Impact of acculturation on oral health among immigrants and ethnic minorities: A systematic review

Objective Cultural changes faced by immigrants and ethnic minorities after moving to a host country may have a detrimental or beneficial influence on their oral health and oral health-related behaviors. Therefore, this paper reviews the literature to see the impact of acculturation on immigrants and ethnic minorities’ oral health outcomes. Methods We searched seven electronic databases up to January 2018. All cross-sectional and longitudinal quantitative studies that examined associations between acculturation and oral health status and/or oral health behaviors among ethnic minority and immigrant population[s] were included. Study selection, data extraction, and risk of bias assessment were completed in duplicate. The Newcastle-Ottawa checklist was used to appraise the methodological quality of the quantitative studies. A meta-analytic approach was not feasible. Results A total of 42 quantitative studies were identified. The studies showed a positive association between acculturation and oral health status/behaviors. The most frequently used acculturation indicators were language spoken by immigrant and ethnic minorities and length of stay at the host country. High-acculturated immigrant and ethnic minority groups demonstrated better oral health outcomes, oral health behaviors, dental care utilization, and dental knowledge. Conclusions According to existing evidence, a positive effect of acculturation on oral health status and behaviors was found. Practical implications Dental practitioners should be culturally competent to provide the appropriate services and treatments to immigrant and ethnic minorities. Policymakers should also be sensitive to cultural diversities and properly address the unique needs of each group in order to maintain oral health equity.


Introduction
Associations between acculturation and general health [14,[17][18][19] have shown that highly acculturated immigrants and ethnic minorities had better physical activity, medication adherence, blood pressure level, and mental health compared with low acculturated individuals [18,19]. Similar correlations have been reported for oral health [6,19,20]. For example, among Haitian immigrants in New York City and Vietnamese immigrants in Melbourne, acculturation was inversely related to OH problems [6,20]. Individuals with high acculturation level showed a low level of decayed teeth and periodontal disease [6,19]. High acculturation status was also directly proportional to positive behavior adaptability and accessibility to OH care services [6,20]. Conversely, acculturation may promote some adverse behavioral practices that affect the OH of immigrant and ethnic minorities, such as the adoption of a cariogenic diet [6].
Although a data review of the OH impacts of acculturation has already been published in 2010 [21], new data might have become available that could challenge its conclusions, especially with the growing interest in this field over the past decade. Therefore, the objectives of the present report are to systematically review the impact of acculturation on immigrant and ethnic minority populations OH outcomes and to update previous evidence-based recommendations with new findings.

Protocol and registration
Neither a review registration nor a review protocol was completed. This systematic review is reported in accordance with Cochrane Handbook [22] and the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statements for reporting systematic reviews of health sciences [23].

Eligibility criteria
Based on the Participants-Intervention-Comparison-Outcome-Study (PICOS) method [24], we included cross-sectional and longitudinal quantitative studies that 1) examined the association between acculturation and at least one OH status (such as dental caries or periodontal disease) or OH behaviors (such as dental care utilization, brushing, flossing, or diet); 2) included a clearly-defined measure of acculturation either by using proxy measures such as language proficiency, country of origin, age at migration, and length of residence or validated scales like Behavioral and Self-identification Acculturation, The Psychological-Behavioral Acculturation Scale, and Acculturation Rating Scale for Mexican Americans-II; a well-described assessment tool for OH status including DMFT, ICDAS, periodontal attachment loss or self-reported OH status; and self-reported OH behaviors; 3) were conducted with at least one immigrant or ethnic group(s). Excluded were literature reviews, conference abstracts, editorials and, qualitative studies.  Table). The search strategy was developed with the assistance of a specialized health sciences librarian at the University of Alberta, Canada. First, we established the search terms on PubMed; next, we applied and adjusted these search terms on different electronic databases (S1 Table). Manual screening, which is checking all the reference lists of the included studies to find any relevant papers that were missed in the electronic searches, was completed by searching through bibliographies and reference lists of the included papers to determine potential papers that were not found in the electronic search. Finally, a grey literature search was conducted by using Google Scholar and Google search engine.

Study selection
Two reviewers (RD and PB) independently screened the list of titles and abstracts to identify the potentially relevant papers based on the inclusion criteria. If the abstracts were judged to contain insufficient information, then the full articles were reviewed to decide whether they should be included based on the selection criteria. When a discrepancy in the selection decision occurred, the two reviewers engaged in discussion until a consensus was reached.

