obesity and dental caries in children: a systematic review and meta-analysis
TRANSCRIPT
Obesity and dental caries inchildren: a systematic reviewand meta-analysis
Hayden C, Bowler JO, Chambers S, Freeman R, Humphris G, Richards D, CecilJE. Obesity and dental caries in children: a systematic review and meta-analysis. Community Dent Oral Epidemiol 2013; 41: 289–308. © 2012 JohnWiley& Sons A/S. Published by JohnWiley & Sons Ltd
Abstract – Objectives: Obesity and dental caries have become increasinglyprevalent challenges to public health. Research results into the relationshipbetween obesity and dental caries in children have been mixed andinconclusive. The aim of this review and meta-analysis was to provide evidenceto quantify the relationship between obesity and dental caries in children usinga systematic approach. Methods: A systematic search for papers between 1980and 2010 addressing childhood obesity and dental caries was conducted and arandom effects model meta-analysis applied. Results: Fourteen papers met theselection criteria. Overall, a significant relationship between childhood obesityand dental caries (effect size = 0.104, P = 0.049) was found. When analysed bydentition type (primary versus permanent), there was a nonsignificantassociation of obesity and dental caries in permanent and primary dentitions,yet on accounting only for standardized definitions for assessment of childobesity using body mass index, a strong significant relationship was evident inchildren with permanent dentitions. Moderating for study country of origin(newly ‘industrialized’ versus industrialized) showed a significant relationshipbetween obesity and dental caries in children from industrialized but not newlyindustrialized countries. Cofactors such as age and socioeconomic class weresignificant moderators. Conclusions: Future analysis should investigate theseconfounding variables, helping shape the future of obesity managementprogrammes and oral health interventions, through determining common riskfactors.
Ceara Hayden1, Jennifer O. Bowler2,
Stephanie Chambers3, Ruth Freeman3,
Gerald Humphris1, Derek Richards4 and
Joanne E. Cecil1
1School of Medicine, University of St
Andrews, Fife, Scotland, UK, 2School of
Psychology, University of East Anglia,
Norwich Research Park, Norwich, UK, 3Oral
Health and Health Research Programme,
Dental Health Services Research Unit,
University of Dundee, Dundee, UK, 4Centre
for Evidence-based Dentistry, Oxford, UK
Key words: children; dental caries; obesity;overweight
Joanne E. Cecil, School of Medicine,University of St Andrews, Medical andBiological Sciences Building, North Haugh,St Andrews KY16 9TF, UKTel.: +44 0 1334 463541Fax: +44 0 1334 467470e-mail: [email protected]
Submitted 16 November 2011;accepted 18 September 2012
Childhood obesity has become a global health
problem and is associated with precursors of adult
illnesses including cardiovascular disease (1) and
type 2 diabetes (2). Worldwide, almost 43 million
children below 5 years of age carry excess body
weight (3). In the United States, 17% of children
and adolescents are currently obese (4). The
increasing risk of obesity for young people is of
particular concern because research has suggested
that childhood obesity predicts adult obesity (5,
6). Obesity is therefore one of the primary chal-
lenges to public health, with consequences affect-
ing many different areas of life, and so the need
for immediate preventative action is warranted.
Although limited, there is evidence to suggest that
the problem extends beyond the peripheries of the
developed world into nondeveloped countries.
For example, in Mexico, classified as a newly
‘industrialized’ country (NIC; referring to coun-
tries between ‘developing’ and ‘developed’ sta-
tus), just over 10% of preschool children and 26%
of school-age children were classed as overweight
or obese in 2006 (7).
Obesity is a multifactorial disorder, influenced
by environmental and genetic risk factors (8),
where a sustained imbalance between energy
intake and energy expenditure facilitates storage of
excess energy as fat. Diet is a primary determinant
doi: 10.1111/cdoe.12014 289
Community Dent Oral Epidemiol 2013; 41; 289–308All rights reserved
� 2012 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd
of obesity. Consumption of energy-dense low-
nutrition foods, which tend to be high in saturated
fats and sugars, and low fruit and vegetable con-
sumption have been linked with increased weight
gain and obesity (9–13). Poor diet can also impact
negatively on health through effects on immune
functioning, growth, development, ageing and oral
health. Poor oral health is typified by the onset of
dental caries, presently the most common chronic
disease found in children (14). In the United States,
27% of preschool children experience dental decay
(15). As in developed countries, dental caries is rec-
ognized as a major problem in NICs where its
prevalence in preschool age children in the devel-
oping world ranges between 27% and 46% and is
as high as 72–95% in Mexico (16).
Diet specifically through the frequent consump-
tion of monosaccharide (e.g. glucose, fructose) and
disaccharide (e.g. sucrose) sugars is the predomi-
nant cause of dental caries (17, 18). Dental caries is
the localized destruction of susceptible dental hard
tissues by acidic by-products from bacterial fer-
mentation of dietary carbohydrates (19). In recent
years, concern has been raised over the consump-
tion of sugar-sweetened beverages in contributing
towards diseases such as obesity and dental caries
(17). In the UK, soft drink consumption provides a
significant source of added sugar to the diet and
contributes around 5% to average daily total
energy intake in men and women aged 19–24 years
(20). In late adolescence and young adulthood,
sweetened beverages provided more than 50% of
total intake of sugars (20). Notably, a greater pro-
portion of children from deprived areas consume
foods high in sugar and saturated fats on a regular
basis compared with those in high socioeconomic
status (SES) areas (21), and both dental caries and
obesity have been associated with low SES (21).
Importantly, dietary behaviours such as frequency
of ingesting cariogenic foods confer risk for dental
caries (22, 23). In a survey study investigating asso-
ciations between diet and caries-associated bacteria
in young children with severe early-childhood car-
ies (ECC), Palmer et al. (22) showed that children
with severe ECC consumed food and beverages
significantly more frequently compared with car-
ies-free children. Children with severe early caries
also ingested more foods that were high in cario-
genic potential compared with caries-free children
(22). The use of fluoride toothpastes (24), fluoride
varnishes (25) and genetic susceptibility (26) are
also important contributors to development of
ECC.
