Brush up on your oral health: Is flossing the key to preserving your brain health?

A growing number of studies have identified a significant link between oral hygiene and dementia risk. 

In 2016 neuroscientists studying a small group of patients with mild to moderate symptoms of dementia found that those with periodontitis, severe gum disease driven by an overgrowth of inflammatory bacteria in the mouth, were experiencing far greater rates of cognitive decline.

Four years later, researchers in the US published a study in the Journal of Alzheimer’s Disease showing that older adults with signs of gum disease and mouth infections were more likely to develop Alzheimer’s. 

And in 2022, a review of 47 different studies on the subject, published in the Journal of the American Geriatrics Society, concluded that poor oral health is associated with brain degeneration over time.

So, what is going on? Dr Eamon Croke, president of the Irish Dental Association, explains there are up to 20bn bacteria and 700 different types of microorganisms in the mouth. 

Fluctuations in our diet and other habits, such as alcohol use, smoking, and irregular brushing, can all have a significant impact on which species tend to flourish and our ability to keep their numbers in check.

“Good oral hygiene along with regular professional assessments to detect and manage disease is important to controlling the total bacterial load in our bodies,” says Croke. 

“The increase in total body load of oral pathogens is potentially harmful to multiple tissues and organs, including the brain.”

We now know that oral microbes can work their way from the mouth into the bloodstream, where they can reach organs ranging from the gut to the heart, joints, and placenta, which is why poor oral health has also been associated with an increased risk of irritable bowel syndrome, diabetes and heart disease. 

But the microbes can also reach the brain, and bacteria, which are single-celled microbes, such as porphyromonas gingivalis (P gingivalis), one of the drivers of periodontitis, have been identified within beta-amyloid plaques taken from autopsies of Alzheimer’s patients.

Scientists are still undecided as to whether these bacteria can actually instigate neurodegeneration, while another theory suggests they may contribute to cognitive decline by driving inflammation in the bloodstream. 

Over many years, this inflammatory assault may ultimately weaken the blood-brain barrier (the protective layer of cells which protects the bloodstream and brain tissue), allowing inflammatory toxins to penetrate and damage the brain. 

Croke says that people with poor oral health have been shown to have increased damage to the delicate architecture of various brain regions.

“In recent years, research has examined the impact inflammation may have on the development of dementia,” says Dr Aoife Fallon, consultant geriatrician at Tallaght University Hospital (TUH), who studies the link between oral and brain health. 

“It is also thought that chronic inflammation could affect the production, deposition and clearance of amyloid, increasing the risk of Alzheimer’s disease and dementia.”

Targeted action

There is no evidence to date showing that dental interventions for patients already diagnosed with dementia could help slow the progression of the disease. 

However, various scientific groups have explored the idea of targeting some of the problematic bacteria linked with the disease, in particular, P gingivalis.

Between 2019 and 2021, the Research Institute for the Care of Older People (RICE) in Bath and San Francisco-based biotech Quince Therapeutics led a clinical trial aimed at blocking the action of P gingivalis.

The bacteria is thought to be particularly harmful because it produces enzymes known as Gingipains, which damage tooth enamel. 

Still, these enzymes can also work their way into other parts of the body, including the brain.

“Evidence of these enzymes have been found in the brain,” says Dr Tomas Welch, research and medical director at the RICE Institute. 

“The pattern of distribution of them in the brain seems to be keyed into the same pattern of change that we see with Alzheimer’s disease. So there are more of these enzymes in the regions that we know are more damaged in Alzheimer’s disease, and less elsewhere.”

Quince Therapeutics has conducted studies in mice, which have shown that infecting them with P gingivalis induces pathological symptoms which are similar to the characteristics of Alzheimer’s disease. 

“They were able to demonstrate in mice that a drug which blocked the activity of Gingipains seemed to slow or arrest the progression of the disease,” says Welch.

Unfortunately, the trial drug subsequently failed when used on human patients, but Welch says the company remains committed to developing new molecules to pursue this potential avenue for treating the disease. 

Such drugs could also be prescribed at a much earlier stage, before the onset of symptoms such as confusion and memory lapse, if they are to prove successful.

Fallon predicts that in the coming years, dental health screening will be increasingly incorporated into brain health and memory clinics in mid-life to identify those who might be at risk at a stage where treatments are still possible.

During the early stage of tooth decay, a thin biofilm (dental plaque), teeming with bacteria develops. As more plaque accumulates, the gums become inflamed as the immune system attempts to fight off the gathering microbes.

According to Welch, gingivitis, the mildest form of periodontal disease, is still highly reversible through brushing and dental treatment to remove the plaque build-up, allowing the gums to heal. 

However, if it is allowed to progress to periodontitis, the entire immune system will be aggravated, creating chronic inflammation, which can ultimately lead to cognitive decline. 

In future, it may be individuals who benefit from new treatments to limit the damage caused by bacterial enzymes such as Gingipains. 

“There has been an increasing interest in brain health and the role risk factors in early and mid-life could play in dementia prevention,” says Fallon. 

“We are focusing on addressing these factors in our Brain Health Clinic at TUH to reduce a person’s risk of developing dementia in the future.”

Nerve tissues

There is much we have yet to understand about the connection between oral health and the ageing brain. 

While research in this area has focused on gum inflammation and the role of specific bacteria, the condition of a person’s teeth may also play a critical role when determining their mental capacities in later life.

A 13-year study in China suggested that older people with more teeth have better cognitive function and a slower rate of cognitive decline. 

Other studies found that the loss of 16 or more teeth is significantly associated with dementia, while conversely, having 20 or more teeth boosts cognition in old age.

Croke says there could be a variety of explanations for these findings. He points out that dental caries and periodontitis can ultimately result in tooth loss, but these conditions tend to be linked with poorer diets and in some cases, it may be a lack of crucial nutrients that is driving cognitive impairment over time.

It also seems that chewing provides unconscious stimulation to specific brain areas. “The role of chewing has become more important in understanding cognitive impairment,” says Croke. 

“Tooth loss is associated with a decrease in nerve tissues in the brain, especially in areas associated with memory and learning. It may be that chewing stimulates brain activity.”

Dementia is also a disease where individuals become markedly more susceptible if they are isolated, and appearance-altering tooth loss may prompt people to stay away from social activities. 

“This is especially the case following the loss of front teeth,” says Croke. “Older people are not immune to the pressures of social norms, especially as portrayed by social media and marketing.”

Croke suggests that in future, we could have a clinically available vaccine against P. gingivalis, which could be given to individuals with signs of gum disease as a preventative measure against cognitive decline. 

Welch hopes that better therapeutics can be developed to target the harmful enzymes which some of these microbes can produce. However, such research projects are still in their infancy, and it is likely to be many years before anything will become clinically available.

While scientists learn more about the connection between oral hygiene and brain health, it seems one of the best steps you can take in the meantime is to keep brushing and flossing.