Introduction
The world is undergoing a transformation of unprecedented scale in a century, with many countries experiencing a significant acceleration in population aging, marking a rapid global demographic shift (1, 2). The rapid increase in older adults brings forth a suite of health-related issues, among which physical frailty stands as one of the most critical concerns (3, 4). Frailty, once considered a complex clinical entity related to aging, has now transformed into a multifaceted bio-psycho-social syndrome (5, 6). In clinical terms, frailty is perceived as a state of heightened vulnerability, marked by a diminution in the functionality of various bodily systems and a depletion of physiological reserves, thus impacting the capacity to manage everyday and acute stressors effectively (7, 8). In the past decade, as frailty has continued to evolve, it has garnered increasing attention (9). Currently, frailty is recognized as a significant public health issue, imposing a substantial burden of disease on the population, particularly among older adults (10, 11). In a study covering 62 countries worldwide, an estimated prevalence of frailty of up to 12% was reported (11). Furthermore, frailty can lead to a cascade of adverse health outcomes such as cardiovascular disease, falls, hospitalization, disability, and death, causing harm to both mental and physical well-being (12–15). Additionally, Ekram and colleagues explored the association between frailty and all-cause mortality in community older adults (16). In light of the aforementioned research findings, there is an urgent need in the future to prioritize the occurrence of frailty in older adults in order to protect and enhance the quality of life for older adults.
Oral health is a critical determinant of overall health and shares a complex, multifaceted relationship with health, especially for older adults, where it is closely linked to physiological health, happiness, life quality, and self-perception (17, 18). However, as age advances, the burden of oral diseases increases, often leading to poor oral health in older adults, which is connected with various conditions encompassing different aspects of frailty (19, 20). For instance, from a physiological perspective, a study conducted by Xu et al. among the older Chinese population suggests that tooth loss might be closely associated with the occurrence of frailty due to its impact on dietary intake (21). Additionally, Hayashi et al. found a positive correlation between having fewer than 20 natural teeth and the incidence of frailty in a Japanese older female population (22). More importantly, on the psychological front, poor oral health may affect older people’s socialization, the incidence of depression as well as, among other things, mental health and lead to physical frailty (14, 23, 24).
Given the above research background, there is a growing interest in the connection between oral health status and frailty among older adults (25). For instance, a study by Dros in the northern Netherlands found a significant correlation between the deterioration of oral health and frailty in older adults over the age of 55 (26). Kang’s research arrived at similar conclusions, indicating that chewing difficulties were significantly associated with frailty in older adults (27). To date, although several meta-analyses on the relationship between oral health status and frailty in older adults have been reported, they all have certain research gaps. For example, Sakai’s study reported limited oral function indicators that included both qualitative and quantitative data and did not focus on a comprehensive analysis of frailty (28). In Kojima’s study, on one hand, only the relationship between chewing function and frailty was analyzed, and on the other hand, it included only self-reported subjective evaluation data (29).
Therefore, the present study aims to encompass a comprehensive set of objective oral health indicators to analyze the association between oral health and frailty in older adults.
Method
The protocol for literature retrieval in this study was thoroughly crafted based on the guidelines set forth by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) in the year 2020 (30).
Search strategy
For this study, a search was conducted in English electronic databases for relevant literature published up to July 1, 2024, focusing primarily on four databases: PubMed, Embase, Cochrane Library, and Web of Science. The main English search terms utilized in this study were as follows: dental, oral, tooth, teeth, mouth, Carious Lesions, Carious Lesion, Lesion, Carious, Lesions, Carious, Carious Dentin, Carious Dentins, Dentin, Carious, Dentins, Carious, Root Caries, Caries, Root, Caries, Cervical, Cary, Cervical, Cervical Cary, Cervical Caries, Periodontal Diseases, Disease, Periodontal, Diseases, Periodontal, Periodontal Disease, Parodontosis, Parodontoses, Pyorrhea Alveolaris, Periodontitis, Periodontitides, Pericementitis, Pericementitides, Gingivitis, Gingivitides, Xerostomia, Xerostomias, Asialia, Asialias, Hyposalivation, Hyposalivations, Dentures, Denture, Toothbrushing, Toothbrushings, Dentifrices, Dentifrice, Mouthwashes AND Frailty, Frailties, Frailness, Frailty Syndrome, Frailty, Debilities. In addition, in the supporting materials, this study also provides a specific search formula for PubMed (Supplementary Table S1).
