protective association between green tea consumption and the number of remaining
There was no protective association between green tea consumption and the number of remaining
teeth among respondents with larger social network size. The protective association of green tea
teeth among respondents with larger social network size. The protective association of green tea
consumption on remaining teeth was remarkable among people with smaller social network size.
consumption on remaining teeth was remarkable among people with smaller social network size.

Figure2. 2.
Interaction
betweengreen
teateaconsumption
andsocialnetwork
size.
FigureInteraction
betweengreen
consumption
andsocialnetworksize.

Discussion
4. 4.
Discussion
Thisisisthethefirst
first study
study tobetweengreen
tea tea
consumption
and andoral healthThis
to investigate
investigatethe
theassociation
association
betweengreen
consumptionoral
outcome,withconsiderationofsocialnetworks.
The
study
demonstrated
that
both
highergreen
health outcome, with consideration of social networks. The study demonstrated that both highertea
consumption
and meeting
friends
perfriends
month weresignificantlywithhaving a withhighergreen
tea consumption
andmoremeetingmore
per month
were associatedsignificantly
associatednumberofremainingteeth.
Notably,theprotective
association
of
green
tea
was
remarkable
among
having a higher number of remaining teeth. Notably, the protective association of green tea wasparticipants
with participants
smaller socialwith
network
size.
remarkable
amongsmallersocial network size.

Int. J. Environ. Res. Public Health 2020, 17, 2052

7 of 10The present study’s results for main effects (green tea and social networks on oral health) aresimilar to previous studies. For green tea consumption, a previous cross-sectional study showed that
consumption of more than one cup of green tea per day was significantly associated with a higher
number of remaining teeth among 40- to 64-year-old individuals [6]. Another cross-sectional study
showed that consumption of green tea 4–5 times per day was significantly associated with a higher
number of remaining teeth among pregnant women in Japan [7]. These beneficial associations of green
tea consumption on teeth are supported by biological mechanisms through fluoride and catechin [21].
For social networks, a previous cross-sectional study showed that meeting ≥ 1 friend per month wassignificantly associated with a higher number of remaining teeth among ≥ 65-year-old individuals inJapan [25].
We first hypothesized that these results in previous studies of green tea and oral health wereconfounded by the effect of social relationships, because Japanese people often drink green tea withothers, as is the Japanese tradition. Even after considering social networks, a significant association ofgreen tea consumption with remaining teeth was still observed. However, the present study revealed
that the significant association of green tea with teeth was observed among respondents with smallersocial network size. There are some possible explanations for this. Participants with larger social
network size may gain larger benefits of social support from their friends. Consistent with the results,
these participants are considered to have better oral health behaviors. In our data, respondents withlarger social networks tended to brush their teeth and visit a dentist more frequently (results not shown).
Therefore, it is possible that green tea is less effective for those respondents, who have good oral healthbehaviors, because of the “ceiling effect.” In contrast, participants with smaller social networks do
not gain large benefits from their social networks. Therefore, among these respondents with worse
oral health behaviors, the protective effects of green tea on dental caries and periodontal disease wererelatively larger. Specifically, the effects of polyphenolic catechins (e.g., epigallocatechin-3-gallate(EGCG), epigallocatechin (EGC), epicatechin-3-gallate (ECG), and epicatechin (EC) [35]) in inhibiting
the growth of oral bacteria such as Streptococcus mutans [36,37] and Porphyromonas gingivalis [38], andof fluoride [8] in facilitating re-mineralization and acid resistance of enamel [39] are considered toreduce the risks of periodontal disease and caries, which are the main causes of tooth loss [40]. Thesebenefits from the green tea could be relatively larger for people with worse oral health behaviors, due
to smaller social networks.
This study had several strengths. Firstly, we used a large sample size of 22,278 subjects from39 municipalities in Japan. Secondly, this was the first study to investigate the effects of green tea
on oral health outcomes with consideration of social networks. Thirdly, this was the first study toinvestigate the association between green tea consumption and the number of remaining teeth amongolder adults (aged 65 years and older).
This study also had several considerable limitations. Firstly, this study was cross-sectional,
and longitudinal studies are required for causal inference. Secondly, this study used self-reported
questionnaire data. However, a previous study reported the validity of a self-reported number ofremaining teeth [41]. Additionally, because of misclassification by self-reported questionnaire data, theestimated 95% confidence intervals of the association between green tea consumption/social network
size, and the number of remaining teeth, are considered to be inflated. Despite this bias, the present
study showed significant associations. Therefore, these results are considered to be robust. Thirdly,
because the municipalities that participated in the JAGES survey were not selected randomly, thegeneralizability of these findings to other populations is limited. Fourthly, we did not ask about
consumption of beverages other than green tea, and thus could not consider them in the analysis.
Finally, we did not measure fluoride exposure. It has been reported that fluoride application was
insufficient in Japan [42]. Even in recent years, when fluoride toothpaste has been widely used, an
additional fluoride mouth rinse program is still effective in Japan [43]. This historical situation may
explain the present beneficial association of green tea with oral health. Therefore, the generalizability