Critically, a number of studies have linked con-
sumption of sweetened beverages with increased
energy intake, obesity (27–29) and dental decay (30
–32). In addition, there are reports of associations
between obesity and dental caries (16, 33–37),although the data are mixed and provide conflict-
ing evidence (38–45). A recent systematic review
(46) reported that only one of seven cross-sectional
(CS) studies with children showed an association
between obesity and dental caries. This review was
limited however, by the narrow search criteria
used in the appraisal of papers reflecting a possible
bias. Conversely, nonsignificant findings for exam-
ple within Chen et al.’s study (38) reflect a skewed
sample of very young children (aged 3 years) with
a high caries prevalence, where the proportion of
children with one or more decayed or filled teeth
(56%) was not different according to body mass
index (BMI) categories. The authors suggest that
the lack of relationship between BMI and dental
caries could be explained by the frequency of sugar
ingestion (resulting in development of dental car-
ies) rather than the amount per se and acknowledge
dietary fat as an important promoter of obesity.
Conflicting findings in the literature to date sug-
gest that the relationship between obesity and den-
tal caries in children is likely to be complex and
potentially hindered by amalgamation of multiple
age groups that differ considerably in growth rate
and expressed phenotype as well as the differential
rates of growth that are expressed in children
within an age group. However, more research
should address these important public health prob-
lems to facilitate prevention and health promotion.
The aim of this systematic review and meta-anal-
ysis was to investigate and quantify the relation-
ship between obesity and dental caries in children.
Methods
Search strategy and selection criteriaA literature search was performed using Embase,
MedLine, ScienceDirect, Ovid and PsychInfo data-
bases. As the prevalence of obesity has increased
dramatically over the past 30 years leading to more
specialized obesity studies, it was decided a priori
that the search would be restricted to studies
published between 1980 and 2010. The identified
keywords and index terms: obes*, child*, paediatric,
weight, overweight, BMI, dental caries, primary
dentition, dft, dmft, dmfs, dfs were entered. Only
studies that were in English and fully accessible
290
Hayden et al.
were included in the meta-analysis. Reference lists
for all relevant articles and review reference sec-
tions were also examined for any further relevant
articles not yet identified. Only published and
accessible papers were considered and thus
unpublished research was not considered. Papers
were filtered by title for relevance and then at
abstract and article level in accordance with inclu-
sion criteria (Table 1).
Data extraction and quality assessmentThe two outcome measures were weight and car-
ies experience. Only participants who fell into the
highest BMI category (‘overweight/obese’
depending on assessment method) or ‘normal’
BMI were included in the analysis. Assessment of
child obesity varies worldwide and relies on dif-
ferential reference data making comparison of
trends difficult (47). BMI-for-age centiles (48), age-
and gender-related international standards recom-
mended by the International Obesity Task Force
(IOTF) (49) and Z-scores (50) are some of the most
often used methods to define a child who is over-
weight and or obese. Some studies thus assign the
term ‘obese’ to the highest BMI category, while
others assign ‘overweight’ to this group of chil-
dren. In this review, ‘obese’ will be used uni-
formly to refer to those children in the highest
BMI category. Children who were ‘underweight’
were not included. Dental caries was measured
using the decayed/missing/filled or decayed/
extracted/filled index.
Relevant abstracts and articles were indepen-
dently reviewed by three investigators (CH, JB and
JC), ensuring that validity was held across all
aspects of the studies. Review of abstracts followed
the PRISMA guidelines (51). Criteria for a valid
paper included defined outcome measures and
sample size above 200 participants. Individual
judgments from these authors were then analysed
to determine inter-rater reliability. Final decisions
on article retrieval and inclusion or exclusion were
made by consensus.
A quality assessment of the selected papers was
independently conducted by two of the authors
(CAH and SC). When assessing the quality of ran-
domized controlled trials or cohort studies, widely
recognized quality appraisal tools, such as the
Cochrane Collaboration’s tool for assessing risk of
bias (52) or the Newcastle-Ottawa scale (53) can be
used. According to a recent review by Sanderson
et al. (54), no such ‘gold standard’ tool exists for
assessing the quality and susceptibility to bias in
observational studies with CS or longitudinal
designs. However, an appraisal checklist devel-
oped by the University of Wales was selected as
Table 1. Inclusion and exclusion criteria for the studies reviewed
Inclusion criteria Exclusion criteria
Population Under 18 year olds with primary and/or secondarydentitions
Over 18s
Male and female No exclusions on genderCross-cultural No exclusions on cultureAny socio-economic class No class restrictions
Outcomes Primary outcome dental caries experience, measuredby dft, dfs, def, dmft, dmfs, DFT, DMFT, DMFS
Tooth decay or dental caries measured inother ways
Studies which did not specify or separatelyanalyse by dentition type
Use of BMI or z-scores as a measure of normal weightand obesity
BMI or z-scores not used as an indicator ofweight categorization
Sample included participants who were in one of twogroups. In the normal range or the highest BMIcategory labelled as either overweight and/orobese
Normal weight not used as control groupand/or obese group not included inanalysis
Clearly defined obesity, overweight and normal BMIcategories
Those only measuring average BMI forentire sample group
Study Design Levels I–IV; cross-sectional, quasi-experimental orcase controlled/cohort design with control groups,observational studies
Level V; qualitative designs and otherreviews
Published papers; unpublished data received directfrom author
Papers where data has not been sent afterrequest was made
Must be in English and accessible Papers in any other language andinaccessible
291
Obesity and dental caries in children
most appropriate for assessing study quality
and bias in the current systematic review and
meta-analysis (55). The checklist assesses study
quality on 12 criteria including bias, follow-up and
appropriate use of statistical methods, with a total
score of 12 indicating the highest quality score
available to each study. The quality assessment
scores for each study (Table 2) from two authors
(CAH and SC) were assessed for agreement to fur-
ther strengthen the quality assurance of this meta-
analysis.
Data extracted for meta-analysis were sample
size, outcome measures and significance. The par-
ticipant numbers for one study (43) were obtained
directly from the author. The papers included in
the analysis were those in which means and signifi-
cance, means and standard deviations, correlation
coefficient and significance or odds ratios and con-
fidence intervals (CI) were reported or calculated
from the available data. Where studies had addi-
tionally presented adjusted values for covariates,
the unadjusted values were included in the review.
Possible publication bias, which can result in the
nonpublication of small studies with negative
results, was assessed by visually evaluating a fun-
nel plot of the mean differences for asymmetry. In
addition to visual assessment of the graph, a
regression asymmetry test was performed to for-
mally determine whether such publication bias
had occurred.
