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“People should really be aware that oral health is really important,” said Anita Visser, professor in geriatric dentistry at the University of Groningen in the Netherlands.
Severe periodontal disease — chronic inflammation of and damage to the gums and bone that support the teeth — affects about 19 percent of people older than 15 or more than 1 billion people worldwide, according to a 2022 World Health Organization report. More research is needed, but recent observational studies have suggested that oral health may be a modifiable risk factor for Alzheimer’s, the most common type of dementia.
Scientists are still working out whether and how the health of our mouths and minds meld, but they have identified two potential culprits that might explain how gum disease could lead to Alzheimer’s disease: bacteria and inflammation.
Connecting dental and mental health
One of the first studies to document the link between gum disease, tooth loss and Alzheimer’s disease focused on a cohort of aging nuns who were part of a larger study on aging. Researchers tracked 144 of the nuns and found that severe tooth loss was associated with dementia risk up to 6.4 times higher than compared with those who lost fewer teeth.
Other more recent longitudinal studies also found that a higher incidence of tooth loss was associated with cognitive decline. In one small 2016 study of 60 patients with mild to moderate dementia, periodontitis was associated with a sixfold increase in cognitive decline.
Another study, in 2017, of almost 28,000 Taiwanese patients reported that having chronic periodontal disease for 10 or more years corresponded to a 1.7 times increase in the risk for Alzheimer’s disease. A 2022 meta-analysis of 47 longitudinal studies reported that tooth loss and poor oral health are associated with both cognitive decline and dementia.
This research paints an emerging picture of an association between poor oral health and dementia, but there are a number of confounding factors that prevent researchers from drawing definitive conclusions of causation.
The higher rate of dental problems among those with dementia may be a symptom rather than a cause of cognitive decline. People with dementia have a difficult time maintaining their oral health and have an increased risk of developing gum disease, meaning that the association between oral and cognitive health may be bidirectional.
Other known risk factors for dementia, such as smoking and lower educational levels, are also associated with worse oral health. Tooth loss has secondary effects that can affect nutrition and overall health, which can also affect cognition, Mario Dioguardi, a researcher in dental science at the University of Foggia, said in an email.
“It’s really complicated,” said Visser, who wrote a recent review on the association of oral health and Alzheimer’s. “And this is why we cannot say, ‘Oh, if you have periodontitis, you will get Alzheimer’s disease.’ But we know now that if you have severe periodontitis, the chance of getting Alzheimer’s disease is bigger.”
Mouth bacteria may infect the brain
Research has found that bacteria that normally reside in our mouth may also infect the brain and potentially contribute to neurodegeneration in Alzheimer’s disease.
A 2019 study published in Science Advances reported that the DNA of P. gingivalis bacteria, a key pathogen in gum disease, could be found in brain autopsies from Alzheimer’s patients. Bacterial DNA was also detected in the cerebrospinal fluid of people living with a probable Alzheimer’s disease diagnosis.
Toxic enzymes from the P. gingivalis bacteria were also found in the Alzheimer’s patients’ brains and correlated with the amount of tau protein pathology, a hallmark of the disease.
Orally infecting mice with the bacteria led to the presence of P. gingivalis DNA as well as the accumulation of amyloid β cellular waste — a hallmark of Alzheimer’s — in their brains.
The researchers were able to inhibit the bacterial enzymes in mice infected with P. gingivalis, which reduced amyloid β production and neuroinflammation. (A recent clinical trial targeting those bacterial enzymes fell short, however, and the Food and Drug Administration placed a hold on future trials.)
“The mechanisms through which periodontal bacteria can access the central nervous system remain unknown” but could reach the brain through circulation in the blood or along peripheral nerves, wrote Dioguardi, who was a co-author on a recent review on the role gum disease and oral bacteria play in Alzheimer’s.
Mouth inflammation can affect the brain
If we do not brush our teeth for several days, each tooth develops a thin biofilm called dental plaque that teems with acid-producing bacteria.
“Your body doesn’t like these bacteria,” Visser said. “They’re on the edge of your teeth and on the edge of your gum.”
With plaque buildup comes inflammation of the gums as our immune system tries to fight off the infection. Gingivitis, the mildest form of periodontal disease, is still reversible; brushing and removing the plaque buildup will allow the gums to heal.
But more severe gum disease, or periodontitis, can follow if the gingivitis is not addressed.
“The whole body is fighting against this bacteria,” Visser said. “The immune system is really provoked and alert and working really hard against these bacteria.”
This chronic inflammation becomes a vicious cycle: more swelling of the gums enlarges the gap between the teeth and gums, allowing more bacteria to enter, which can lead to inflammation of not only the gums but also the underlying bone. Continued unabated, the body will reject the tooth, causing loosening and eventual loss of the tooth.
This chronic inflammation can spill from the mouth into the rest of the body. Gum disease is linked to an increase in pro-inflammatory molecules in the blood, Dioguardi said.
Chronic inflammation in the body can, in turn, lead to chronic neuroinflammation in the brain, which induces neurodegeneration and plays a key role in Alzheimer’s.
Visser is conducting a longitudinal study collecting oral health data — X-rays of teeth and samples of bacteria — from several hundred patients with cognitive impairment to get more information about how their oral health affects their cognitive risk.
Already, “we saw some really severe cases of oral health problems which are missed by the doctors,” she said.
The challenge of untangling the relationships between our lifestyle, our teeth and our brains remains. “There are so many confounders, lifestyle, smoking, educational level diet,” Visser said. “So it’s really hard to do this research.”
Until more is known, researchers stress that oral hygiene remains one of the simplest and most important ways you can take care of yourself.
“Raising awareness among the population about the increased risk of Alzheimer’s associated with tooth loss and periodontitis can lead to heightened attention towards oral health,” Dioguardi said in an email.
For better health — and possibly for a healthier brain — keep brushing.
Do you have a question about human behavior or neuroscience? Email [email protected] and we may answer it in a future column.
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