Older adults with more natural teeth are better able to perform everyday tasks such as cooking a meal, making a telephone call or going shopping, according to researchers from UCL and the Tokyo Medical and Dental University.
The study published in the Journal of American Geriatrics Society, analyzed data from 5,631 adults from the English Longitudinal Study of Aging (ELSA) aged between 50 and 70.
Previous studies have shown the link between tooth loss and reduced functional capacity but did not establish a causal link. In this study the research team wanted to investigate the causal effect of tooth loss on someone’s ability to carry out daily activities.
After considering factors such as participants’ socioeconomic status and poor general health, they still found there was an independent link between tooth loss and the ability to carry out everyday tasks.
For the study, participants were asked how many natural teeth they had, with older adults usually having up to 32 natural teeth that are lost over time. Then, using data collected in 2014-2015, the researchers measured the effect of tooth loss on people’s ability to carry out key instrumental activities of daily living (IADL).
The activities included preparing a hot meal, shopping for groceries, making telephone calls, taking medications, doing work around the house or garden, or managing money.
Senior author, Professor Georgios Tsakos (UCL Institute of Epidemiology and Health), explained: “We know from previous studies that tooth loss is associated with reduced functional capacity, but this study is the first to provide evidence about the causal effect of tooth loss on the instrumental activities of daily living (IADL) among older adults in England. And this effect is considerable.
“For example, older adults with 10 natural teeth are 30% more likely to have difficulties with key activities of daily living such as shopping for groceries or working around the house or garden compared to those with 20 natural teeth.
“Even after taking in factors such as participant’s education qualification, self-rated health and their parent’s education level for example, we still found a positive association between the number of natural teeth a person had and their functional ability.”
The team of researchers note that having more natural teeth is associated with delaying the onset of disability and death and that tooth loss can also hamper social interactions, which is linked to poorer quality of life. They also suggest tooth loss could be linked to having a poorer diet with less nutrients.
The researchers say the results must be interpreted with caution because of the complex design and further studies are needed to investigate the casual relationship between tooth loss and functional ability.
First author, Dr. Yusuke Matsuyama (Tokyo Medical and Dental University) said: “Preventing tooth loss is important for maintaining functional capacity among older adults in England. Given the high prevalence of tooth loss, this effect is considerable and maintaining good oral health throughout the life course could be one strategy to prevent or delay loss of functional competence.
“The health gain from retaining natural teeth may not be limited to oral health outcomes but have wider relevance for promoting functional capacity and improving overall quality of life.”
An optimal cognition status is essential to lead a quality of life (QoL) in young and old people. In 2010, 35.6 million people worldwide were estimated living with dementia and reach of 115.4 million people by 2050, is expected (Marchesi, 2011). Changes in cognition occur with normal aging (Harada et al., 2013), but they can be maximized by intrinsic and extrinsic risk factors (Marchesi, 2011), thus triggering severe cognition impairment that affect social and occupational tasks (Hugo and Ganguli, 2014).
Cognitive changes are mostly related to declines of learning, memory, thinking, reasoning, and judgment abilities, such cases of dementia, among others neurocognitive disorders (Murman, 2015).
Influenceable external risk factors identified to the onset of neurocognitive disorders include genetic factors, hypertension, diabetes mellitus, hyperlipidemia, vascular disease, demographic, and lifestyle factors—such as smoking and alcohol use—and oxidative stress (Marchesi, 2011).
On the other side, factors such as education, physical exercise, and active social engagement help to maintain cognition (Campbell et al., 2013). However, all the factors related to cognitive changes are still unknown and subject to study.
In this scenario, optimal mastication has been appeared as a factor to preserve cognitive functions, in addition to its well-known important role in food intake and swallowing (Teixeira et al., 2014).
Masticatory dysfunction (MD), as a general term, refers to an impairment in the masticatory function triggered either by a structural factor, such as tooth loss; or by functional factors such as weaker bite force or a poorer masticatory performance (Ikebe et al., 2012; Lin, 2018), for example as in advance loss of periodontal support and tooth movement (Kosaka et al., 2014).
