Ito et al. BMC Oral Health (2015) 15:34
DOI 10.1186/s12903-015-0020-z

RESEARCH ARTICLE

Open Access

Individual- and community-level social
gradients of edentulousness
Kanade Ito1,2*, Jun Aida1, Tatsuo Yamamoto3, Rika Ohtsuka4, Miyo Nakade5, Kayo Suzuki6,7, Katsunori Kondo6,8,
Ken Osaka1 and for the JAGES group

Abstract
Background: Community-level factors as well as individual-level factors affect individual health. To date, no studies
have examined the association between community-level social gradient and edentulousness. The aim of this study
was to investigate individual- and community-level social inequalities in edentulousness and to determine any
explanatory factors in this association.
Methods: We analyzed the data from the Japan Gerontological Evaluation Study (JAGES). In 2010-2012, 112,123
subjects aged 65 or older responded to the questionnaire survey (response rate = 66.3%). Multilevel logistic
regression analysis was applied to determine the association between community-level income and edentulousness
after accounting for individual-level income and demographic covariates. Then, we estimated the probability ofedentulousness by individual- and community-level incomes after adjusted for covariates.
Results: Of 79,563 valid participants, the prevalence of edentulousness among 39,550 men (49.7%) and 40,013
women (50.3%) were both 13.8%. Living in communities with higher mean incomes and having higher individuallevel incomes were significantly associated with a lower risk of edentulousness (odds ratios [ORs] by 10,000 USD
increments were 0.37 (95% confidence interval [CI] [0.22-0.63]) for community-level and 0.85 (95% CI [0.84-0.86]) for
individual-level income). Individual- and community-level social factors, including density of dental clinics, partiallyexplained the social gradients. However, in the fully adjusted model, both community- and individual-level social
gradients of edentulousness remained significant (ORs = 0.43 (95% CI [0.27-0.67]) and 0.90 (95% CI [0.88-0.91]),
respectively). One standard deviation changes in community- and individual-level incomes were associated with 0.78 and0.84 times lower odds of edentulousness, respectively. In addition, compared to men, women living in communities withhigher average incomes had a significantly lower risk of edentulousness (p-value for interaction < 0.001).
Conclusions: Individual- and community-level social inequalities in dental health were observed. Public health policies
should account for social determinants of oral health when reducing oral health inequalities.
Keywords: Dental public health, Edentulous/edentulism, Income inequality, Gender differences, Multilevel analysis

Background
Severe tooth loss is the 36th most prevalent condition
among 291 diseases and it caused a loss of 106 disabilityadjusted life-years per 100,000 population [1]. Prevalenceof severe tooth loss increases with age. Approximately
20% of the older population experienced severe tooth loss
* Correspondence: kanade-i@umin.ac.jp
1
Department of International and Community Oral Health, Tohoku University
Graduate School of Dentistry, Sendai City, Miyagi, Japan
2
Division of Oral Health Sciences, Department of Health Sciences, School of
Health and Social Services, Saitama Prefectural University, Koshigaya City,
Saitama, Japan
Full list of author information is available at the end of the article[2]. Severe tooth loss causes chewing difficulties and poor
nutritional status [3]. It also affects general health status.
For example, tooth loss predicts the onset of future comorbidities such as dementia [4] and mortality [5].
Recent studies showed that the prevalence of severe
tooth loss differed by socioeconomic group [6-8]. These
health inequalities are caused by social determinants ofhealth and can be observed on social gradients [9].
Adverse social conditions such as lower income andlower educational attainment affect the health of not
only the most disadvantaged people, but also the entire
population within a society [10,11]. The differences in© 2015 Ito et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution License(http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium,provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://
creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Ito et al. BMC Oral Health (2015) 15:34

social conditions create a stepwise gradient of healthconditions between social groups [12,13]. The total lossof teeth (i.e., edentulousness) reflects the social determinants of an individual’s life-course, as it is the result oforal health behavior, oral diseases, and the community
health care system [14]. Reducing oral health inequalities
is an urgent matter for both researchers and policymakers [15-17]. Furthermore, determining the factorsthat affect oral health inequalities is important for future
public health interventions.
Recent studies have demonstrated that not only individual factors, but also community-level social determinants such as income inequalities or community-levelmean income affect the health of individuals and facilitate health inequalities [7,8,18]. Because community
factors potentially affect the health of all residents in an
area, it is important to understand their effects on
health. However, to the best of our knowledge, no study
has examined both the individual- and community-levelsocial gradients of edentulousness. Thus, the aims of thepresent study were: 1) to investigate the association
between individual- and community-level incomes andedentulousness, 2) to determine the explanatory factors
for edentulousness inequalities, and 3) to investigate
gender differences within the socioeconomic inequalities
of edentulousness.