Inclusion and exclusion criteria
The literature screening component of this paper was conducted independently by the authors based on the following inclusion and exclusion criteria. The inclusion criteria established for this study were founded on the Population, Intervention, Comparison, Outcome (PICO) principle, to more accurately identify articles pertinent to this research. Inclusion criteria: (1) Study population: Older adults as defined by the original studies; (2) Four types of oral health indicators: I: Dental-related; II: Oral hygiene care; III: Oral function; IV: Comprehensive oral health score. Specific oral health status indicators and their definitions can be found in Supplementary Table S2. (3) Criteria for the assessment of frailty: I: Cardiovascular Health Study (CHS); II: Japanese version of the CHS (J-CHS); III: Fatigue, Resistance, Ambulation, Illnesses, and Loss of weight (FRAIL) Scale; IV: Frailty Index (FI); V: Kihon Checklist (KCL); VI: Groningen Frailty Indicator (GFI). (4) Type of study: Cross-sectional study, and prospective cohort study. Detailed scoring or definitions of the above six methods of evaluating debilitation are shown in Supplementary Table S3. Exclusion criteria: (1) Experimental animal studies; (2) Reviews, meta-analyses, case reports, conference abstracts, editorial materials, trial registry records, and guidelines; (3) Non-English language literature; (4) Studies with incompatible themes.
Data extraction
Regarding the data collection process, after sifting through and selecting all the eligible papers, researchers meticulously reviewed the full text of each publication and independently and diligently gathered various data indicators. Below was the data collected from each literature source: Publication year, author, study type, sample size, gender ratio, average age of the study population, methods of frailty assessment, prevalence of frailty, and oral health-related indicators.
Approaches of literature quality assessment
In the present investigation, different scales were employed to assess the quality of literature for different study types. For cross-sectional studies, the assessment was conducted using the Joanna Briggs Institute (JBI) critical appraisal tool, which encompasses a spectrum of 8 criteria, each offering a binary “Affirmative,” “Negative,” or “Ambiguous” response (31). In the instance of cohort studies, the Newcastle-Ottawa Scale (NOS) was applied, which assigns a cumulative score out of 9, delineating study quality into low (0–3 points), fair (4–6 points), and high (7–9 points) tiers (32). The NOS scale consists of three main entries: “Choice,” “Comparability,” and “Outcome,” and each study can have up to one “*” for each of the entries for “Choice” and “Outcome,” and up to two entries for “Comparability.” Each study can have up to one “*” per entry for “choice” and “outcome” and up to two “*” for “comparability.” “*” for “Comparability” and up to two “*” for “Comparability.” Specific entries for both scales are provided in Supplementary Tables S4, S5.
Statistical analysis
Continuous data were analyzed employing the standardized mean difference (SMD) as the index of statistical measure, and categorical data were subjected to the odds ratio (OR) as the measure of association. Effect sizes were presented with 95% confidence intervals (CIs). Assessment for heterogeneity was conducted for every outcome metric, utilizing the I2 statistic; where the I2 value exceeded the threshold of 50%, analyses were performed using a random-effects model, and conversely, a fixed-effects model was engaged. Furthermore, stratified analyses were executed according to the methodologies of frailty assessment and the regions from which the studies were derived. Sensitivity analyses were applied across all endpoints, and in instances where the corpus of included literature surpassed a count of 10 publications, an examination for publication bias was instituted. The suite of analytical processes was performed utilizing Stata 15.1 software, with statistical significance ascertained at the p < 0.05.
