For this study, we determined the strength of the association (OR) of the oral health categories at baseline (t0) and development of frailty over a 12-month period (t1) in older adults. The bivariate analysis found that the strength of the association is greater in edentulous older adults and those with poor oral health. After adjustment for the other variables under study, the association between edentulism, poor oral health, and developing frailty in that period remained.
In total, 18.0% (n = 97) of the older adults included in this study developed frailty over a period of 12-months. This incidence rate was higher than those reported by Castrejón et al. and Ramsay et al. (14.8% and 10.0%, respectively) in older adults, in their respective three-year follow-up studies. Likewise, the incidence rate in our study surpassed that found by Iwasaki et al. (14.9%) in their five-year follow-up study in adults over age 75, as well as the overall incidence reported by Ofori-Asenso (13.6%) in the meta-analysis of 46 studies with a median follow-up of three years in populations aged 60 years and over [14, 18, 34, 35].
Comparing this study with one conducted by Castrejón et al., there is a difference in the development of frailty of 3.2% (14.8% cumulative incidence of frailty). They also used frailty phenotype, but unlike this work, they evaluated unintentional weight loss of 5 kg in the last 6 months, the slowness of gait, and grip strength, which is evaluated subjectively. Therefore, there could have been a reporting bias and probably an underestimation of the results due to the subjectivity of the aforementioned frailty criteria. In addition, the population of this study are residents of a district that is part of a Mexico City government program, so it is possible that they are under constant review and receive some type of care .
The difference in the development of frailty in this study compared with Ramsay et al. is 8 points. Although they also used the frailty phenotype, this difference may be due to the fact that they evaluated grip strength subjectively and slow walking speed was determined by self-reporting of usual running rate. In addition, unintentional weight loss was ascertained by self-reporting of weight loss in the past 4 years. Therefore, there may have been an information bias and possible underestimation because of the subjectivity of the assessment regarding the above-mentioned frailty criteria [18, 36, 37].
According to the LCA, a methodology to determine oral health typology, we considered three classes of oral health related to the studied parameters . Doing so, we observed a stronger association between older adults with poor oral health and development of frailty over a 12-month period. Consequently, the LCA can be considered a useful tool for determining typology in other studies that need to classify older adults according to their oral health status .
Oral health ailments begin at early ages; without proper care, they can evolve into major problems at advanced ages. For example, tooth loss, edentulism, loss of clinical insertion, coronal and root caries, use of non-functional dental prosthesis (partial or total dentures), and chewing problems, among other conditions .
Oral health is generally evaluated in terms of deficits and indices, which does not provide a complete measure of oral health, like the patterns in this study . Using latent class analysis for oral health conditions, we obtained a model of three classes in addition to the group of edentulous older adults. This is consistent with both Sánchez et al. and Ortiz et al., who also identified three classes in addition to edentulous participants, in a population covered by the social security system. It should be mentioned that, although the oral health deficits used to perform the latent class analysis were not the same among the aforementioned studies, similarities were observed so far as the highest percentage of the population was classified with favorable oral health [9, 10].
Regarding edentulism and dental loss, several reports [15, 16, 34] have identified tooth loss as a possible early indicator of frailty [39,40,41]. Dental loss, edentulism, and absent or inadequate dental prosthetic rehabilitations in older adults can affect several factors, such as nutrition [42,43,44,45], socialization, and quality of life [46, 47], which could probably lead to the development of frailty. Studies are needed to clarify the relationship between edentulism and tooth loss with the components of the frailty phenotype.
On the other hand, although a high probability of periodontal disease is observed among people with acceptable oral health, a high probability is not observed for other deficits. The probability of periodontal disease is higher among people who have more teeth; tooth loss cancels that probability. Also, edentulous people probably lost their teeth to periodontal disease, but this cannot be accurately determined. As for poor oral health condition, periodontal disease represents an important component.
Periodontal disease does not begin at an early age. It is an inflammatory process of periodontal tissues that is exacerbated when the disease is active. This can trigger higher levels of inflammatory markers, damage to tissue, or pathogenic bacteria can cause the gingival epithelium to initiate the inflammatory response in which epithelial cells release pro-inflammatory mediators such as macrophages, mast cells and polymorphonuclear cells, with secretion of interleukins IL-1 IL-6, IL-8 and tumor necrosis factor alpha (TNF-α) and histamine, which amplify inflammation and could contribute to the development of frailty [48, 49].
Among the limitations of the study is the uneven distribution of participants in the different classes. It would thus be necessary to increase the size of the sample or implement other designs in which the number of participants in each group remains fixed. Another limitation is the loss to follow-up of some participants (18.7%). The main reasons were not being located (48.4%), followed by not agreeing to be interviewed or examined (31.5%), which could have occurred because of illness or frailty. The literature has reported that the loss of participants during follow-up is associated with data missing not at random, so biased estimations may occur . For these reasons, we suspect that the incidence of frailty could have been underestimated. Furthermore, being a community of older adults with social security coverage, the results cannot be generalized to the entire population of that age group.
An additional limitation was the evaluation of periodontal disease, which was defined as presence of clinical attachment loss ≥ 4 mm in at least one tooth. This measurement of periodontal disease generally considers people with at least moderate severity . Therefore, a large number of individuals were classified as having the disease (88.2%) without the possibility of identifying those with severe periodontal disease, who may have had the highest risk of frailty because of the constant inflammatory process [5, 8]. Thus, the impact of each latent class could have modified the final result. It is necessary to continue studying the possible relationship between the inflammatory processes originated by periodontal disease and other diseases, in this case the development of frailty.
This study did not evaluate masticatory capacity with and without the use of prostheses. Therefore, other studies should consider it to determine if it has any implication in the development of frailty in older adults.
Hakeem et al.  and Tórres et al.  conducted literature reviews of studies with both cross-sectional and longitudinal designs, in which they report the association between clinically assessed oral health deficits and perception of oral health with frailty or any of its components in older adults. The number of teeth is among the main oral conditions associated with frailty, which is related to chewing problems and a need for rehabilitation with dental prostheses. This could limit food selection and processing, impacting nutritional status, which is one of the most recognized factors associated with the development of frailty. This could be a possible mechanism underlying the relationship between oral health and frailty . There is a need for further research that explores the role of nutrition as a mediator between oral health and frailty.
With respect to the strengths of the study, one is its case-cohort design, in which cases develop according to the same chronological sequence of the case–control design nested in a cohort. The difference between these two designs is that controls in the case-cohort design are selected from the cohort with which the study began . A longitudinal study can help establish a causal relationship between oral health and development of frailty over a short period of time. More years of follow-up would certainly show a greater incidence of frailty and the effect would remain significant in the fit analysis. Finally, using the frailty phenotype for the evaluation of the event of interest allows comparison with other studies. Yet, as far as we know, this is the first study to use a comprehensive approach to oral health and examine its relationship with the development of frailty.
Edentulism and poor oral health can be indicators of the presence or development of frailty, which in turn is considered as a potential public health problem in the older adult population [54, 55], given its association with adverse health outcomes such as falls, impaired mobility, functional dependence, disability, hospitalization, and institutionalization [5, 21].
Further research will be needed to confirm the association between oral health and the development of frailty in older adults, using longitudinal studies with other populations, such as those who are institutionalized or do not have social security. In this way, the causal relationships of highly preventable diseases can be explored.
Lastly, changes in operational definitions of oral health deficits may probably modify the distribution of groups, so it would be advisable to hypothesize using different operational definitions or different cutoff points to evaluate behavior when predicting frailty.