Data synthesisComprehensive Meta-analysis II software (56) was
used to generate standard mean differences using
random effects meta-analysis. A random effects
approach to account for within-study and
between-study variation is more conservative than
a fixed effects model and is recommended by pres-
ence of high heterogeneity (56). Heterogeneity of
data was evaluated using the I2 statistic. Standard
mean differences were represented as a point esti-
mate and 95% CIs on a forest plot.
It was decided a priori that subgroup analyses
would be conducted on the studies according to
caries index, definition for assessment of child
obesity using BMI and the study country of origin.
Partitioning the data in this way also enabled age
and caries index to be examined in relation to
childhood obesity prevalence. The studies were
partitioned according to whether they were con-
ducted in NICs (Brazil, India, Iran, Thailand and
Mexico) or industrialized countries (France, Ger-
many, Sweden, Taiwan and USA) to assess the
impact of economic development on the relation-
ship between obesity and dental caries in children.
Role of the funding sourceThere was no funding source for this study. The
corresponding author had full access to all the
data in the study and had final responsibility for
the decision to submit for publication.
Results
Study characteristicsTwo hundred and twelve potential studies were
identified from the initial literature search. Thirty-
eight full-text articles were accessed, and 14 stud-
ies met the criteria and were included within the
meta-analysis (Fig. 1). A systematic review of the
appraised studies was completed (Table 2). One
study was longitudinal in design (35), while 13
were CS (16, 33, 36–39, 42–45, 57–59). Eight stud-ies were conducted in industrialized countries
(33, 35, 37, 38, 42, 43, 57, 58), with six conducted
in NICs (16, 36, 39, 44, 45, 59). Six studies investi-
gated permanent dentitions only (33, 35, 36, 39,
44, 58), five studies investigated primary denti-
tions only (16, 38, 43, 57, 59) and three studies
investigated both (37, 42, 45), although data relat-
ing to permanent dentition from the Kopycka
study (42) were insufficient for inclusion in the
meta-analysis. Six studies (16, 36, 42, 43, 45, 57)
assessed child obesity using the BMI-for-age cen-
tiles from the 2000 Centers for Disease Control
and Prevention (CDC) growth charts for children
and adolescents from 2 to 20 years of age (48),
while one paper (38) used standardized centiles
derived from the First National Health and Nutri-
tion Examination Study, 1971–1974 (60). Three
studies (33, 35, 58) assessed child obesity using
age and gender appropriate international stan-
dards for child obesity (>30 kg/m2 at 18 years)
(49) recommended by the IOTF. Two studies (39,
59) used Z-scores (50), where a Z-score > 2 was
used to classify overweight. In two studies, non-
standardized measures for assessment of child
obesity were used (37, 44). Four studies analysed
decayed or filled teeth (DFT/dft) (35, 37, 38, 45),
five studies analysed decayed, missing or filled
teeth (dmft/DMFT) (39, 43, 44, 57, 58), one study
incorporated decayed, extracted or filled teeth
(deft) (16) and surfaces (defs) (16), one study
assessed decayed, missing or filled surfaces
(dmfs) (59) and two decayed or filled surfaces
292
Hayden et al.
Tab
le2.
Literature
review
ofpap
ersincluded
within
themeta-an
alysis
Article
characteristics
Sam
ple
characteristics
Methoddetails
Resultsdetails
Qualityscore
(Max
12)
Studydesign
Participan
tdem
ographics
andch
aracteristics
Obesityoutcomemeasu
reDen
talcaries
outcome
measu
re
Analysis
Results
Conclusion
Referen
ce:A
lmet
al.(33
)8
Cross-sectional
(CS)
Swed
enn=40
2isoBMI>25
(n=64
)isoBMI<25
(n=33
8)13
.5–16.4years
Proportional
representation
ofsocio-dem
ographics
BMI(kg/m
2),categorized
accord
ingto
IOTFcu
t-off
values
(49)
Low
isoBMI(isoBMI<25
at18
years)
Overweight
(isoBMI25–29.9
at18
years)
Obesity(isoBMI>30
at18
years)
Total
man
ifest
caries
–D
mFa
Unpaired,
two-
sample
t-test
Adolescen
tsoverweight
(incl.o
bese)
had
higher
caries
than
low
weight
participan
tsPositive
relationsh
ipbetween
obesityan
dden
talcaries
‘Overweight
andobese
adolescen
tshad
more
approxim
alcaries
than
norm
al-w
eight
individuals’
Referen
ce:C
hen
etal.(38
)6
CS
Taiwan
n=51
333yearolds
BMI;less
than
5thpercentile
(verylow
BMI)
5th–25thpercentile
(low
weight)
25th–75th(m
edium
BMI)
75th–95th(highBMI)
>95
th(obese)
First
National
Health
andNutrition
Exam
inationStudy,
1971
–197
4
dft
Chi-square
analysis
Kruskal–
Wallis
tests
Therewereno
differences
amongBMI
groupsin
regardsto
theden
tal
caries
reported
among
participan
ts(K
ruskal–
Wallis,H
=6.45
,P=0.17
)Meanscoresof
dftwithin
thisgroupof
participan
tswas
4.1+2.7for
those
obese,
4.1+3.1for
overweightan
d4.2+3.0forthose
ofnorm
alBMI
‘Wecould
notfind
theassociation
betweenden
tal
caries
indeciduousteeth
andbodymass
index
of3-year-
old
children’
‘Nev
ertheless,
since
obesity
andden
tal
caries
are,in
principle,
causedbythe
poordietary
hab
it,further
studiesincluding
themealpattern
andthe
compositionof
thedietare
needed
for
evaluatingthe
293
Obesity and dental caries in children
Tab
le2
Continued
Article
characteristics
Sam
ple
characteristics
Methoddetails
Resultsdetails
relationsh
ips
betweenthese
twomost
prevalen
thealth
problemsin
childrenan
dad
olescen
ts’
Referen
ce:G
erdin
etal.(35
)11
CSlongitudinal
Swed
enn=23
034–10
years
Clustered
by
socioeconomic
group
(I–V
;high–low)
BMI(kg/m
2),categorized
accord
ingto
IOTFcu
t-off
values
(49)
BMIforoverweight
(>25
at18
years)–
BMIforobese(>30
at18
years)
deft(6
years)
DFT(10an
d12
years)
DFSa
One-way
ANOVA
Chi-square
linear
tren
dtest
Usinggen
der
and
socioeconomic
statusas
covariates,
BMIwas
significant
inaffecting
den
talcaries
prevalen
cein
12yearolds
Positive
relationsh
ipbetween
obesity
andden
tal
caries
‘Overweight
andobese
adolescen
tshad
more
approxim
alcaries
than
norm
alweight
individuals’
Referen
ce:G
ranville-G
arcia
etal.(39
)4
CS
Brazil
n=26
51ag
e1–5years
z-scoresalongsideW
HO
criteria
andNational
Cen
treforHealth
Statisticsguidelines.