Discussion
Frailty is highly prevalent among older adults, leading to adverse outcomes, affecting the quality of life for older adults, and posing challenges to the sustainability of the healthcare system (10). Besides, older people often have more oral health issues compared to younger people, which can cause harm to both physical and mental well-being (19, 20). Therefore, based on four types of oral health indicators, this paper comprehensively explores the relationship between oral health and frailty in older adults, with the following key findings: (1) Dental-related indicators: Number of teeth, functional dentition (≥21 teeth), no false teeth and ≤20 teeth/with a denture were associated with frailty; (2) Oral hygiene care: Lower brushing frequency was related to frailty; (3) Oral function: The relationship between tongue pressure/decreased tongue pressure, occlusal force/occlusal force reduced ODK hypofunction, and poor mixing ability were associated with frailty, while no significant statistical association has been found between oral moisture, oral dryness, and masticatory function with frailty; (4) Comprehensive oral health score: OHAT scores ≥4 was correlated with frailty.
Older adults who have experienced tooth loss might frequently be at risk of frailty due to inadequate nutritional intake (61). Additionally, the loss of teeth can lead to social barriers, causing psychological distress that increases the likelihood of developing depression (62, 63). This, in turn, causes older adults to resist social interactions, thereby reducing their level of physical activity, which can ultimately result in frailty (64, 65). The results of the present study on oral dental health indicators support the above conclusions. Consistent with Zhang et al.’s study on the association between the number of teeth and frailty in older adults’ care homes, our research found that older adults in the frail group had fewer teeth than those in the non-frail group (66). Moreover, subgroup analyses conducted based on frailty assessment methods or study regions both suggest that the number of teeth was conceivably associated with frailty. The term “functional dentition” refers to the minimum number of natural teeth required to provide adequate oral function, albeit not under ideal conditions (67). In this investigation, a lower functional dentition in older adults was likely to be associated with an increased risk of frailty, possibly due to the compromised nutritional value of the diet that results from a reduced functional dentition. When exploring the relationship between wearing dentures and frailty, the present investigation, in line with the research by Zhang and colleagues, indicated that older adults without dentures were potentially more susceptible to progressing toward frailty (59). This suggests that properly wearing dentures could potentially help older adults prevent frailty. The above outcomes could be attributed to the fact that the use of dentures in older adults can improve masticatory function, leading to a more varied diet and enhanced nutritional intake, as well as addressing speech difficulties and boosting confidence, which fosters social engagement (68, 69). Finally, this investigation also perceived that ≤20 teeth, with dentures may be associated with the onset of frailty. The aforementioned findings suggested that even with the presence of prosthetic teeth, a lower count of natural teeth can still predispose an individual to frailty, which is consistent with the research outcomes of Kimble et al. (44) and Zhang et al. (60). Therefore, when examining the relationship between dental health and frailty, it is not sufficient to draw conclusions based solely on one or two indicators. Instead, a comprehensive exploration of the association between dental health and frailty, incorporating multiple indicators, is required. The above approach can help establish effective frailty prevention strategies for older adults from the perspective of protecting dental health.
In the realm of oral hygiene care, the research revealed that older adults with a lower frequency of daily tooth brushing were perhaps likely to be at a higher risk of developing frailty, which was in line with the findings of Zhang: Older adults who brushed their teeth regularly were less likely to be frail compared to those who do not brush their teeth, demonstrating that regular tooth brushing may help reduce the risk of frailty (66). However, due to the lack of comprehensive information in this survey, more indicators related to oral hygiene care were not included. Therefore, future research can explore the association between oral health care and frailty further, with the aim of proposing additional preventive measures from a public health perspective to safeguard the physical and mental well-being of older adults.
Almost seven categories of oral function indicators were used to analyze the relationship between oral health and frailty in this investigation. Contrasting with Sakai’s research but aligning with the study by Tanaka et al., our study found that in older adults, reduced tongue pressure/low tongue pressure may be related to frailty (28, 70). This could be due to the fact that decreased tongue pressure is believed to lead to a decline in intake and swallowing functions, resulting in malnutrition and inadequate nutritional intake, thereby affecting the onset of frailty in older adults (48). However, there are evident conflicts in the conclusions of the aforementioned studies, and future research necessitates larger-scale longitudinal studies to substantiate these findings. Next, occlusal force/reduced occlusal force was also revealed to be possibly associated with frailty in this survey, which is consistent with Nakamura’s research (46). Anterior tongue movements reflected in ODK hypofunction are oral functions that play a key role in effective eating and maintaining adequate nutrient intake (71). In our research, ODK hypofunction was shown to possibly correlate with frailty, a finding that was consistent with the research conducted by Tani et al. However, Satake’s research did not corroborate this association (48, 72). Variations in study populations and sample sizes may cause differences. Once more, a relationship between poor mixing ability and frailty was observed, which was consistent with Horibe’s study, revealing that mixing ability was associated with frailty progression (73). Consequently, regarding oral moisture, oral dryness, and masticatory function, a statistical association with frailty was not demonstrated. However, Dibello’s research indicated that these three indicators possessed a certain correlation with frailty (17). We speculate that the reason for the discrepancy might be due to the different frailty assessment methods, different definitions of the study populations in the involved literature, and the lack of stratified analysis. Hence, more extensive and comprehensive research is needed in the future to explore the specific causal relationship between the aforementioned indicators and frailty.
