Introduction
Childhood overweight and obesity are increasingly significant public health issues, and the World Health Organization (WHO) has classified both as key risk factors for chronic diseases such as type 2 diabetes, cardiovascular disease, musculoskeletal disorders, certain cancers and psychological comorbidities and subsequent mortality in adulthood.1-4 The WHO estimates that nearly 41 million children under the age of five were either overweight or obese in 2016.2 The Global Burden of Disease Study examined the global, regional and national prevalence of obesity in children from 1980 to 2013 and reported that the prevalence of obesity has increased substantially in children from both developed and developing countries.5
Dental caries (i.e. tooth decay) is one of the most common chronic diseases worldwide and presents a persistent public health challenge for young children.6-9 The consequences of untreated dental caries can include severe pain, infection and abscesses, all of which lead to repeat prescription of antibiotics, distress and altered sleep patterns.10,11 Furthermore, untreated dental caries can compromise children's growth and development, especially in relation to their poor dietary intake.7 There is also evidence that decay in primary (i.e. baby) teeth is a strong risk predictor of tooth decay in the permanent dentition.6
The causes of overweight and obesity, and dental caries are multi-factorial, and share some common risk factors including high-sugar diet, low socio-economic status, limited use of health services and low health literacy.12-14 The common risk factors for these health conditions seem to increase the prevalence of both conditions and appear to be the most accepted explanation for the relationship between overweight and/or obesity, and dental caries.15 Several researchers have postulated that overweight and/or obesity may be a marker of dental caries,16-18 but the evidence remains inconclusive. It would be beneficial to clearly identify the common risk factors that are associated with both of these conditions. This will assist clinicians, researchers and policy makers to implement health promotion programs based on the common risk factor approach, to reduce the burden of overweight and/or obesity and dental caries in young children.19,20
Multiple scoping searches of the literature were conducted to assess previous systematic reviews on overweight and/or obesity and dental caries. The authors of this review also searched for registered (e.g. PROSPERO)21 or published protocols of any new or ongoing reviews. These searches revealed six systematic reviews on this research topic. The assessment of these reviews identified various knowledge gaps. Most systematic reviews draw evidence predominantly from cross-sectional studies15,22-24 except for the reviews by Li et al.25 and Hooley et al.,26 which reported evidence from multiple observational study designs (i.e. cohort, nested cross-sectional and case-control studies). However, the literature search of Li et al.25 was limited to 2014, and that of Hooley et al.26 was only until 2011, which highlights the need for an updated review. Then, we appraised the methodological quality of all previous systematic reviews using the AMSTAR 2 tool27 and all were rated as “critically low”. None of the reviews performed gray literature searches or contacted field experts to gather information on ongoing and/or unpublished studies.15,22-26 In addition, none of the reviews identified the common risk factors for overweight and/or obesity and dental caries,15,22-26 which is the most accepted explanation for the inter-relationship12-14 between the two health conditions. Furthermore, previous reviews searched the evidence relating to both children and adolescents (under 18 years of age),15,22-26 and the majority of reviews did not distinguish the results specifically for children under six years with primary (baby) dentition,15,24-26 which highlights a need to focus on evidence in early childhood (under six years), as the health status at a young age is a strong predictor for wide-ranging health outcomes later in life.28,29 Moreover, all reviews conducted the literature search on limited databases,15,22-26 with some restricted to English language only,23,25 which might lead to missing important evidence and language bias, respectively.30,31 Finally, none of the reviews distinguished the results depending on the design of included studies15,23,24,26; they considered the evidence drawn from cross-sectional, cohort and case-control studies as equal rather than weighing the evidence as per the levels of quality of evidence.32 These gaps identify a need for a high-quality systematic review to ascertain if overweight and/or obese children are at increased risk for dental caries.
The objectives of this review are i) to identify whether overweight/obese children aged under six years experience increased dental caries compared to non-overweight/non-obese children, and ii) to identify the common risk factors associated with overweight and/or obesity, and dental caries. Our planned search strategy will include studies of overweight and/or obesity, and dental caries measures reported, and will synthesize available evidence using rigorous methods. The strengths of this review is that the updated and high-quality evidence will be based on a truly exhaustive literature search. Furthermore, this review will also identify the common risk factors for overweight and/or obesity, and dental caries in early childhood to inform policy and practice in health promotion.