Those
withaz-score
>2forweight–
heightrelationsh
ipwereconsidered
overweight
DMFT
Descriptive
statistics
Chi-square
analysis
Man
n–
Whitney
U-tests
Obesity
appearedto
berelatedto
inflated
decayed
values
(P=0.00
7),
butnotmissing
(P=0.74
3),fi
lled
(P=0.72
1),o
roverallvalues
(P=0.09
4)Those
withlow
DMFTindexes
wereless
obese,
howev
er,this
‘Norelationsh
ipwas
found
betweenden
tal
caries
and
obesity.S
uitab
lehealthpolicies
should
be
adoptedso
asto
minim
izethe
highprevalen
ceofden
talcaries
amongthis
population’
294
Hayden et al.
Tab
le2
Continued
Article
characteristics
Sam
ple
characteristics
Methoddetails
Resultsdetails
was
confounded
bysocial
class
Referen
ce:K
opycka-Ked
zieraw
ski
etal.(42
)10
CS
USA
n=10
180(756
8completed)
2–18
years
BMI;less
than
5th
percentile
(underweight)
5th–85thpercentile
(norm
alweight)
85th–95th(riskof
overweight)
95th
(overweight)
CDC
dfs
(2–11
years)
DMFS(6–18
years)
Bivariate
analyses
with
chi-square
statistics
Logistic
regression
Nostatistically
significant
associations
betweencaries
prevalen
cean
dweightstatus
between19
99an
d20
02
Noev
iden
ceto
suggestthat
overweight
childrenareat
anincreased
risk
forden
tal
caries.A
lthough
nodifferencesin
caries
ratesby
weightwere
foundin
younger
children,
suggestions
conferthat
being
overweightmay
beassociated
withdecreased
ratesofcaries
inolder
children
Referen
ce:M
acek
andMitola
(43)
10CS
USA
n=76
17BMI-for-ag
e�
95th
percentile
=19
.8m
BMI-for-ag
e5th–85th
percentile
=83
m2–17
years
BMI-for-ag
ecentiles
(CDC20
00)
Underweight=BMI-for-ag
e<5thpercentile
Norm
al=BMI-for-ag
e5th–
<85
thpercentile
Atrisk
overweight=85
th–
<95
thpercentile
Overweight=BMI-for-ag
e�
95th
percentile
Datafrom
NHANES
1999
–200
2dft/DMFT
Multiple
linear
regression
Noassociation
foundbetween
overweightan
dden
talcaries
Overweightwas
foundto
be
associated
with
lower
geo
metric
meanDMFT
‘Althoughitwas
hypothesized
that
BMI-for-ag
ewould
be
associated
with
increasedden
tal
caries
prevalen
ce…
thisassociation
was
notfound’
‘Given
the
importan
ceof
overweightas
apublichealth
problem,
howev
er,
295
Obesity and dental caries in children
Tab
le2
Continued
Article
characteristics
Sam
ple
characteristics
Methoddetails
Resultsdetails
cliniciansare
encouraged
tocontinuegiving
healthed
ucation
anddietary
counsellingto
overweightch
ild
paren
ts’
Referen
ce:N
arksawat
etal.(44
)5
CS
Thailand
n=86
212–14years
BMICen
tiles.Nutritionstatus
calculatedaccord
ing
toThai
MinistryofPublic
HealthMan
ual
using
weightforheightin
Thai
children.S
tandard
dev
iationsfrom
themed
ian
wereusedas
the
cut-offpoints
forfivegroups
Verythin
=<median�
2SD
Thin=median�
2SDto
median�
1.5S
DNormal
=median�1
.5SDto
median+1.5SD
Overw
eigh
t=median
+1.5to
+2SD
Obese
=>median+2S
D
DMFT
Man
n–
Whitney
U-test
Multiple
logistic
regression
Aneg
ative
relationsh
ipwas
foundbetween
theobesityan
dthe
DMFTindex,such
that
asweight
decreased
caries
prevalen
ceincreased
‘Norm
alweight
childrenhad
ahigher
risk
of
den
talcaries
than
obese
childrenag
ed12
–14years
inThailand’
‘Sch
oolhealth
promotion
activitiessh
ould
emphasize
eatinghab
itim
provem
ent
toreduce
the
inciden
ceof
caries’
Referen
ce:O
liveira
etal.(59
)10
CS
Brazil
n=10
1812–59months
BMI-for-ag
e;W
HO
childgrowth
stan
dardsreference
Nutritional
status
calculatedbyZ-scores
Atrisk
underweight=
Zscore<�2
Norm
al=�2
�Z-score
�+2
Atrisk
overweight=
Zscore>+2
dmfs
Unconditional
logistic
regression
Association
foundbetween
nutritional
status,
socioeconomic
factors
andcaries
experience
‘Underweight
childrenan
dthose
with
adverse
socioeconomic
conditionswere
more
likelyto
experience
caries’
296
Hayden et al.
