To the best of our knowledge, this is the first longitudinal study to examine the associations between swallowing function and dementia in Japanese elderly people using data from the NDB. The results showed that participants with dementia after two years of follow-up had a higher proportion of poor swallowing function at baseline than those without dementia. The results of logistic regression analysis showed that, after adjusting for gender, age, hypertension, regular dental checkups, brushing frequency ≥ twice/day, number of present teeth ≥ 20, decayed teeth, and tongue and lip function, the presence or absence of dementia after two years was associated with swallowing function at baseline. From these results, it was predicted that a decrease in swallowing function was associated with a higher risk of the onset of dementia in the future.
There are several possible mechanisms for the relationship between swallowing function and dementia. Swallowing actions activate brain function and improve learning and memory skills [15, 16]. It was also reported that swallowing actions increase cerebral blood flow and the partial pressure of oxygen in the brain [17]. Decreased cerebral blood flow is a known risk factor for dementia [18]. Thus, participants with swallowing difficulties might have a greater risk for dementia because of reduced cerebral blood flow and even ischemia. In addition, people with poor swallowing function tend to consume less fruit and vegetables and more high-energy foods than those with good swallowing function [19]. High-calorie diets rich in carbohydrates and saturated fatty acids tend to increase the risk of dementia [20]. Therefore, these may be also among the mechanisms. However, further research is needed to clarify the mechanisms by which poor swallowing function is associated with dementia.
In the present study, chewing function was not associated with dementia. A previous report showed an association between chewing function using the evaluation of chewing function was based on muscle activity of the masticatory muscles and dementia [21]. The results in the previous and present studies may have been different due to their different measurement methods. In the future, we would like to consider judging chewing function by methods such as assessing the distribution of crushed particles in chewing samples (e.g., gummy jellies), the amount of elution of contents in chewing samples, or the activity of the masticatory muscles.
In addition, a cross-sectional study reported an association between poor tongue and lip function and dementia using the same research methodology as the present study [22]. This finding differs from the present study. This may be related to differences in research methods (cross-sectional vs. longitudinal studies). It may also be related to the bias in the proportion of participants with poor tongue and lip function at baseline. In the previous study, most participants possessed good tongue and lip function [22], unlike the present study. Therefore, external validity should be considered in this study.
In the present study, lack of regular dental checkups and decayed teeth were associated with dementia. Past reports showed that recommendations for regular dental checkups reduced the risk of dementia onset [23]. Furthermore, it was reported that mutans bacteria that cause tooth decay adhere to the walls of blood vessels in the brain and reduce cognitive function [24]. These previous studies support the results of the present study. These studies reported that maintaining a good oral environment and controlling oral bacteria decrease the risk of dementia onset. In the present study, brushing frequency ≥ twice/day was associated with dementia. It is widely known that proper brushing habits are important for preventing decayed teeth and maintaining a good oral environment [25, 26]. Therefore, participants in the present study who brushed ≥ twice/day may have engaged in proper brushing habits, had a good oral environment, and may not have had decayed teeth, and thus had a lower risk of dementia onset. Therefore, brushing habits may indirectly contribute to delaying the onset of dementia via maintaining a good oral environment.
A major strength of the present study is its sample size of more than 7000 Japanese elderly people. In addition, it was a longitudinal study, which is useful for establishing a causal relationship between dementia and poor swallowing function and for inferring factors that contribute to pressure on social security costs in Japan. Furthermore, it was possible to gather study population data from multiple locations in Gifu, Japan (Gifu City, Kagamihara City, Kani City, and Ogaki City).
In the present study, the Hosmer-Lemeshow fit test was used in a multivariate logistic regression analysis model. The Hosmer-Lemeshow fit test is used to examine the fit of a multivariate logistic regression analysis model and tests whether the observed event rate in a subgroup model fits the expected event rate. The Hosmer-Lemeshow test is considered to show a good fit with p-values > 0.05 [27]. In the present study, the p-value was 0.288, suggesting a good fit.
However, there are several limitations to the present study. First, since participants of the present study visited to dental checkups, they may have been a highly health-conscious population. Second, the presence or absence of diseases not in the database is unknown, since the NDB was used.
In conclusion, the present study showed that Japanese elderly people with poor swallowing function have a higher risk for future dementia.