Review questions
To what extent do overweight/obese children, aged under six years, experience more dental caries compared to non-overweight/non-obese children?
An additional review sub-question is: Are there any common risk factors for both health conditions (i.e. overweight and/or obesity and dental caries) in the study population?
Inclusion criteria
Participants
Studies examining children under six years of age and with a complete primary dentition (baby teeth) at the time of dental caries assessment will be included. Studies comparing children with: normal adiposity score, no dental caries, high adiposity score and with dental caries, will be included regardless of sex, country of origin, ethnicity or socio-economic status. Studies that include children with other medical conditions will be excluded. If studies report outcomes for children and adolescents, they will only be included in this review if data for children aged 72 months and younger are reported separately.
Exposure
The exposure of interest is the overweight and/or obesity (excluding birth weight) status of children under six years of age. Various assessment measures will be considered such as body mass index (BMI), BMI z-score, BMI z-score for age, BMI percentile for age, and weight-for-height z-score.33-35 These are some of the most accurate measures for assessing adiposity change in growing children.33 The adiposity status will be clearly defined into categories of normal weight, overweight and obese based on widely recognized standardized criteria.35-37
Outcome
The outcome of interest is dental caries in children under 72 months of age with complete primary dentition. The outcome must be measured after the exposure assessment. The American Association of Pediatric Dentistry definition will be used for early childhood caries, which is the presence of cavitated or non-cavitated carious lesions in children under the age of six years.38 Only validated measures of dental caries will be considered39: dmft (number of decayed, missing due to decay, and filled primary teeth); dmfs (number of decayed, missing due to decay, and filled primary teeth surfaces); deft (number of decayed, extracted due to caries, and filled primary teeth); defs (number of decayed, extracted due to caries, and filled primary teeth surfaces); dft (number of decayed and filled primary teeth); dfs (number of decayed and filled primary teeth surfaces); the number of dmfs according to International Caries Detection and Assessment System (ICDAS) codes 1–2 (non-cavitated lesions), codes 3–6 (cavitated lesions), or 1–6 (non-cavitated and cavitated lesions).40
The outcome variables will be considered for inclusion if dental caries is assessed by a minimum of one qualified dental professional and/or professionally trained and calibrated non-dental examiner. Studies using data on self-reported experience of dental caries will be used carefully and study authors will be contacted for clarity, if necessary. Studies will be omitted if they only report on self-reported or non-validated outcome measures.
The eruption of complete primary dentition will be the eligible starting point for measuring the primary outcome. Birth cohort studies with a minimum follow-up of three years will be considered to account for the eruption of all primary teeth. Dental caries assessments at follow-up time points after the initial three-year examination will also be eligible.
There will be no restriction on the type of study settings.
Types of studies
Only specific human observational study designs will be included: longitudinal/cohort studies (prospective and retrospective), cross-sectional studies nested within cohort studies, and case-control studies. Cross-sectional studies, case series and case reports will be excluded due to their low level of scientific evidence.32 Randomized controlled trials and quasi-experiments will also be excluded because this review is not examining the role of any intervention related to overweight/obesity and/or dental caries outcomes.
Methods
This protocol will be reported based on the Preferred Reporting Items for Systematic Reviews and Meta-analysis Protocol (PRISMA-P) checklist41,42 and will follow the JBI methodology for systematic reviews of etiology and risk.43
Search strategy
The search strategy will aim to find both published and unpublished studies. A combination of specific medical subject headings (MeSH) terms and text words related to overweight, obesity, dental caries, and children under six years of age will be drafted and pre-tested in the MEDLINE database. Two reviewers (NM and AA), having experience in database searching, with assistance from an expert health sciences librarian, will initially conduct a pilot search on two databases. Thereafter, the same two reviewers will independently complete all remaining literature searches. A draft MEDLINE search strategy is provided in Appendix I. Once the MEDLINE strategy is finalized, it will be adapted to the syntax and subject headings of all other databases.
Additionally, a manual search of the reference lists of all the eligible studies (matching the inclusion criteria) and previously published systematic reviews will be performed. Furthermore, topic specialists and experts identified by the review team will also be contacted to obtain information on unknown and ongoing research studies. If a response is not received after three contact attempts, it will be considered that the respective expert is unaware of any ongoing and/or unpublished studies on the research topic. For this review, we will include articles with no restriction on language. Relevant non-English articles will be translated into English.