Ultimately, OHAT was employed in the study, a structured and effective method for oral health assessment that has been validated as suitable for non-dental healthcare professionals to evaluate the oral health of older adults (74). The tool was utilized to reflect the overall state of oral health and explore its relationship with frailty in older adults. Rapp’s study has indicated a significant correlation between OHAT scores and frailty, and a similar finding was reported by Kuo et al. (75) and Rapp et al. (76). Their research among older adults in rural China, suggested that OHAT scores can predict frailty. Both of these research conclusions were likewise the findings of the current survey, which also demonstrated that an OHAT score ≥ 4 was possibly associated with frailty. Nonetheless, aside from the OHAT, there are several other oral health evaluation tools capable of providing a comprehensive reflection of the oral health status of older adults, such as the Brief Oral Health Status Examination (BOHSE) and the Oral Assessment Guide (OAG) (77, 78). Different assessment tools perhaps yield varying evaluation outcomes. This study only explored the OHAT, which presents certain limitations, and future researchers might consider focusing on this aspect.
In the preliminary survey, the articles included in this study exhibited substantial variation in terms of geographic region and frailty assessment methods, which may suggest potential confounding factors. To ensure the robustness of the study results, we conducted subgroup analyses for different outcomes based on region and assessment methods. The findings revealed that the results of most subgroups were consistent with the overall results, further demonstrating the stability of the study. Moreover, the factors influencing the occurrence of frailty in older patients are diverse. For instance, age, body weight, and educational level are all associated with frailty in older patients (79). Additionally, the study by Liang et al. indicated that malnutrition was closely related to frailty in patients aged 65 years or older (80). The overall health status or underlying medical conditions of older adults are also potential risk factors for frailty (81, 82). As mentioned above, there are confounding factors that may affect the relationship between oral health and frailty in the older adult. However, due to limitations in the included primary studies, we were unable to extract additional relevant data for meta-analysis. Taken together, these observations highlight the need for future research that can better control for confounding factors, such as prospective cohort studies or randomized controlled trials, to investigate the impact of various factors on frailty.
Within the context of existing research, to gain a more comprehensive and objective understanding of the relationship between oral health and frailty in older adults to a certain extent, this study endeavored to search for and include a greater number of objectively measured oral health indicators to explore their connection with frailty. We hope that this study will contribute to urging older adults to take oral diseases seriously to prevent frailty, encourage clinicians to incorporate more oral health-related indicators in future medical check-ups, and achieve sustainability in the healthcare system. This study has certain strengths: Firstly, it utilizes objective oral health indicators. Secondly, despite the issue of high heterogeneity among studies, subgroup analyses were conducted to trace the sources in all high-heterogeneity analyses. However, like other studies, our analysis also has some limitations. Firstly, the included studies are mostly cross-sectional, thus precluding the inference of causal relationships between oral health status and frailty. Secondly, older adults often have poor physical conditions, and the present study did not include the medical history of the analyzed patients, which could affect frailty. Thirdly, due to limitations in the number of studies for different outcomes, publication bias analysis was not performed in this study. Therefore, caution should be exercised when extrapolating the study conclusions. Further in-depth research is needed to validate our findings. Lastly, the included studies defined older adults differently, so caution should be exercised when extrapolating the results. In summary, large-scale, high-quality longitudinal studies are needed in the future to establish the causal relationship between oral health and frailty, thereby providing scientific support for geriatric care in clinical practice.