Int. J. Environ. Res. Public Health 2020, 17, 2052

8 of 10of the effects of green tea on tooth loss to countries with water fluoridation or sufficient topical fluoride
application is unclear.
5. Conclusions
Green tea consumption was positively associated with the number of remaining teeth amongolder Japanese. The association between green tea consumption and the number of remaining teeth
was affected by the social networks. The protective association of green tea consumption on teeth waslarger among the people with smaller social network sizes than those with larger network sizes.
Author Contributions: Conceptualization: M.H. and J.A.; data curation: M.H., J.A., T.K., T.Y., S.K., T.Y., T.O., K.K.,
and K.O.; formal analysis: M.H., J.A.; funding acquisition: J.A., T.Y., T.O., K.K., and K.O.; investigation: M.H. andJ.A.; methodology: M.H. and J.A.; project administration: J.A., T.Y., T.O., and K.K.; resources: J.A., T.Y., T.O., K.K.,
and K.O.; software: M.H. and J.A.; supervision: J.A., T.K., T.Y., S.K., T.Y., T.O., K.K., and K.O.; writing–originaldraft: M.H. and J.A. All authors have read and agreed to the published version of the manuscript.
Funding: This study used data from the Japan Gerontological Evaluation Study (JAGES), which was supported
by Grants-in-Aid for Scientific Research (22330172, 22390400, 22592327, 23243070, 23590786, 23790710, 24390469,
24530698, 24653150, 24683018, 25253052, 25870573, 25870881, 26285138, 26882010, 15H04781, 15H01972,
16H05556, 16K15851, 16K19267, 18KK0057, and 19H03860), a Health and Labour Sciences Research Grant,
and grants for Comprehensive Research on Aging and Health (H22-Choju-Shitei-008, H24-Junkankitou-Ippan-007,
H24-Chikyukibo-Ippan-009, H24-ChojuWakate-009, H25-Kenki-Wakate-015, H25-Irryo-Shitei-003[Fukkou],
H26-Choju-Ippan-006, H27-Ninchisyou-Ippan-001, H28-Choju-Ippan-002) from the Ministry of Health, Labourand Welfare, Japan, the Research and Development Grants for Longevity Science from AMED (Japan Agency
for Medical Research and Development) (JP19dk0110034, JP19dk0110037), the Research Funding for Longevity
Sciences from the National Center for Geriatrics and Gerontology (29–42), the Personal Health Record (PHR)
Utilization Project from AMED, Japan Foundation for Aging and Health Research Support Grant Open Innovation
Platform with Enterprises, Research Institute and Academia (OPERA), as well as grants from the National Center
for Geriatrics and Gerontology.
Acknowledgments: This study used data from the Japan Gerontological Evaluation Study (JAGES).
Conflicts of Interest: The authors declare no conflict of interest.

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