Maximum bite force and masticatory ability scores are lower in older subjects with <20 remaining teeth (Tatematsu et al., 2004), suggesting a directly proportional association between tooth loss and inappropriate mastication (Ikebe et al., 2012).
MD such as a result from structural or functional factors, acting as a source of chronic stress, triggers functional and morphological changes on the hippocampus—a brain area crucial for learning and memory abilities (Azuma et al., 2017).
These cognitive changes are promoted through the activation of the hypothalamic-pituitary-adrenal (HPA) axis and its glucocorticoid production (Azuma et al., 2017), similar to those changes induced by long term exposure to excessive corticosterone (Kubo et al., 2010).
In this regard, there are several studies around this association with different variables and outcome measures which become complicated the interpretation of the results in many systematic reviews made before (Tada and Miura, 2017).
Thus, in order to reduce heterogeneity, the aim of this study was synthesizing only those studies that made a comparison between subjects with presence/absence of tooth loss as a structural factor triggering MD and its association with CD; and also demonstrate, through a quantitative analysis, whether MD might be adequately considered as a risk factor for CD, measuring its impact.
Discussion
This systematic review and meta-analysis aimed to synthetize only those studies comparing the cognition performance between subjects having ≥20 remaining teeth (mild or no tooth loss) and <20 remaining teeth (severe tooth loss), considering the latter as a structural factor that promote MD. In total, 14 studies with a low risk of bias were systematically selected, in which nine were included in the meta-analysis.
The quantitative analysis of findings performed by our MA shows that individuals with inappropriate mastication have a greater chance of presenting a lower cognitive performance, suggesting that MD might be considered as a risk factor of CD.
Frequently, in order to synthesize the highest level of evidence existing in literature, rigorously systematic reviews, and meta-analysis are conducted in healthcare research. They are made following a well-designed methodological protocol, thus providing a precise estimate of the effect of risk factors for diseases. The estimate is obtained measuring the heterogeneity of the studies through statistical analyses. Therefore, this sort of reviews can provide the most trustworthy evidence for clinical decisions (Haidich, 2010).
In this context, we conducted this SR based on a search strategy previously designed and seven leading databases in medical research were accessed. Our literature search strategy and criteria for the inclusion/exclusion of studies were guided with the PECO question.
According to our inclusion criterion, the final selection to evaluate the association of MD with the risk of CD, included only those studies in which was possible to extract the data concerning the comparison of the effect in cognition between subjects having ≥20 remaining teeth (mild or no tooth loss) and <20 remaining teeth (severe tooth loss) (Table 1), being the latter a structural factor that promote MD (Zhang et al., 2019).
MD, in general, refers to an impairment in the masticatory function due to structural factors, such as tooth loss, or functional factors such as weaker bite force or a poorer masticatory performance (Ikebe et al., 2012; Lin, 2018). Several studies have reported a decrease of masticatory function with increasing degree of tooth loss (Boretti et al., 1995; Savoca et al., 2010; Lexomboon et al., 2012).
Tooth loss and also inappropriate vertical dimensions of crowns, bridges, or dentures can induce MD in humans (Miura et al., 1997). Studies that evaluated tooth loss and its impact in QoL have demonstrated that subjects having fewer than 20 teeth get more impact in QoL than those having 21–32 (Petersen et al., 2005; Tsakos et al., 2006; Akpata et al., 2011). Moreover, limited food choices due to dental function were significantly associated with having fewer remaining teeth (<20 remaining teeth) (Wang et al., 2014).
Although there are also functional factors regarding MD as occlusal disharmony, bite force, soft-diet feeding, and salivary (Azuma et al., 2017), literature supports that multiple tooth loss (<20 remaining teeth) by itself promotes an inappropriate chewing function (Tatematsu et al., 2004; Savoca et al., 2010; Kosaka et al., 2014).