Methods
Data collection

We used cross-sectional data from the Japan Gerontological Evaluation Study (JAGES) cohort study in Japan.
The JAGES project is an ongoing prospective cohort study
investigating social and behavioral factors associated withthe loss of health related to functional decline or cognitive
impairment among individuals aged 65 years or older[6,19,20]. Between August 2010 and January 2012, a totalof 169,215 community-dwelling people aged 65 years andolder were randomly selected from 31 municipalities in 12
prefectures in Japan and mailed a set of questionnaires.
In total, 112,123 people in 31 municipalities participated(response rate = 66.3%). We used data from 79,563 participants without missing responses.
Outcome variable

The outcome variable for the present analysis was edentulousness (i.e., edentulous or dentulous). Current dental
status was measured by a self-administered questionnaire. Respondents were asked “What is the status ofyour dental health?” with four choices: 1) I have 20 ormore natural teeth, 2) I have 10 to 19 natural teeth, 3) I
have 1 to 9 natural teeth, or 4) I have no natural teeth.
We categorized answers 1-3 as “dentulous” and answer
4 as “edentulous.”

Page 2 of 8

Main predictors

We used two income variables as the main predictors.
The individual-level equivalent household income wasobtained and calculated from the questionnaire. The
community-level mean income was obtained from national census data [21]. Both income variables were used
as continuous variables and the unit used was 10,000
USD (1 USD = 100 JPY).
Individual-level socio-demographic covariates

Sex, age (65-69, 70-74, 75-79, 80-84 and >84 years old),
marital status (currently married, widowed, divorced, nevermarried, and other), and educational attainment (years ofschool education received (<6, 6-9, 10-12, >12 years, andother)) were used as individual-level socio-demographic
covariates. Marital status [22-24] and educational attainment [25,26] were associated with general and oral healthstatus. In addition, both variables in this study wereassociated with income level. Therefore we included thesevariables as covariates.
Community-level socio-demographic covariate

Density of dental clinics is a proxy for access to dental
care in communities. A previous study in Japan indicated that density of dental clinics was an appropriate
proxy for access to care [27]. Dental status is associatedwith access to dental care [27]. Density of dental clinics
is likely to be higher in urban areas than rural areas [28].
Generally, urban areas are richer than rural areas [29].
Thus, we used density of dental clinics as a covariate ofcommunity-level income in this analysis. Density ofdental clinics in each municipality in 2010 were obtained
from the census data and used as the community-levelvariable [30].
Data analysis

In our dataset, 79,563 individuals (individual-level) were
nested across 30 municipalities (community-level). We
have hypothesized that oral health is affected not only
by individual-level socioeconomic status but also bycommunity-level social conditions. To examine the contextual effect of community-level income on edentulousness, we applied a 2-level multilevel logistic regression
analysis with random intercepts and fixed slopes. To
determine explanatory factors in the association between
individual- and community-level incomes and edentulousness, we built the models as follows. Model 1 tested
the association between individual- and community-level
incomes and edentulousness. Model 2 tested the association between income variables and edentulousness after
adjusting for age, sex, and marital status. Model 3 added
educational attainment into Model 2. Model 4 was the
fully adjusted model, adding the community-level variable(density of dental clinics) into Model 3. To determine

Ito et al. BMC Oral Health (2015) 15:34

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gender differences in the effect of both individual- andcommunity-level incomes on dental health, interaction
terms were included in the fully adjusted model. To evaluate the degrees of individual- and community-level variances in edentulousness, median odds ratios (ORs) were
calculated [31]. To compare the degrees of the association
between individual- and community-level income variables and edentulousness, we constructed a fully adjusted
model with standardized income variables. When nonstandardized income variables were included into themodels, they were grand mean centered. Analysis wereconducted using MLwiN version 2.28 (Centre for Multilevel Modelling, University of Bristol, UK).
Ethical considerations

Ethical approval for the study was obtained from the
Ethics Committee at Nihon Fukushi University, Japan(Approval number: 10-05).

