We don’t yet fully understand the causes of dementia, but scientists are making progress.
Even more encouraging, research suggests that nearly half of dementia cases could be reduced, at least temporarily, through modifiable risk factors that can help prevent or delay the onset of the disease.
But a new study reveals that we may have overlooked a serious risk factor for cognitive decline, and it’s hiding right under our noses.
According to a study carried out in the Journal of the American Association of Medical Directorspoor oral health could contribute much more to dementia than we ever imagined.
Although a number of studies have linked oral health to dementia, it was not included among the 14 modifiable risk factors in the influential study Lancet The Commission’s findings were updated in 2024, which helped inform new WHO guidelines published this year on reducing the risk of dementia.
This might be worth rethinking, based on new evidence discovered by Japanese scientists.
In the study, researchers analyzed data from 47,293 adults aged 65 or older who participated in the Japan Gerontological Assessment Study (JAGES), following participants for approximately nine years, from 2013 to 2022.
During this follow-up period, 8,247 (17.4%) of the participants developed dementia, and the researchers wanted to study the extent to which poor oral health – measured by the number of teeth – was linked to dementia.
compared to established risk factors listed by the Lancet Commission (excluding two which could not be assessed using JAGES data: high LDL cholesterol and air pollution).
“We used the number of remaining teeth as an indicator of poor oral health, because tooth loss usually reflects the endpoint of a lifelong history of oral diseases, such as advanced dental caries and severe periodontal disease,” the research team, including first author and medical student Maho Iwata of Tohoku University, explains in their paper.
In their analysis, the researchers found that having fewer teeth was linked to a significantly higher incidence of dementia, increasing from an incidence rate of 1.63 (per 100 person-years) for people with 20 or more teeth, to 2.31 (per 10 to 19 teeth), 2.97 (per 1 to 9 teeth), and 4.34 (for people with no teeth left).
After adjusting for gender, age and equivalent income, the risk of dementia was found to increase with reduced number of teeth, being 14% higher for people with 10 to 19 teeth, 12% higher for 1 to 9 teeth, and 27% higher for people without teeth, compared to people with 20 or more teeth.
But the most striking result came when researchers compared poor oral health to modifiable risk factors recognized by Lancet Commission.
Using a statistical method called population attributable fractions (PAFs) to estimate how much risk factors might contribute to disease in a population if they played a causal role, the researchers calculated a PAF of 7.2 percent for tooth loss, meaning it could explain 7.2 percent of dementia cases in the study.
What’s surprising is that, across the entire population, this estimate of 7.2 percent was the third-highest PAF overall, after depressive symptoms (which hypothetically accounted for 10.5 percent of dementia cases) and social isolation (9.4 percent).
Even more notable, when the results were filtered by only men, tooth loss had the highest PAF of all risk factors in the study (at 12%), while for women it was fourth (6.1%).
That’s quite a finding, and researchers say it suggests tooth loss may account for a larger share of dementia cases than known risk factors such as physical inactivity, smoking and low education.
“To our knowledge, this is the first study to quantify and compare the PAFs of poor oral health and a large number of other dementia risk factors within the same population setting,” the team writes.
“Notably, among men, the PAF for tooth loss in our study was substantial and comparable to the top-ranked modifiable risk factors, such as depressive symptoms and social isolation, reported by the study. Lancet Commission, indicating its relatively significant contribution to the onset of dementia.
The researchers acknowledge a number of limitations to the study, including that the JAGES data relies on participants’ self-reported information, which may be subject to bias or error.
It should also be noted that PAFs are speculative, and while the method estimates how risk factors might contribute to disease, the study does not actually demonstrate that tooth loss causes dementia, nor that preventing tooth loss would necessarily prevent cases of dementia.
That said, the team says there are several pathways that could support this link. Having fewer teeth could impact the ability to chew and eat food, which could lead to poor nutrition, linked to cognitive decline.
Tooth loss is also linked to social isolation, which is another risk factor for dementia. And tooth loss is also linked to oral inflammation, which can lead to systemic inflammation, which can lead to cognitive decline.
Related: Study reveals effect of lifetime exposure to fluoride in drinking water on cognition
Although more research is needed, the study authors say their findings show that oral health has major public health implications, and urge that it be integrated into dementia prevention strategies.
“This study demonstrates that tooth loss makes a substantial contribution to the incidence of dementia, comparable to the major modifiable risk factors established and identified by the Lancet Commission,” conclude the researchers.
“With the growing body of evidence, these findings argue for a re-evaluation of oral health as a key risk factor for dementia and its inclusion in future study updates. Lancet Report of the Commission.”
The results are reported in the Journal of the American Association of Medical Directors.
This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. Although we are proud of our process, we are only human. If you spot an error, please let us know.
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