Tab
le2
Continued
Article
characteristics
Sam
ple
characteristics
Methoddetails
Resultsdetails
Referen
ce:S
adeg
hian
dAlizadeh
(45)
4CS
Iran n=10
076–11
years
BMI-for-ag
ecentiles
(CDC20
00)
Norm
al=5th–<
85th
percentile
Atrisk
Overweight=85
th–
<95th
percentile
Overweight=
�95
thpercentile
DFT/dft
Multiple
linear
regression
Therewas
no
significant
association
betweenBMI-
for-ag
ean
dDFT,andBMI-
for-ag
ean
ddft
‘Overweight
childrenhad
higher
DFT/dft
scoresthan
did
norm
alch
ildren,
buttherewas
no
association
betweenBMI-for
agean
dDFT/
dftindices’
Referen
ce:S
harmaan
dHeg
de(36)
6CS
India
n=50
08–12
years
BMI-for-ag
ecentiles
(CDC20
00)
DMFS/dfs
ANOVA
Children
whowere
overweight
andobesehad
increased
prevalen
ceofcaries
inboth
primary
andperman
ent
den
tition
Apositive
association
was
also
found
betweencaries
prevalen
cean
dsw
eetfood
preference
‘Thereishigher
prevalen
ceof
den
talcaries
inoverweightan
dobesech
ildren’
‘Theim
portan
ceofnutrition
should
notonly
beem
phasized
withresp
ectto
gen
eral
diseases
butalso
with
regardsto
cariouslesions’
Referen
ce:S
helleret
al.(57
)7
CS
USA
n=29
32–5years
BMI-for-ag
ecentiles
(CDC20
00)
dmft
Reg
ression
BMIpercentile
did
notcorrelate
withdmft
‘Inthissample
of
children,theBMI
percentile
was
notcorrelated
withdmft’
Referen
ce:T
raminiet
al.(58)
8CS
France
n=83
512
years
BMI(kg/m
2),categorized
accord
ingto
IOTFcu
t-off
values
(49)
DMFT
Kruskal–
Wallis
Test
Noassociationwas
foundbetween
DMFTan
dBMI,
‘DFMTscoreswere
notsignificantly
different
297
Obesity and dental caries in children
Tab
le2
Continued
Article
characteristics
Sam
ple
characteristics
Methoddetails
Resultsdetails
Logistic
regression
althoughan
associationwas
foundwithsu
gar
consu
mption
accord
ingto
BMIlevels,as
defi
ned
for12
yearold
children’
‘Howev
er,itis
now
well
docu
men
ted
that
obesityan
dden
talcaries
hav
ecommon
determinan
ts,so
theopportunity
ofpreven
tive
measu
res
concerning
nutrition,
especially
amongch
ildren
andad
olescen
ts,
appears
tobe
essential’
Referen
ce:V
azquez-N
avaet
al.(16)
9CS
Mexico
n=11
604–5years
BMI-for-ag
ecentiles
(CDC20
00)
Norm
al=5th–85th
percentile
Atrisk
Overweight=
�85
th–<
95th
percentile
Overweight=
�95
thpercentile
deft/defs
Logistic
regression
Therewas
asignificant
association
between
atrisk
overweight,
overweightan
dcaries
inthe
primary
den
tition
‘Childrenwith
obesityhav
emore
caries
than
childrenwith
norm
alweight’
‘Theprevalen
cedocu
men
tedfor
each
ofthese
variables
indicates
that
thereisaclear
needfor
establish
ing
community
healthprograms
toiden
tify
and
limitrisk
factors
298
Hayden et al.
Tab
le2
Continued
Article
characteristics
Sam
ple
characteristics
Methoddetails
Resultsdetails
forden
talcaries
andobesityin
children’
Referen
ce:W
illershau
senet
al.(37
)5
CS
German
yn=12
906–11
years
BMI(nutritionstatus
calculatedwith
reference
toArbeitsgem
einschaft
Adipositasim
Kindes-
undJugen
dalter,20
02)
low
weight;norm
alweight;
highweight;obese
DFT/dft
T-test
ANOVA
Logistic
regression
Therewas
asignificant
association
betweenhigh
weightan
dcaries
prevalen
cein
the
firstan
dperman
ent
den
tition
‘Theresu
ltsofthis
studyindicatea
possible
associationof
highweightan
dcaries’
‘Infuture
preven
tive
programs,the
importan
ceof
nutritionsh
ould
notonly
be
emphasized
withresp
ectto
gen
eral
details
butalso
with
regardto
carious
lesions’
IOTF,International
ObesityTaskForce.
299
Obesity and dental caries in children
(dfs) (35, 42). A single report examined total ap-
proximal caries prevalence and fillings (33).
Population characteristicsThe age of participants from the studies included
in the meta-analysis spanned from 1 to 18 years.
All studies sampled both boys and girls.
Data extraction and quality assessmentAn intraclass correlation (ICC) was calculated to
assess the inter-rater reliability of the papers that
were excluded after appraisal by three authors
(CH, JB and JC). A value of 0.63, indicating moder-
ate consensus, was found. Using the University of
Wales Quality Assessment tool for observational
studies (55), studies were assigned quality scores
ranging from 4.0 to 10.5 (Table 2). Reliability of
study quality appraisal was robust, and reliability
between raters was strong (ICC = 0.91). Four of the
14 studies scored <6.0 (half of the total possible
quality score) (37, 39, 44, 45), while seven studies
scored 8.0 or above in relation to study quality (16,
33, 35, 42, 43, 58, 59). The most frequent threats to
quality were a failure to take confounding and bias
into account and drawing conclusions not sup-
ported by the data presented. Study quality scores
assisted in the interpretation of meta-analysis
results.
A funnel plot (Fig. 2) used to assess publication
bias among the papers illustrates a symmetrical
spread of the studies with regard to the stan-
dard errors reported within each paper. The plot
Potentially relevant studies identified on literature search and screened for
retrieval n = 212
Study abstracts identified and eligibility analysed
n = 45
Studies excluded by title due to non-access or irrelevance n = 167 Elimination of duplicates n = 70
Did not satisfy inclusion criteria n = 97
Full text articles appraised n = 38
Studies included in meta-analysis n = 14
Studies excluded by abstract only n = 7No measure for caries n = 1
Review/Thesis n = 2 No direct comparison n = 3
No weight measure n = 1
Studies excluded unanimously n = 11 No comparison between BMI and caries n = 5
Wrong type of data n = 4 Inadequate measures n = 1 Inadequate statistics n = 1
Full text articles further appraised, discussed amongst investigators and
awaiting author clarification n = 27 Studies excluded n = 13
No comparison between BMI and caries n = 2 Wrong type of data n = 5
No BMI classifications n = 3 Inadequate statistics n = 3
Fig. 1. PRISMA flow diagram illus-trating the literature review process.
–2.0 –1.5 –1.0 –0.5 0.0 0.5 1.0 1.5 2.0
0.0
0.1
0.2
0.3
0.4
Stan
dard
err
or
Std diff in means
Funnel plot of standard error by std diff in means
Fig. 2. Funnel plot indicating anyelements of publication or selectionbias within the studies included inthe meta-analysis.
300
Hayden et al.
confirms a low level of publication bias, further sup-
porting the reliability of the overall findings.