Results
The average ages of 39,550 men (49.7%) and 40,013
women (50.3%) were 73.5 (SD = 5.97) and 73.7 (SD = 6.17)
years old, respectively. The prevalence of edentulousness

was 13.8% for both men and women. Table 1 shows thedemographic distribution of the variables by dental status.
Edentulous individuals had significantly lower incomes
and lived in communities with lower mean incomes(p < 0.001).
Table 2 shows the results of the multivariate multilevel analysis. In the intercept-only model (not shown),
there was a significant difference in edentulousness
between municipalities (community-level variance: Ωγ =0.262, SE = 0.069). The median OR in the model was
1.629, which indicated that if a person moved to anothermunicipality with a higher probability of poor dental status, their median risk of edentulousness would increase
1.629 times.
Univariate analyses showed that both individual- andcommunity-level incomes were associated with lower riskfor edentulousness; ORs of individual- and communitylevel incomes were 0.84 (95% confidential interval [CI][0.84-0.86]) and 0.33 (95% CI [0.19-0.60]), respectively.
Then, we included both income variables into the samemodel. Having a 10,000 USD higher income and living in
a community with a 10,000 USD higher mean income
were associated with 0.85 times and 0.37 times lower risk

Table 1 The demographic distribution of variables by dental status (n = 79,563)
Dentulousnessn (%)

Edentulousnessn (%)

p-value

Male

34,083 (86.2)

5,467 (13.8)

0.798†

Female34,507 (86.2)

5,506 (13.8)

65-69 ys

23,239 (94.6)

1,327 (5.4)

70-74 ys

21,560 (90.3)

2,314 (9.7)

75-79 ys

14,212 (83.2)

2,877 (16.8)

80-84 ys

6,899 (72.8)

2.573 (27.2)>84 ys2,680 (58.7)

1,882 (41.3)

Married

52,769 (88.1)

7,115 (11.9)

Widowed

12,185 (78.6)

3,311 (21.4)

Divorced

2,007 (86.7)

307 (13.3)

Never married

1,316 (88.4)

173 (11.6)

Other

313 (82.4)

67 (17.6)<6 ys

1,120 (61.70

694 (38.3)

6-9 ys27,979 (82.7)

5,853 (17.3)

Categorical variables
Sex

Age

Marital status

Educational attainment

10-12 ys

25,428 (89.4)

3,023 (10.6)>12 ys13,650 (91.3)

1,299 (8.7)

Other413 (79.9)

p < 0.001†

p < 0.001†

p < 0.001†

104 (20.1)
Mean (SE)Continuous variables
Density of dental clinics (per 10 thousand population)

4.45 (±0.837)

4.31 (±0.699)

p < 0.001‡

Individual income (10 thousand US dollars*)

2.39 (±1.553)

1.95 (±1.467)

p < 0.001‡

Community income (10 thousand dollars*)

3.18 (±0.297)

3.09 (±0.285)

p < 0.001‡†

p-value for chi-squared test.
p-value for t-test.
*1 US dollar = 100 Japanese Yen.

‡

Ito et al. BMC Oral Health (2015) 15:34

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Table 2 Association of edentulousness with individual- and community-level variables determined by multilevel logistic
regression (n = 79,563)
Model 1Model 2Model 3Model 4Model 5Model 6

OR (95% CI)

OR (95% CI)

OR (95% CI)

OR (95% CI)

OR (95% CI)

OR (95% CI)

0.85 (0.84-0.86)

0.87 (0.86-0.88)

0.90 (0.88-0.91)

0.90 (0.88-0.91)

0.90 (0.88-0.92)

0.90 (0.88-0.91)

Fixed effectIndividual-variables
Individual income (10 thousand US dollars)
Educational attainment (ref:>12 ys)

1.00

1.00

1.00

1.00

<6 ys

2.19 (1.94-2.47)

2.19 (1.94-2.47)

2.19 (1.94-2.47)

2.19 (1.93-2.48)

6-9 ys

1.61 (1.50-1.73)

1.61 (1.51-1.73)

1.62 (1.51-1.73)

1.62 (1.51-1.73)

10-12 ys

1.15 (1.07-1.24)