Data extraction and data items
Two reviewers (RD and PB) independently extracted data from the selected papers on the following items: host country, participants' origins and ages, sampling, sample size, type of study, acculturation measure, association with OH outcomes, and results. Inconsistencies were discussed and resolved between the two authors. Missing or unclear information was sought from the authors of the selected papers. The Newcastle-Ottawa Scale assessed (NOS) the quality of the quantitative studies by scoring three main categories, which are group selection (four items), comparability (one item), and outcome (two items) [25]. A study can be awarded maximum of five stars for selection, a maximum of two stars for group comparability, and a maximum of three stars for outcome categories. The highest methodological quality is indicated by the maximum score; which is 10 points. Studies scored less than 3 are considered low quality, between 3 and 8 are medium quality and above 8 are high quality studies. Although the NOS is easy to apply and an adaptable tool, it has some limitations as there is no manual tool to use as a guide and it's not validated for cross sectional studies [26,27].

Risk of bias in individual studies
Two reviewers (RD and PB) independently assessed the methodological quality of the selected studies by using The Newcastle-Ottawa Scale [25] for cohort, cross-sectional, and case control studies.

Synthesis of results
Due to the heterogeneity of the included studies, findings were evaluated in a descriptive manner. It was not possible to conduct a meta-analysis.

Study selection
The electronic search of seven databases resulted in 641 studies. Of these studies, 168 were found eligible for a full-text review and 30 met our inclusion criteria. With the additional 14 studies found by manual screening, a total of 42 studies were included in our review. The selection process of the included papers is presented in Fig 1.

Study characteristics
Regarding the study design, 42 were cross-sectional, 1 was cohort, 1 was case-control, and all were written in English. Among the included studies 64% were conducted in the United States and 36% were conducted in other countries including: Canada, Japan, UK, Germany, Norway, China, Australia, New-Zealand, and Sweden. The characteristics of the included studies are presented in Table 1.
Orofacial pain. The association between acculturation attributes and orofacial pain was examined in 1 study [43]. High acculturated Hispanic immigrants more frequent usage of health and dental care for orofacial pain and symptoms [43]. Interestingly, nativity or longer time of residency and English language proficiency were negatively associated with orofacial pain, eating problems, sleeping difficulty, and depression, while recent immigrants had fewer sleep problems [43].
Oral health knowledge and behaviors. Five papers reported that immigrants and ethnic minorities with high acculturation level, local language proficiency, and long period of residency had better knowledge of the etiology of caries and periodontal disease, as well as better understanding of how to prevent dental caries [20,29,34,37]. In addition, among 12 studies investigating the association between acculturation and OH behaviors [6,20,29,37,40,44,47,58,60,62], 2 showed significant associations between high acculturation and healthier behaviors including frequent brushing [6] and mouth rinsing, while 2 studies reported no significant association between these variables [20,58]. One study reported that separators exhibited better OH behaviors than marginalized individuals [11][12][13]60]. On the contrary, more brushing frequency was seen among assimilators and integrators [11][12][13][14]62]. Living in a new country was associated with better OH behaviors and adaptability [29]; however, place of birth was not significantly correlated with any oral health-related behaviors [37]. Mother's language proficiency was also associated with enhanced oral hygiene practices [40] and use of sealant [44] for immigrants' children, but another study [47] found no association between host country's language use and OH-related behaviors. Length of residency was reported by some immigrants to be associated with increased consumption of sugary foods and drinks.
Oral health-related quality of life. The impact of acculturation on oral health-related quality of life was explored by 2 studies. Both studies reported that oral health-related quality of life was significantly associated with the length of living in the host country and high social acculturation [35,56].

Risk of bias in the included studies
Overall, the studies included in this systematic review attained medium-high methodological quality, according to the grading method used [25]. Table 2 presents the quality assessment of included paper.