Data synthesis
Relationship between BMI and dental caries. Hetero-
geneity of the included studies was present as
indicated by the significant I2 value (Q = 55.701,
P < 0.001), and therefore, a random effects model
was employed. This model provides support for
a significant relationship between obesity and
dental caries. In synthesizing the results of the
selected studies (16, 33, 35–39, 42–45, 57–59), thefirst forest plot (Fig. 3) segregates the data into
dentition type, which is correlated with age
(older children associated with DMFT, younger
with dft). Overall, a significant relationship
between childhood obesity and dental caries
(effect size = 0.104, P = 0.049) was found. When
analysed by dentition type, there was a nonsignif-
icant association of obesity and dental caries in
permanent (effect size = 0.124, 95% CI: �0.053 to
0.301, P = 0.17, ns) and primary dentitions (effect
size = 0.093, 95% CI: �0.033 to 0.220, P = 0.149,
ns). Subgroup analyses were conducted to
explore the role of caries index, definition for
assessment of child obesity using BMI, and the
study country of origin on the relationship
between obesity and dental caries.
Relationship between BMI and dental caries by measure
of obesity assessment. As Fig. 4 confirms, different
results are derived dependent on the measure used
to assess child weight status. A significant relation-
ship between dental caries and obesity is noted in
the studies using standardized measures for
assessing child obesity such as BMI-for-age centiles
(effect size = 0.189, 95% CI: 0.060–0.318, P = 0.004)
or IOTF cut-offs (effect size = 0.104, 95% CI: 0.060–0.180, P = 0.008). Interestingly, those that used Z-
scores (effect size = �0.147, 95% CI: �0.396 to
0.102, P = 0.248) provided nonsignificant findings,
along with studies using nonstandardized scales
(effect size = �0.030, 95% CI: �0.436 to 0.375,
P = 0.884). In exploring the impact of including
both Willerhausen’s (37) and Narksawat’s (44)
studies that employed nonstandardized measures
for assessment of child overweight and obesity, it
was appropriate to run a meta-analysis by denti-
tion type on data purely based from standardized
measures (i.e. excluding the two aforementioned
papers (37, 44). Contrary to previous analysis, a
significant positive relationship between obesity
Group byDentition type
Study name Statistics for each study Std diff in means and 95% CI
Std diffin means
Lowerlimit
Upperlimit P-Value
0.0831.010–0.0620.474Alm et al., 2008Permanent
Permanent
Permanent
Permanent
Permanent
Permanent
Permanent
Permanent
Permanent
0.0170.1750.0170.096Gerdin et al., 2008
0.8360.228–0.282–0.027
0.000–0.252–0.809–0.530Narksawat et al., 2009
0.0130.6610.0780.370Sharma and Hedge, 2009
0.5990.450–0.2590.095Tramini et al., 2009
0.1700.301–0.0530.124
0.5620.148–0.0800.034Chenetal., 1998Primary
Primary
Primary
Primary
Primary
Primary
Primary
Primary
Primary
0.1240.319–0.0380.140Macek and Mitola, 2008
0.0500.000–0.563–0.282Oliveira et al., 2008
0.6810.541–0.3530.094Sheller et al., 2009
0.0000.5440.1790.362Vazquez-Nava et al., 2009
Granville-Garcia et al., 2008
Sadeghi and Alizadch, 2007 (permanent teeth) 0.394 0.176 0.612 0.000
Willerhausen et al., 2007 (permanent teeth) 0.239 0.053 0.425 0.012
Kopycka-Kedzierawski et al., 2008 (primary teeth) –0.123 –0.386 0.140 0.360
Sadeghi and Alizadch, 2007 (primary teeth) 0.227 0.010 0.445 0.040
Willerhausen et al., 2007 (primary teeth) 0.159 –0.027 0.345 0.093
0.093 –0.033 0.220 0.149
Overall 0.104 0.001 0.206 0.049
–1.00 –0.50 0.00 0.50 1.00
Favours non-caries Favours caries
Meta analysis
Fig. 3. Forest plot illustrating overall relationship between BMI and dental caries within permanent and primarydentitions.
301
Obesity and dental caries in children
and dental caries was revealed within permanent
dentitions (effect size = 0.198, 95% CI: 0.046–0.350,P = 0.011; Fig. 5).
Relationship between BMI and dental caries bycountry (industrialized versus NIC)Subgroup analyses indicated that compared with
normal weight children, obese children from
industrialized countries (effect size = 0.122,
CI = 0.047–0.197, P = 0.001) had a significant rela-
tionship between obesity and caries in contrast to
those from NIC countries (effect size = 0.079,
CI = �0.106 to 0.264, P = 0.264; Fig. 6).
Discussion
The aim of the systematic review and meta-analy-
sis was to investigate and quantify the relationship
between obesity and dental caries in children. The
principal findings indicate a small overall associa-
tion between obesity and level of caries in the
permanent dentition when standardized defini-
tions for the assessment of child obesity are used,
such that caries is more prevalent in obese children
than normal weight children. No association
between obesity and caries was found in the
primary dentition. These results reflect previous
findings indicating that different levels of obesity
exist in school-age (6–18 years) and preschool chil-
dren, where obesity tends to be more prevalent in
the older age group (42) and suggest that obesity
and childhood caries may have a joint cause. There
may be a number of candidate variables. The con-
sumption of foods high in sugar and refined carbo-
hydrates would be a strong contender. Subgroup
analyses provided a deeper understanding of this
relationship and possible explanation of previous
conflicting reports.
BMI classificationsAssessment of child weight status was subject to
nonuniformity across studies used in the meta-
analyses. Some studies used the most recent CDC
2000 centiles for children of ‘underweight’, ‘nor-
mal’, ‘at risk of overweight’ and ‘overweight’
(overweight > 95th percentile) developed for the
United States (48), to determine weight status. In
the United States, a child BMI-for-age and gender
tracking at or above the 95th centile has been rec-
ommended and related to identify obesity (61).