Information sources
The following electronic databases will be searched, without any restriction on publication date (i.e. from the time of database inception to present), type, language or region: MEDLINE (OVID), Web of Science (ISI), Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library), Embase (OVID), PsycINFO, ProQuest Central, Scopus, Cumulative Index to Nursing and Allied Health Literature (CINAHL) (EBSCO) and Google Scholar. The electronic database search will be supplemented by searching OpenGrey and Grey Literature Report (1999–2016) databases for gray literature, and ProQuest Dissertations and Theses Global, and the International Association for Dental Research conference website for unpublished studies.
Study selection
Studies identified through the electronic databases, gray literature databases, theses, conference website, and manual searches, will be uploaded into reference manager software, EndNote X8 (Clarivate Analytics, PA, USA), and duplicates removed. Prior to the formal screening and selection process, a calibration exercise between the two reviewers (NM and AA) will be performed on a pilot group of studies to refine the screening questions and ensure consistency across reviewers for screening and selecting eligible studies. The titles and abstracts of the identified studies will be screened independently by two reviewers (NM and AA). Articles that may meet the inclusion criteria will be retrieved in full and their details imported into the JBI System for the Unified Management, Assessment and Review of Information (JBI SUMARI; Joanna Briggs Institute, Adelaide, Australia). Two reviewers (NM and AA) will then independently assess the full-text articles and decide whether these meet the eligibility criteria. To resolve questions about eligibility, the study authors will be contacted to seek additional information. If disagreements arise, they will be resolved through discussion including a third reviewer (AH). If there are multiple reports published from a single study, multiple reports from the same study will be linked together. We will also record the reasons for excluding studies. None of the review authors will be blind to the journal titles, study authors or institutions. Non-English studies will be translated into English with assistance from translation services and/or volunteers. We anticipate that the translation process of non-English articles will be completed in a period of four weeks. The study selection process will be presented in the format of Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) flow diagram.44
Assessment of methodological quality
The methodological quality of each paper selected for retrieval will be assessed by two reviewers (NM and AA) independently using standardized critical appraisal instruments from JBI for the following study types: cohort studies (retrospective and prospective) and case-control studies.43 Any disagreements will be resolved through discussion and consensus with a third reviewer (AH). Study authors will be contacted in the event of insufficient details to confidently assess the methodological quality. If a response is not received after three contact attempts, we will assess the study based on its available information. The results of JBI tool assessment will be defined in a contextualized (descriptive) format by indicating what issues of methodological quality are present in each study and how these may influence the interpretation of study evidence.
The risk of bias of observational studies (i.e. cohort and case-control studies) will be assessed by two reviewers (NM and AA) independently using the Risk Of Bias In Non-randomized Studies of Interventions (ROBINS-I) tool.45 The tool evaluates biases based on confounding, selection of participants into the studies, missing data, and the measurement of outcomes. The adjustment of confounding variables will be considered during the conduct of review and what potential for bias that might bring. Any disagreements will be resolved through discussion and/or consensus with a third reviewer (AH). Study authors will be contacted in the event of insufficient details to confidently assess the risk of bias. If we do not receive a response after three contact attempts, we will assess the study based on its available information.
The purpose of using two different instruments is to ensure a comprehensive critical appraisal of the included studies, as the JBI tool will assess the methodological quality of included studies whereas the ROBINS-I tool will primarily assess overall the risk of bias. All included studies will be narratively reported in the review regardless of their methodological quality and risk of bias score.
Data extraction
A standardized data extraction form has been developed based on a checklist presented in the Cochrane Handbook for Systematic Reviews of Interventions.31 The data extraction form will be pilot tested (on two studies), and subsequently refined to ensure that we capture all relevant data (see Appendix II) and a calibration exercise will be performed to ensure consistency across reviewers. Data from each included study will be extracted independently by two reviewers (NM and AA), and any subsequent discrepancies will be resolved through discussion with a third reviewer (AH).