1.15 (1.07-1.24)

1.16 (1.08-1.24)

1.16 (1.08-1.25)

Other

1.79 (1.41-2.28)

1.80 (1.42-2.28)

1.80 (1.42-2.28)

1.80 (1.42-2.29)

Community-variables
Community income (10 thousand US dollars)

0.37 (0.22-0.63)

0.39 (0.25-0.61)

0.41 (0.27-0.63)

Density of dental clinics (per 10 thousand population)

0.43 (0.27-0.67)

0.43 (0.27-0.67)

0.53 (0.33-0.85)

0.96 (0.78-1.19)

0.96 (0.78-1.19)

0.96 (0.78-1.18)

Interaction term (Sex*Individual income)

0.98 (0.95-1.02)

Interaction term (Sex*Community income)Random effects (SE)Median OR

0.63 (0.54-0.73)
0.148 (0.039)

0.105 (0.028)

0.095 (0.026)

0.095 (0.026)

0.095 (0.026)

0.095 (0.026)

1.443

1.362

1.342

1.342

1.342

1.342

Model 1: Adjusted for individual- and community-level incomes.
Model 2: Model 1 + age, sex, and marital status.
Model 3: Model 2 + educational attainment.
Model 4 (full model): Model 3 + community-variable (density of dental clinics).
Model 5,6: Model 4 + each interaction term.
*1 US dollar = 100 Japanese Yen.

for edentulousness, respectively (Model 1). Individual
characteristics mediated these relationships by 13.5%(individual-level income) and 3.4% (community-level
income), respectively (Model 2, calculated from the ORs[32]). Educational attainment further attenuated the ORsof individual- and community-level income variables by
20.5% and 3.8%, respectively (Model 3). The communitylevel covariate, density of dental clinics, only mediated theassociation between community-level income and edentulousness (2.1% reduction of the OR, Model 4). Even after
considering all covariates, there remained significant
geographical differences and individual- and communitylevel social gradients for edentulousness (Model 4). Whenstandardized income variables were included in Model
4 instead of non-standardized income variables, ORs
for individual- and community-level income variables
were 0.84 (95% CI [0.82-0.87]) and 0.78 (95% CI [0.680.89]), respectively. There was a significant interaction
between gender and community-level income, although
the interaction between gender and individual-level income was non-significant (Models 5 and 6). Compared to men, women living in areas with higher
community-level incomes had a lower probability ofedentulousness (Figure 1). For individual-level income,similar social gradients were observed among both men
and women (Figure 1).

Discussion
To the best of our knowledge, this study was the first to
examine both the individual- and community-level social
gradients of edentulousness using a multilevel analysis.
This large-scale multilevel analysis demonstrated that
not only individual-level income but also communitylevel income showed social gradients for edentulousness.
Present study also investigated the factors which associated between income inequalities and dental status. Individual socioeconomic characteristic partially mediated theassociation between both individual- and community-level
incomes and edentulousness. The community’s sociodemographic characteristic also partially mediated theassociation between community-level income and edentulousness. However, even after adjusting for all covariates,individual- and community-level social gradients remained.
In addition, compared to men, women living in municipalities with higher community-level incomes derived greateroral health benefits from the social environment.
The present study reports similar result to previous
studies using non-oral health outcomes, which havesuggested that community-level social factors affectpopulation health [18]. Those systematic review and metaanalysis showed that poor community-level socioeconomic
environment increased resident mortality [18]. A metaanalysis of 11 studies with smaller sample sizes indicatedIto et al. BMC Oral Health (2015) 15:34

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Figure 1 Gender difference in the association between individual- and community-level incomes and probability of edentulousness.
Compared to men, women living in areas with higher community-level incomes had a lower probability of edentulousness.