Discussion
The association between acculturation and health, in general, and oral health (OH), in particular, has received increased attention in the past decade because of growing migration worldwide. Therefore, this paper is considered as an extention of the previous systematic review as we systematically reviewed the existing reports on the impact of acculturation and its attributes on OH outcomes of immigrants and ethnic minorities [21]. overall, acculturation has been proven to positively influence dental services utilization and OH behaviors of migrants such as brushing frequency and increased flossing. Acculturation was also associated with immigrant and ethnic minoritys' improved OH status, improved OH knowledge, and reduced orofacial pain.
The panel for updating guidance for systematic reviews (PUGs) consists of "review authors, editors, statisticians, information specialists, related methodologists, and guideline developers met to develop guidance for people considering updating systematic reviews" [68]. According to the (PUGs) an update of a systematic review is defined as: ". . .a new edition of a published systematic review with changes that can include new data, new methods, or new analyses to the previous edition" [68]. A similar systematic review was conducted by Gao and McGrath in 2010 [21]. Although the evidence reported by the this review was relatively comprehensive at that time, much more knowledge has been added to the literature since then. The increase in the number of papers in this review proves a greater attention of researchers to this topic including the application of acculturation scales that were not used by the older studies included in the previous review [21] like the East Asian Acculturation Measure [60], Berry's bi-directional model framework; general ethnicity questionnaire [62], Suinn-Lew Asian Self Identity acculturation scale [48], Acculturation Rating Scale for Mexican Americans (ARSMA)-II [64], and the Acculturation Scale for Hispanics [56]. Furthermore, in the present review, the New Castle Ottawa (NOS) checklist was used for quality assessment of nonrandomized studies. This tool has a valid content and inter-rater reliability [25]. The quality assessment tool used in the previous review was non-validated and developed based on guidelines proposed by previous authors [21]. In addition, long time has elapsed since the previous review search ended in January 2010.
In this paper, we have systematically reviewed the existing reports on the impact of acculturation and its attributes on OH outcomes of immigrant and ethnic minority populations and found that, overall, acculturation has been proven to positively influence dental services utilization and OH behaviors such as brushing frequency and increased flossing. Acculturation       was also associated with immigrants' improved OH status, improved OH knowledge, and reduced orofacial pain. Thirty two of the included studies conducted a multivariate analysis to explore the association between different variables with adjusting some socioeconomic and demographic factors. While interesting, none of the included studies examined the relationships between the acculturation indicators themselves like the possible relationship between time since immigration and language proficiency; however, age was reported to have the most significant effect on dental care utilization in one study [69]. The potential for collinearity between age and age at immigration and between age and length of stay in the United States was also examined in another study and no correlation was found between years in the United States and age at immigration [4].
The papers reviewed in this study used different measures of acculturation. Acculturation proxies such as length of living in the host country, age at migration, language proficiency, or country of origin were used by 25 studies (60%), certain scales were solely used in 12 studies (28%), and certain scales combined with proxy measures were used in 5 studies (12%). The main scales used included the Psychological-Behavioral Acculturation (P-BAS), [61,69]Acculturation Rating Scale for Mexican Americans (ARSMA) [57], or (ARSMA-II) [64], which consisted some questions about language use and preference, ethnic and cultural identity, ethnic interaction, and values. However, the overreliance on acculturation proxies used by the majority of the studies has caused inconsistencies among their findings mainly because these proxies A study can be awarded one star " � " or a maximum of two stars " �� " (representing "yes") for each numbered item within the selection, comparability, and outcome categories. https://doi.org/10.1371/journal.pone.0212891.t002 Impact of acculturation on oral health among immigrants and ethnic minorities are unidimensional in nature and therefore reveal only one direction of findings [70,71]. In other words, unidimensional proxies are unable to explain the extent to which immigrants retain their own culture or adapt to their host culture [71][72][73]. Furthermore, proxy measures give only a snapshot of immigrants' cultural changes rather than presenting acculturation as a process [70][71][72], and they do not consider the psychological domain of acculturation [71,74]. Therefore, the use of proxies instead of validated measures across most of the reviewed papers led to methodological heterogeneity that precluded us from conducting a metaanalysis.
Host language proficiency, one of the acculturation proxies used by 19 reviewed papers, was found to be significantly associated with improved OH knowledge [20,29], oral hygiene practices [40], dental attendance [30,42,45,51,52,55], preventive services utilization [30], and OH outcomes such as dental caries and periodontal disease [31,49,55]. Similar to our findings, limited English language proficiency was associated with lower use of necessary mental health care services and general health care utilization [75,76]. These findings reveal that language proficiency is one of the most influential behavioral acculturation indicators. It is possible that individuals who speak the host country's local language gain more confidence which allows them to socialize with native people. In turn, this may lead to immigrants' increased awareness of OH knowledge and available services. Therefore, providing culturally appropriate services are crucial for culturally and linguistically diverse immigrants to overcome certain barriers [77]. For example, cultural competency training for health-care providers and presence of an interpreter has significantly increased health and dental care utilization, improved patients' outcomes by facilitating communication and providing better understanding [77][78][79][80][81][82][83].
Length of residency in the host country is one of the most important contributing factors of dental care utilization [36,38,42,48,51,54]. Likewise, immigrants who had stayed longer in USA and Canada demonstrated better access to health care and increased service utilization [75]. The association between length of stay and OH was not only limited to OH outcomes, but also positively correlated with OH-related quality of life [35]. The longer immigrants had stayed in the host country, the more likely they had become aware of the health care system and ways to overcome structural barriers to health care such as language, social, or cultural differences [4,49].