The data extracted will include information on the study design and settings, participant demographic details, sample size, follow-up period, methods used to measure overweight and/or obesity and dental caries, statistical analysis employed, key study findings, common risk factors identified for both the health conditions, and study funding sources. We will record any additional information considered to be relevant, and modify our data extraction form accordingly. If such a situation arises, it will be duly reported in the intended systematic review manuscript. We anticipate that data will be presented as text, tables and graphs, and that numerical values will be mentioned within the actual graphs for better understanding for the readers. The study authors will be contacted in case of missing data and/or to resolve any uncertainties.
In case of studies in non-English language, we anticipate that a single translator will transcribe the full-text article followed by the aforementioned data extraction process. We anticipate that measures used to assess overweight and/or obesity and dental caries outcomes in children would differ across studies. If so, we will caution readers on the impact of such measures on the review evidence.
Data synthesis
Individual studies,46 where possible, will be pooled in a meta-analysis using STATA v15 (StataCorp, College Station, TX, USA). Some degree of heterogeneity is expected across the studies; therefore, the random effects model for meta-analysis will be applied. Furthermore, data from cohort studies and case-control studies will not be pooled in a single meta-analysis. Effect sizes will be expressed as risk ratios or odds ratios or prevalence ratios (for dichotomous data) and weighted (or standardized) mean differences (for continuous data) and their 95% confidence intervals (CI) will be calculated for analysis. Cohort estimates will be presented as risk ratios/prevalence ratios with 95% CI, and case-control estimates will be presented as odds ratios with 95% CI. We anticipate using adjusted values for the analysis; however, if not available, unadjusted values will be considered and readers will be cautioned on the review findings. The degree of statistical heterogeneity will be assessed using standard I-squared statistics, with statistical significance values based on the statistical guidelines of the Cochrane Handbook for Systematic Reviews of Interventions.31 Where statistical pooling is not possible and/or there is substantial heterogeneity,31 a narrative synthesis of the study findings will be provided. The sources of heterogeneity and reasons for which it was inappropriate to pool the data will be specified in the main systematic review.
Subgroup analysis will be performed when there is sufficient data (if over 10 studies) as recommended by the Cochrane Handbook for Systematic Reviews of Interventions,31 based on the following:
- estimates (adjusted or crude) for the outcome measure
- country classification based on income level (high-income countries versus middle-and low-income countries)
- study design (cohort studies versus case-control studies).
Sensitivity analyses will be performed in order to explore the impact of risk of bias of included studies on outcomes. Where statistical pooling is not possible, the findings will be presented narratively in the form of text, tables and figures. The readers will be cautioned about the interpretation of the results and recommendations will be made for future research.
Publication bias will be assessed using a funnel plot of all included studies (if over 10 studies).47 To assess funnel plot asymmetry, the Egger test48 (for continuous outcomes) and the Arcsine test49 (for dichotomous outcomes) will be performed. Study authors will be contacted via email for missing data and a maximum of three attempts will be made to acquire specific numerical data. If data are still unavailable, a narrative description of these studies will be provided, and the potential impact of these studies will be elaborated on in the review.
Assessing certainty in the findings
The quality of evidence for all the outcomes will be assessed using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach,50 which may require some level of adaptation as our aim is to assess if overweight/obese children are more at risk of dental caries compared to non-overweight/non-obese children. Two reviewers will perform this process independently (NM and AA). Any unresolved disagreements will be resolved through discussion and/or consensus with a third reviewer (AH). The quality of the evidence will be assessed based on risk of bias, indirectness, inconsistency, imprecision, and publication bias.
Acknowledgments
The authors would like to thank Professor Gregory Kolt (School of Science and Health at Western Sydney University), Mr. Paul Fahey (Western Sydney University) and Ms. Katrina Chaudhary (School of Science and Health Librarian at Western Sydney University) for their support and aid in this review.
This study is being completed as part of Doctor of Philosophy (PhD) for NM.
Funding
This study is financially supported by the Australian National Health and Medical Research Council Grants (1033213, 1069861, 1134075) and Western Sydney University. The University will also provide services of an on-site librarian, access to commercial databases to obtain relevant literature, and software to manage or analyze data.
Appendix I: Search strategy for MEDLINE (Ovid)
Appendix II: Data extraction
Data extraction form 1: Source and eligibility
Data extraction form 2: Methods, participants, exposures, and outcomes
Data extraction form 3: Miscellaneous
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