that living in areas with low socioeconomic status increased mortality 1.11 times compared to areas with highsocioeconomic status [18]. Another meta-analysis of sevenstudies with larger sample sizes also demonstrated that living in low socioeconomic status areas increased mortality
1.07 times [18]. In oral health studies, regardless of
individual-level income, adults living in affluent areas had ahigher number of remaining teeth than those living in
deprived areas, after adjusting for age, sex, and educational attainment [8]. Dental health is considered to beaffected to a greater extent by community-level factors.
Previous studies conducted in one Japanese prefecturereported larger geographical differences in the dental
outcome of number of remaining teeth compared toself-rated health [7]. Therefore, a public health intervention considering community-level social determinants
would be more effective.
There are at least three possible pathways between
community-level income and oral health. First, access todental care could explain the mechanism. Second, individual health behaviors are formed by the surrounding
environment. Third, people living in affluent communities are less likely to have psychosocial stress than thoseliving in deprived communities. In relation to the firstpathway, although we considered access to dental clinics
in the models, there might be unexplained variance ofthe outcome associated with access to dental care. A
previous study demonstrated that low-income individuals had less access to dental clinics than high-income
individuals [33]. Moreover, access to dental clinics was significantly associated with area-level income after adjusting
individual income [28]. This study suggested that people

living in affluent areas were more likely to visit a dental
clinic than those in deprived areas, regardless of individual
socioeconomic status. Although the variable we used,
density of dental clinics, could change throughout the lifecourse of each respondent, we could not consider possible
changes in this variable. Therefore, this might have caused
the unexplained variance of the outcome, which was associated with access to dental care. For the second pathway,
compared to deprived communities, affluent communities
tend to have positive social environments, including sufficient grocery stores with fresh and healthy food, public
safety, and good access to hospitals and dental clinics[34,35]. People living in affluent communities tend to eat
more fruits and sugar-free foods because they can easilypurchase healthy foods at grocery stores in their communities [36,37]. In addition, people living in affluent communities are also more likely to drink healthy beverages,
such as non-sugared teas rather than sodas [38]. Sugar
is an established risk of dental caries [39]. Moreover, recent study also indicated that sugar associated with riskof periodontal diseases [40]. Healthy lifestyles can help
prevent them. For the third pathway, people living in affluent communities are less likely to have psychosocial
stress because of increased safety, good social capital(e.g., social connections and social networks), and social norms than those living in deprived communities[41]. Psychosocial stress is also associated with smoking status, which affects periodontal diseases causing
tooth loss [42]. In addition, community public safety
affects oral health by reducing the possibility of dental
injuries. Dental injury was affected by community social
environment [43].

Ito et al. BMC Oral Health (2015) 15:34

Present study showed that community explanatory
variable partially mediated the association between
community-level income and edentulousness. To examine the possibility of the pathway “access to dental care”,
we include the variable into the model. However, variable on access to dental care explained only 2.1% of theassociation between community-level income and edentulousness. Further studies that consider the wider rangeof variables related to the pathway, such as social capitaland geographical clustering of dental health behaviors,
are needed.
In the present study, women’s dental health was affected by community-level income to a greater extent
than men’s health. Previous studies on other health outcomes have reported similar results. Compared to men,
the self-rated health of women is considered to be affected
to a greater extent by the neighborhood social environment [44]. Another study on self-rated health reported
similar findings and the authors suggested that this might
be because women tend to spend more time at home andin the community [45]. Thus, women were more likely
than men to communicate with neighbors. Therefore,
women’s health behaviors are more likely to be affected byneighbors though informal social control and social influence. A previous study in Japan demonstrated that, forolder women, the distance to a dental clinic was animportant factor for dental attendance, while distance
was not significantly associated with access to dental
care among older men [27]. Because many older womenin Japan do not have a driver’s license, public transportation is considered an important factor for dental clinic
access [27].
Public health implications

Community factors are important because they potentiallyaffect the health of all residents in a given area. The
present study revealed the importance of community-levelsocioeconomic status on oral health. Therefore, interventions should focus not only on individual efforts but also
consider community-level social determinants underlying
the oral health of a population. Therefore, after relevant
factors are determined by future studies, upstream approaches including structural and environmental interventions for improving various social determinants ofcommunities (e.g., smoking policies for public spaces, foodpolicies for reducing sugar consumption, health care system reforms for improving access to preventive and curative care, and access to fluoride in the water system or inschools) are necessary for reducing oral health inequalities[46-48]. In addition to these upstream approaches, building society which focuses on not only economic growth,
but also fair distribution of well-being of individuals are
required [34]. As various socioeconomic environment ofcommunity affect health of residents, broader social andPage 6 of 8

economic policies should consider health and well-beingof residents [34].
Limitation and strengths

This study has some limitations. First, this was a crosssectional study; thus, we cannot rule out the possibilityof reverse causation. Consequently, prospective followup studies are required. Second, though the validity ofself-reporting number of remaining teeth was validated,measurements were obtained from a self-administeredquestionnaire [49]. If we can obtain clinical measurements of remaining teeth, which are more accurate thanself-administered questionnaires, the association between income variables and edentulousness will bestrengthened. Third, there might have a potential bias
because of a lack of many cases. The main strength of
this study was its large sample size. In addition, oursurvey was conducted across an adequate number ofmunicipalities with various characteristics and we usedappropriate statistical analysis. Therefore, the present
study could legitimately describe the effects of community
factors.