Other studies employed the international age and
gender appropriate data sets for assessment of
child obesity recommended by the IOTF (49),
Group byBMI scale
Study name Statistics for each study Std diff in means and 95% CI
Std diffin means
Lowerlimit
Upperlimit P-Value
0.5620.148–0.0800.034Chen et al., 1998BMI for age centilesBMI for age centilesBMI for age centilesBMI for age centilesBMI for age centilesBMI for age centilesBMI for age centilesBMI for age centilesBMI for age centiles
0.1240.319–0.0380.140Macek and Mitola, 2008
0.0130.6610.0780.370Sharma and Hedge, 20090.6810.541–0.3530.094Sheller et al., 20090.0000.5440.1790.362Vazquez-Nava et al., 20090.0040.3180.0600.1890.0831.010–0.0620.474Alm et al., 2008IOTF cut offs
IOTF cut offsIOTF cut offsIOTF cut offs
0.0170.1750.0170.096Gerdin et al., 20080.5990.450–0.2590.095Tramini et al., 20090.0080.1800.0280.1040.000–0.252–0.809–0.530Narksawat et al., 2009Other
OtherOtherOther
Kopycka-Kedzierawski et al., 2008 (primary teeth) –0.123 –0.386 0.140 0.360
Sadeghi and Alizadch, 2007 (primary teeth) 0.227 0.010 0.445 0.040Sadeghi and Alizadch, 2007 (permanent teeth) 0.394 0.176 0.612 0.000
Willerhausen et al., 2007 (primary teeth) 0.159 –0.027 0.345 0.093Willerhausen et al., 2007 (permanent teeth) 0.239 0.053 0.425 0.012
0.8840.375–0.436–0.0300.8360.228–0.282–0.027Granville-Garcia et al., 2008Z-scores
Z-scoresZ-scores
0.0500.000–0.563–0.282Oliveira et al., 20080.2480.102–0.396–0.1470.0010.1670.0420.105Overall
–1.00 –0.50 0.00 0.50 1.00
Favours non-caries Favours caries
Meta analysis
Fig. 4. Forest plot illustrating overall relationship between BMI and dental caries, with measure for assessment of childobesity as a moderator.
302
Hayden et al.
based on a reference population developed from
heterogeneous CS growth studies across six
nations. Two studies used other nonstandardized
data for assessment of child obesity (37, 44) which
may be more limited in their representation of pop-
ulation variability compared with the CDC growth
charts and IOTF cut-offs. Interestingly, the effect
size varied as a function of measure to assess
Group byType of country
Study name Statistics for each study Std diff in means and 95% CI
Std diffin means
Lowerlimit
Upperlimit P-Value
0.0831.010–0.0620.474Alm et al., 2008Industrialised
0.0170.1750.0170.096Gerdin et al., 2008
0.1240.319–0.0380.140Macek and Mitola, 2008
0.5990.450–0.2590.095Tramini et al., 2009
0.0010.1970.0470.122
0.5620.148–0.0800.034Chen et al., 1998Newly-Industrialised
Industrialised
Newly-Industrialised
Industrialised
Newly-Industrialised
Industrialised
Newly-Industrialised
Industrialised
Newly-Industrialised
Industrialised
Newly-Industrialised
Industrialised
Newly-Industrialised
Industrialised
Newly-Industrialised
Newly-Industrialised
Newly-Industrialised
0.8360.228–0.282–0.027Granville-Garcia et al., 2008
0.000–0.252–0.809–0.530Narksawat et al., 2009
0.0500.000–0.563–0.282Oliveira et al., 2008
0.0130.6610.0780.370Sharma and Hedge, 2009
0.6810.541–0.3530.094Sheller et al., 2009
0.0000.5440.1790.362Vazquez-Nava et al., 2009
0.4040.264–0.1060.079
Overall
Kopycka-Kedzierawski et al., 2008 (primary teeth) –0.123 –0.386 0.140 0.360
Willerhausen et al., 2007 (primary teeth) 0.159 –0.027 0.345 0.093
Willerhausen et al., 2007 (permanent teeth) 0.239 0.053 0.425 0.012
Sadeghi and Alizadch, 2007 (primary teeth) 0.227 0.010 0.445 0.040
Sadeghi and Alizadch, 2007 (permanent teeth) 0.394 0.176 0.612 0.000
0.116 0.046 0.185 0.001
–1.00 –0.50 0.00 0.50 1.00
Favours non-caries Favours caries
Meta analysis
Fig. 6. Country type used as a moderating variable to distinguish between BMI and dental caries relationships.
Group byDentition type
Study name Statistics for each study Std diff in means and 95% CI
Std diffin means
Lowerlimit
Upperlimit P-Value
0.0831.010–0.0620.474Alm et al., 2008Permanent
Permanent
Permanent
Permanent
Permanent
Permanent
Permanent
0.0170.1750.0170.096Gerdin et al., 2008
0.8360.228–0.282–0.027
0.0130.6610.0780.370Sharma and Hedge, 2009
0.5990.450–0.2590.095Tramini et al., 2009
0.0110.3500.0460.198
0.5620.148–0.0800.034Chen et al., 1998Primary
Primary
Primary
Primary
Primary
Primary
Primary
Primary
0.1240.319–0.0380.140Macek and Mitola, 2008
0.0500.000–0.563–0.282Oliveira et al., 2008
Granville-Garcia et al., 2008
Sadeghi and Alizadch, 2007 (permanent teeth) 0.394 0.176 0.612 0.000
Kopycka-Kedzierawski et al., 2008 (primary teeth) –0.123 –0.386 0.140 0.360
Sadeghi and Alizadch, 2007 (primary teeth) 0.227 0.010 0.445 0.040
0.6810.541–0.3530.094Sheller et al., 2009
0.0000.5440.1790.362Vazquez-Nava et al., 2009
0.080 –0.068 0.228 0.289
Overall 0.138 0.032 0.243 0.011
–1.00 –0.50 0.00 0.50 1.00
Favours non-caries Favours caries
Meta analysis
Fig. 5. A forest plot illustrating those studies using standardized definitions for assessment of child obesity, in relationto BMI and dental caries. Data moderated into permanent and primary dentitions.
303
Obesity and dental caries in children
child obesity using BMI (standardized versus
nonstandardized), and this variation in measure
may partly explain the inconclusive reports on the
relationship between dental caries and obesity in
the literature to date. In their recent investigation
into adult periodontitis and obesity, Suvan et al.
(62) have suggested that variation in BMI thresh-
olds contribute to the heterogeneity of meta-analyt-
ical data that can directly influence the resulting
effect size. The current meta-analysis was success-
ful in separating the different measures for assess-
ment of obesity to provide a more robust analysis
of the data to determine the relationship between
obesity and dental caries. It is also possible that dif-
ferential growth rates between children at similar
ages (49) and genetic susceptibility (26) may also
help to explain the mixed results into the relation-
ship between obesity and dental caries in children.
Another confounding variable that may have influ-
enced this relationship was socio-economic class,
and thus, the investigation into industrialized and
NICs was highly relevant.
