What your mouth has to do with your risk of dementia

In 2024, The Lancet published a paper naming 14 modifiable risk factors that could prevent nearly half of all dementia cases worldwide. Our mouths – our oral health – were not among those 14 factors.

However, a new study builds on a growing body of evidence linking our oral and cognitive health.

The study suggested that our oral microbiome (the 100 billion or so bacteria that live in our mouths) might predict cognitive decline before symptoms start.

What we put in our mouths and how we care for them matters for our cognitive health too.Credit: Getty Images

Taking samples of 120 older adults, researchers found that people with certain types of bacteria in their mouths performed better on cognitive tests. Other types of bacteria were associated with mild cognitive impairment.

It comes off the back of a 2020 study of 6000 adults, which suggested the bacteria that cause gum disease are also associated with the development of Alzheimer’s disease, and a 2023 analysis of the data of more than 30 million people, which found a two-fold increase in Alzheimer’s disease risk in the poor oral health cohort compared to the normal oral health group.

“Multiple studies have found differing microbiota profile … in patients with cognitive impairment, memory decline and Alzheimer’s Disease,” says Dr Amanda Phoon-Nguyen, an oral medicine specialist and Australian Dental Association (ADA) spokesperson.

The question is whether it’s the chicken or the egg, says Scientia Professor Henry Brodaty, co-director of the Centre for Healthy Brain Ageing at the University of NSW.

“People with poor cognitive health don’t look after their teeth as well,” says Brodaty. “There’s a high rate of caries and gum disease, particularly in nursing homes where oral health is not looked after.

“The more sensational story is whether it goes the other way around and people with poor oral health are more susceptible to dementia, particularly Alzheimer’s disease.”

It is, of course, possible that it goes both ways, he adds.

There are several theories about why poor oral health might increase the risk of dementia.

Bacteria may travel through the bloodstream to the brain, and they (or their byproducts) may cause an inflammatory response. There is, Brodaty explains, “quite a lot of inflammation” in the brains of people with Alzheimer’s disease.

Another theory is that the amyloid beta protein, which is the toxic protein that builds up in the brain in Alzheimer’s disease, may provide some protection against infection. When bacteria like porphyromonas gingivalis (the main pathogen in the gum disease periodontitis, which affects up to 62 per cent of adults) get into the brain, it may provoke more amyloid.

Finally, what we put in our mouths affects our risk of dementia both directly and indirectly.

Smoking, excessive alcohol, cholesterol, obesity, hypertension and diabetes are all among the 14 modifiable risk factors (the others are education, traumatic brain injury, depression, physical inactivity, hearing loss, air pollution, social isolation and loss of sight).

Periodontitis is common and easily treated.Credit: Getty Images

Research suggests that a Mediterranean-style diet rich in specific nutrients may protect the brain through anti-inflammatory and antioxidant properties. This style of diet focuses on fruits, vegetables, whole grains, legumes, fish and other seafood, unsaturated fats such as olive oils, and minimal amounts of red meat, eggs and sweets.

Though the association between our oral and cognitive health is likely multifactorial – dependent on age and genetics, for instance – and needs more research, it is a compelling reminder to care for our mouths and what goes in them.

Currently, more than 55 million people have dementia worldwide and about 10 million new cases are diagnosed each year.

There is no good evidence that probiotics can improve our oral microbiome, and long-term use of mouthwash may negatively impact the oral microbiome. There is, however, plenty that we can do.

The ADA recommends brushing twice daily with fluoridated toothpaste, flossing daily, eating a diet low in sugar, and having regular dental visits to care for your oral health.

“According to data from the ADA, shockingly, 74 per cent of Australians rarely or never floss, and only 55 per cent brush their teeth twice a day,” says Phoon-Nguyen. “One in four Australians have inflammation of the gums.”

If you have gum disease, such as periodontitis (symptoms include swollen or bleeding gums), Phoon-Nguyen’s advice is not to ignore it, as it can have long-term consequences on your health.

“Other ways you can also protect your oral and systemic health is making sure your medical conditions are managed (such as diabetes or heart conditions) and to stop smoking.”

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