Conclusion
In conclusion, community-level income, as well as
individual-level income, formed social gradients foredentulousness, even after accounting for individualand community-level factors. The oral health of women
living in municipalities with higher community-level incomes benefited from the social environment.
Availability of supporting data
Raw data is available from corresponding author.
Standards of reporting

This study was prepared according to STROBE checklist for cross-sectional studies.
Consent

Present study was an observational study and not using
human biological specimens. We explained all relevant
details regarding the study to be carried out and provide
each prospective subject an opportunity to refuse inclusion in the research. Subject who consented to participate in the study wrote the self-reported questionnaireand send it by mail.
Abbreviations
JAGES: Japan Gerontological Evaluation Study; OR: Odds ratio; CI: Confidence
interval.
Competing interests
The authors declare that they have no competing interests.

Ito et al. BMC Oral Health (2015) 15:34

Authors’ contributions
KI participated in the acquired the data and the study design, performed thestatistical analysis, and drafted the manuscript as the principal author.
JA helped to develop the idea of the study, participated in the acquired
the data, advised the statistical analysis, and edited the manuscript.
TY participated in the acquired the data and the study design, and edited
the manuscript. RO and MN participated in the acquired the data andreviewed the manuscript. KS helped to edit the manuscript, participated inits design, and advised the statistical analysis. KK is the principal investigatorof the JAGES project, helped to develop the idea of the study, participatedin the acquired the data and the study design, and edited the manuscript.
KO participated in the acquired the data, helped with data analysis, andcritically revised the manuscript. All authors read and approved thefinal manuscript.

Page 7 of 8

8.
9.

10.
11.

12.

13.
Acknowledgments
This study used data obtained from the Japan Gerontological Evaluation
Study (JAGES), conducted by the Nihon Fukushi University Center for
Well-Being and Society as one of their research projects. This study was
supported in part by Grants-in-Aid for Scientific Research (A) (KAKENHI
23243070), (B) (22390400 and 24390469), and (C) (22592327) from the Japan
Society for the Promotion of Science. The study was also supported by a
Health and Labour Sciences Research Grant, and grants for Comprehensive
Research on Aging and Health (H24-Choju-Wakate-009) and Comprehensive
Research on Lifestyle-Related Diseases including Cardiovascular Diseases andDiabetes Mellitus (H24-Junkanki [Seishu]-Ippan-007) from the Ministry of
Health, Labour and Welfare (MHLW), Japan. The sponsors of the study had
no role in the study design, data collection, data analysis, data interpretation,
or writing of the paper.
Author details
1
Department of International and Community Oral Health, Tohoku University
Graduate School of Dentistry, Sendai City, Miyagi, Japan. 2Division of Oral
Health Sciences, Department of Health Sciences, School of Health and Social
Services, Saitama Prefectural University, Koshigaya City, Saitama, Japan.
3
Department of Dental Sociology, Graduate School of Dentistry, Kanagawa
Dental University, Yokosuka City, Kanagawa, Japan. 4Research Team for
Human Care, Tokyo Metropolitan Institute of Gerontology, Itabashi Ward,
Tokyo, Japan. 5Department of Nutrition, Faculty of Health and Nutrition,
Tokaigakuen University, Nagoya City, Aichi, Japan. 6Center for Well-being andSociety, Nihon Fukushi University, Nagoya City, Aichi, Japan. 7Department of
Policy Studies, Aichi Gakuin University, Nisshin City, Aichi, Japan. 8Center for
Preventive Medical Sciences, Chiba University, Chiba City, Chiba, Japan.

14.

15.

16.
17.

18.

19.

20.

21.

22.

Received: 12 December 2014 Accepted: 20 February 201523.
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