Maximum bite force and masticatory ability score in a study in patients having <20 remaining teeth, were lower; there was a strong positive correlation between the number of remaining teeth and the occlusal supporting score in this population (Tatematsu et al., 2004). In addition, other study in which occlusal force was significantly associated with masticatory performance, the number of residual teeth was significantly associated with masticatory performance in subjects having contact in four support zones and those having one to three support zones in the anterior region only (Ikebe et al., 2012).
However, it has also been suggested that it is important not only the number of remaining teeth but also the location of missing teeth for masticatory function assessment (Zhang et al., 2019). An unfavorable chewing ability is more prevalent in those individuals with <10 natural teeth in each jaw than those having all-natural teeth or more than 10 teeth in each jaw and the presence of three to four premolars can still provide good mastication function (Zhang et al., 2019). After variables controlled, the masticatory function is significantly associated with posterior occlusal contact (Ikebe et al., 2012).
Despite the studies included in this review did not report the location of missing teeth, several studies assessing the incidence of tooth loss have demonstrated that the posterior occluding units are the most affected and the front teeth the least affected (Montandon et al., 2012; Minja et al., 2016); being premolars less affected than molars (Silva-Junior et al., 2017), and this may interfere in the prediction of MD. Thus, future studies should include the location of missing teeth in dental records, considering that the presence of three to four premolars can still provide good mastication function, as well as the presence of removable full or partial dentures (Zhang et al., 2019), fixed dentures and implants (Prithviraj et al., 2014; Neves et al., 2015).
Although, there are also functional factors which can trigger MD, we decided to use only the data about the structural factor, multiple tooth loss (<20 remaining teeth)—in which literature supports that by itself promote MD (Tatematsu et al., 2004; Savoca et al., 2010; Kosaka et al., 2014)—as a strategy to reduce heterogeneity in the measure of the chance of CD, due to there are different variables and outcome measures among the studies evaluating tooth loss and/or MD and cognition.
Mastication is essential not only for food intake and swallowing, it has been reported that optimal chewing is also important preserving and promoting general health, including cognitive functions (Azuma et al., 2017). A study described the association between the loss of molars in senile rats and spatial memory deficits. Animals without molar teeth showed worse performance in behavioral tests than control animals maintained on a solid diet.
Thus, the results of this experimental study suggested that the failure of spatial memory may be caused by inappropriate mastication and not only by tooth loss (Kato et al., 1997). MD associated with few residual molars, promote morphologic changes in the hippocampus and cerebral cortex, through increased blood flow in the bilateral lower frontal and parietal lobes during mastication activating various areas related with memory and learning ability (Kubo et al., 2010).
MD acting as a source of chronic stress activates the HPA axis. The main neuroendocrine response to stress is via activation of the HPA axis, which stimulates—as a final product—the synthesis, and secretion of glucocorticoid (GC) from the adrenal cortex. Appropriate activation of the HPA axis is critically important for stress adaptation but repeated or prolonged HPA axis hyperactivity is linked with cognition disorders (Azuma et al., 2017). This process is similar to that induced by long term exposure to excessive corticosterone (Kubo et al., 2010).
The hypothesis of oxidative stress involvement and mitochondrial dysfunction in the pathogenesis of cognitive impairment and other diseases has been reported in several studies (Kann and Kovács, 2007). Oxidative phosphorylation occurring in the mitochondria is a major source of energy and this process produces free radicals or reactive oxygen species (Pero et al., 1990), in which, moderate or low levels are considered essential for neuronal development and function, whereas high levels are perilous (Wang et al., 2014).
A community-based study in China, included in our review, found that the number of missing teeth and gene variations in the mitochondrial DNA D-loop region (including SNPs and haplogroups) were correlated with cognitive function in this population (Gao et al., 2016).
Nonetheless, one study reported no difference between subjects with many missing teeth compared to those with few missing teeth after adjusted cognition scores. The same study reported that subjects with periodontal inflammation (PI) obtained lower mean cognition scores compared to subjects without PI and the association of PI with cognitive scores was dependent on the number of missing teeth (Kamer et al., 2012).