Immigrants' country of origin was another indicator of acculturation used by a number of studies in our systematic review [4,53]. Country of origin has been correlated with other aspects of health as well. For instance, a higher prevalence of hypertension was reported among immigrants from Puerto Rico and Dominican Republic compared to Mexican-Americans [84]. Country of origin may reflect immigrants' cultural background and their attachment to specific beliefs, attitudes, and practices. For example, some cultures have specific diets consisting of high fibre and low refined carbohydrates, or they have defined oral hygiene practices or well-established use of preventive measures such as fluoride [4]. On the other hand, some immigrants are more susceptible to OH problems due to inadequate access to dental care and insurance coverage in their country of origin [4]. In addition, some studies reported that country of origin is one of the most important acculturation measures as it acts as a baseline of immigrants 'cultural, historical and geographical characteristics that will consequently affect their acculturation level [70,85,86]. Biological differences such as genetics, tooth morphology, and oral microflora may also affect the vulnerability of immigrants to OH problems [4]. Moreover, children who were born outside of the United States showed a lower rate of dental visits and higher rate of dental caries compared to their U.S.-born counterparts [50,52,55]. Perhaps immigrants' children born in the host country had better coverage and access to dental care services including school-based programs than their foreign-born counterparts.
In the reviewed studies, age at migration was identified as another acculturation proxy measure affecting immigrants and ethnic minorities' OH. Those of a younger age at immigration had more advantages than their older counterparts [4,33,41,50,53,55]. Migration during old age may be associated with late adaptation to the host country's services, including the health care system, or perhaps immigrants may not prioritize OH problems over other resettlement issues [4,53]. Also, preventive dental programs in the host country are usually offered through school programs that are more likely to benefit younger immigrants [41].
Some aspects of OH known to be culturally relevant, like orofacial pain, have been underreported in the acculturation literature [43] while the association between general pain and acculturation has been widely documented [87][88][89][90][91][92][93][94][95][96]. For example, chronic pain was more prevalent among low acculturated South Asians in UK [87]. High pain intensity was reported by low acculturated Chinese Americans in one study [91]; however, in other studies [92,93] high pain intensity was reported by high acculturated Chinese and Latino American immigrants. Three studies did not find any associations between chronic pain and level of acculturation [88,[94][95][96]. These inconsistencies may be caused by different study designs, sample characteristics, and interpersonal, cultural, and psychological differences among participants [92]. Moreover, the perception of pain may differ from one culture to another, which could in turn affect the degree of reported pain by immigrants [93]. Acculturation proxies such as language proficiency may also limit immigrants' ability to understand and respond properly to the questions asked by a health professional. Consequently, this language barrier may lead to inconsistent answers [93].
The quality of the reviewed studies ranged between medium and high. For example, a study conducted among Portuguese-speaking immigrants' children to assess their caries experience and dental care utilization was attained medium quality due to some reasons such as, unjustified sample size, no description of the response rate or the characteristics of the responders and the non-responders, and no description of the measurement tool [53]. On the other hand, another study investigated the impact of acculturation on Haitian immigrants' oral health was considered as a high quality as the sample size was justified and satisfactory, a validated measurement acculturation tool was utilized, the study controls for different factors, the statistical test was clearly described and appropriate, and the measurement of the association is presented, including confidence intervals and the probability level (p value) [6].
This study has some limitations that need to be acknowledged. In this review, we included studies that involved immigrant and ethnic groups since it was not possible to distinguish between ethnicity and immigration history (being a newcomer), and we had to rely on the definitions and main categories applied in the included studies [97]. In addition, the diversity among the immigrants at the individual and social level, such as their reasons for immigration, origin and host countries, timing of migration within a political, social environment, and individual life stage makes it hard to lump them together for analysis in one systematic review. Moreover, the restriction to English language among the included studies limited our findings mainly to North America which may penalize information on migratory movements in South America, Africa, Middle East, and Asia. The inconsistency in determining acculturation level and given the quality and quantity of the bibliographic sources identified as a result of the review, does not favor the recommendation of a meta-analysis. While acculturation is an ongoing process of adaptation, most studies included in this review used a cross-sectional design that is unable to show the causal nature of the observed relationships over time. Although the Newcastle-Ottawa Scale was used for quality appraisal of the included studies, there was no validated methodological assessment tool designed specifically for observational studies.

Conclusion
According to existing evidence, a positive effect of acculturation on OH status and behaviors was found. High acculturated immigrants and etnhic minorities with a longer time of residency in the host country, local language proficiency, and younger age at migration had better OH status and behaviors than their counterparts. Therefore, dental practitioners should be sensitive to cultural differences when providing services to immigrant and ethnic minority groups. Policymakers should also be mindful of cultural barriers and adequately address the unique needs of these individuals to maintain OH equity. Further qualitative and longitudinal studies are needed to better understand acculturation influence on OH. Using validated multidimensional scales instead of acculturation proxies will generate more comprehensive and comparable data. Finally, greater attention should be given to understudied aspects of OH and its association with acculturation.
Supporting information S1