Socioeconomic statusThe differential pattern of childhood caries and
obesity observed between industrialized and NICs
might be explained by the excessive consumption
of foodstuffs, such as soft drinks (17) that contain
high quantities of refined carbohydrates, in indus-
trialized countries. Soft drink consumption has
been shown to be a risk factor for obesity and den-
tal caries (63). Previous reports have concluded an
inverse relationship to the one found within this
meta-analysis and propose that the generally
poorer eating practices in NICs are promoted by
adverse economic conditions and associated with
an increase in caries prevalence development (64).
Interestingly, Dye et al. (64) found that being Mexi-
can-American predicted increased caries preva-
lence among preschool children and associated this
with poor eating practices such as not eating suffi-
cient fruit and vegetables and missing breakfast
(64). Likewise, the growing trend of soft drink con-
sumption in NICs has been particularly associated
with hypocalcaemia among school-age children
(65), which may in turn lead to poor enamel devel-
opment or hypoplasia (66). Such conflicting results
highlight the caveats of meta-analysis to explore
multiple correlating factors. Thus, in the present
review, the moderating influence of SES to assist
explanation of the significant obesity–caries rela-
tionship may be a result of the comparative surplus
of supply of such unrefined foodstuffs in the
higher Gross Domestic Product (GDP) nations and
is likely to be a function of specific measures for
assessment of child weight status in these studies
(as above).
Dental caries in the permanent and or primary
dentition was also a moderating factor. Seen within
the context of previous research, the tentative link
between tooth development and dental caries
further highlights the notion that age may be a
significant confounding variable. One possible
explanation for this apparent age-dependent rela-
tionship could be a reflection of the increasingly
sedentary lifestyles led by children, particularly
within older children (43). Increased television
viewing habits in older children have been associ-
ated with an increasingly self-moderated and
unhealthy diet (67–69) and increased meal fre-
quency (70), particularly snack consumption (71),
which is often highly processed and high in sugar.
This specific sedentary behaviour provides an
opportunity for increased energy intake from
energy-dense poor-nutrition foods that can increase
the risk of weight gain and obesity (71) and also pro-
vides a window for increasing risk of dental decay
through an increased contact time between ferment-
able carbohydrates, in snack foods high in sugar
and dentition (72). Importantly, however, obesity
and dental caries represent multifactorial conditions
and thus, the notion that they represent a cause–effect relationship should be viewed with caution.
LimitationsThe main limitation to the present analysis is the
level of weighting of the studies included in the
meta-analysis. The study by Macek and Mitola (43)
was the most heavily weighted study based on par-
ticipant numbers, and this showed no significant
association between BMI-for-age and dental caries
in both the primary and permanent dentitions.
Thus, as there is one disproportionately strong
negative study, the true representativeness of these
results within the meta-analysis may be ques-
tioned.
Despite a stringent selection process, the quality
of the studies selected for the meta-analysis may be
subject to criticism. Many papers appeared incon-
sistent in both their definitions of dental caries as
well as assessment of child obesity; an indication
of possible measurement error across all studies.
For example, some interchanged between mea-
sures of reporting dental decay, using dft, DMFS,
DMFT or a combination of measures (16), which
could lead to variation in the levels of dental caries
304
Hayden et al.
identified. Our use of unadjusted data rather than
adjusted data should also be noted as a potential
limitation. However, not all studies gave adjusted
results and, where available, use of adjusted results
could also introduce bias through variation in
number and type of covariates adjusted for, there-
fore introducing variation of approach.
At the study level, Vazquez-Nava et al. (16)
recruited participants from a nursery school
engaged in a preventive dentistry programme.
This implies that the nursery was in an area identi-
fied as having a high incidence of tooth decay and
worthy of intervention. Thus, while the relatively
high prevalence of untreated childhood tooth
decay in Mexico has been noted (36, 37), sampling
bias might be partially responsible for the compar-
atively large effect observed in this study. Sam-
pling bias could also have occurred in the studies
by Sharma and Hegde (36) and Sheller et al. (57),
who both recruited from a single department of
paediatric dentistry.
As part of the analysis, studies were critically
appraised, and an average quality score was
attached to each. This provided some guidance as
to which papers were of better quality than others,
and the use of a scale provided a linear measure of
quality, rather than categorizing papers as ‘good’
or ‘bad’. The study by Gerdin et al. (35) was a lon-
gitudinal study and received the highest quality
score. This study found that BMI had an indepen-
dent effect on caries prevalence. There was no clear
pattern found in terms of study quality and signifi-
cant or nonsignificant relationships between obes-
ity and dental caries within the CS studies.
There were two potential sources of bias in the
studies themselves: the role of SES and the samples
used within the studies. Substantial research has
investigated the relationship of SES as a moderat-
ing variable to the onset of dental caries and BMI
(63, 73, 74). Separating papers into industrialized
and NICs attempts to account for SES, yet more
stringent assessment of SES would be advanta-
geous for future investigations. In this current
review, only two studies looked at SES directly (35,
59), and so separate analysis for this is limited.
Finally, potential sources of bias in the review
process itself can also be identified: selection bias,
validity bias and the use of meta-analysis as a pro-
cess. Primarily, papers were only selected from
online databases, and therefore, only published
and accessible papers written in English were con-
sidered, meaning unpublished studies were auto-
matically excluded. Often reports are not
published because of nonsignificance, which, in
this case, would have further confirmed the non-
significance of the results.
These results confirm a positive association
between obesity and dental caries in the permanent
dentition, although it is unclear what is the causa-
tive direction of this relationship? Future research
investigating the complexities of this relationship
is warranted and should employ experimental
designs to assess the common risk factors of obes-
ity and dental caries using more systematic and
universal measures of both obesity and permanent
dentitions. Health promotion interventions and
health education programmes promoting healthy
eating, targeting diets high in sugars and cario-
genic foods that are associated with weight gain,
within a common risk factor approach would
therefore assist in preventing obesity and dental
caries.
AcknowledgementsThere was no funding source for this study.
Author contributions
JC, RF and GH planned the study. CH and JB con-
ducted the literature search. CH, JB, JC and SC
undertook the data extraction. CH, JB and JC con-
ducted the meta-analysis. CH, JB and JC drafted
the manuscript. All authors critically revised the
manuscript.
Conflicts of interest
The authors declare that there are no conflicts of
interest.
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