Furthermore, other study reports an effect of periodontal status and occlusal support on masticatory performance (Kosaka et al., 2014). In this regard, periodontitis has been also associated with cognitive functions due to the inflammatory and immune pathways involving in the pathogenesis of periodontitis (Noble et al., 2014; Nascimento et al., 2019). Tooth loss and gingival bleeding were markers of poorer executive function among dentate people (Naorungroj et al., 2013).
In this regard, a study included in our selection found an association between severe tooth loss and vascular cognitive impairment, even including inflammatory markers as a covariate (Zhu et al., 2015). Although there exists the possibility of periodontitis interfering in the risk of CD, considering that tooth loss is also a consequence of severe periodontitis, many studies evaluating the prevalence of tooth loss reported the tooth decay as a main cause of tooth loss (60–70%) being consistent among the studies compared with other reasons (Minja et al., 2016); periodontal disease has been reported as a minor cause (around 15%) (Caldas et al., 2000).
In contrast, a study classifying subjects according to number of teeth (Group 1 <20 teeth vs. Group 2 > 20 teeth), reported a positive association between fewer remaining teeth and lower cognitive status as represented by MMSE scores <25; but a score 25 indicated normal cognitive function, suggesting that the elderly in both groups had normal cognitive function (Wang et al., 2014).
When confounding factors were controlled, lower MMSE scores were not associated with the subjects in Group 1 (<20 remaining teeth). It is unclear whether having total or partial dentures was controlled in its analysis, considering that subjects in this group had more partial dentures (Wang et al., 2014). Moreover, one study in elders living in rural Ecuador, reported that persons with <10 remaining teeth scored significantly worse in MoCA test when compared with those with ≥10 teeth (Del Brutto et al., 2014).
Cognition, as such, is about information processing performed by our brain and in this processing, the hippocampus plays an important role for cognitive functions, such as learning and memory abilities. When the hippocampus suffers some alterations, may trigger cognitive deficits, in which worst cases affect the ability to pay attention, process information and respond to information quickly, remember and recall information, think critically, plan, organize, solve problems and even inability to initiate speech (Trivedi, 2006). In order to detect cognitive impairments, some tests are widely used for measure cognition, such as MMSE and MoCA—30-point questionnaire mainly assessing memory abilities, executive capacity, attention, concentration, language functions, and orientation (Siqueira et al., 2018).
Despite the heterogeneity of the studies, the positive trends of association were consistent among all studies. Nine studies were included in our meta-analysis, and seven found an association between MD and CD, using the MMSE and MoCA. Well-known risk factors for cognitive deficits such as demographic and lifestyle characteristics, as well as some health conditions, were considered to avoid potential confounding factors in most of the studies included (Ishimiya et al., 2018). One study included dietary habits and using reduced rank regression analysis showed that tooth-loss related dietary pattern was associated with cognitive impairment. However, the diet of this study population cannot be generalized to other populations (Ishimiya et al., 2018).
Gil-Montoya et al. (2015), used Clinical Attachment Loss (CAL) in its correlation analysis—besides the other common variables—and the association between tooth loss and CD disappeared when all variables were adjusted. In addition, this study included CAL, because it also attempted to evaluate the association of periodontitis with CD, not tooth loss only, and as a result periodontitis actually appeared associated after controlling adjustments (Gil-Montoya et al., 2015).
Thus, our meta-analysis demonstrates that MD can be considered as a risk factor for CD. However, the level of evidence was rated as low by GRADE approach. First, due to the observational studies start as low-quality evidence to support interventions but also the amount of heterogeneity reported among the studies, mainly about the outcome measures of cognition. Therefore, according to GRADE parameters for grading, our evidence level regarding the estimated effect is not strong enough about direct effects of MD in cognition (Guyatt et al., 2011).
reference link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6618904/
More information: Yusuke Matsuyama et al, Causal Effect of Tooth Loss on Functional Capacity in Older Adults in England: A Natural Experiment, Journal of the American Geriatrics Society (2021). DOI: 10.1111/jgs.17021
