In 2019 a group of scientists reported finding a bacterium from the gums in the brains of people who died with Alzheimer’s, and the headlines have not calmed down since.

I get asked about gum disease and Alzheimer’s more than almost any other mouth-body topic, and I understand why. It’s terrifying and strangely hopeful at the same time, because flossing is easier than most things you can do for your brain.

This science deserves a careful telling. You’re getting the exciting part and the sobering part, in that order.

Is there a link between gum disease and Alzheimer’s?

Yes, there is a consistent link, and nobody has proven that gum disease causes Alzheimer’s. Both of those are true, and the gap between them is where most of the bad headlines live.

What did the P. gingivalis Alzheimer’s study find?

The paper, by Stephen Dominy and colleagues in Science Advances, looked at brain tissue from people who had died with Alzheimer’s. In a large majority of samples, they found Porphyromonas gingivalis, the main culprit in periodontitis.

They also found gingipains, the toxic enzymes P. gingivalis uses to chew through tissue. The amount of gingipain tracked with the amount of tau pathology.

Then they infected the mouths of healthy mice with the bacterium and watched it show up in the brain weeks later, along with a rise in amyloid beta. Blocking the gingipains with an experimental drug reduced the bacterial load and the brain inflammation in those mice.

Why would a gum bacterium end up in the brain?

Because amyloid, the protein everyone blames for Alzheimer’s, may be part of the brain’s immune defense.

That theory has been building for a decade: amyloid beta, the sticky protein at the center of Alzheimer’s plaques, may be an antimicrobial peptide. Rudolph Tanzi and Robert Moir’s lab at Massachusetts General showed amyloid trapping and killing microbes in mice.

If that holds, plaque could be the brain’s response to chronic infection. Think of it as flypaper the brain puts up when something keeps getting in. And a mouth bug that leaks into the bloodstream every time you brush bleeding gums becomes a very plausible something.

Can gum disease cause dementia?

The population data say gum disease and dementia travel together. Whether one causes the other is still open.

A Taiwanese cohort of thousands of adults found that people with chronic periodontitis for ten years or more had about 1.7 times the rate of Alzheimer’s diagnoses.

An analysis of the NHANES data out of the National Institute on Aging linked antibodies to P. gingivalis with higher rates of Alzheimer’s diagnosis and death.

And a small King’s College London study found that people with Alzheimer’s who also had gum disease declined cognitively about six times faster over six months than those with healthy gums.

The sobering part

Every one of those is an observational study. People with early dementia brush and floss less, so some of the association runs backward.

The drug that worked in mice, atuzaginstat, went into a large human trial called GAIN, and in 2021 it did not meet its primary endpoints. A subgroup with detectable P. gingivalis in their saliva seemed to decline more slowly, but that was not the pre-planned test, and the program later stalled on liver safety concerns.

A failed drug trial leaves the bacterial theory alive. It also means nobody has yet shown that removing the bug changes the disease in humans.

Causation is unproven, and anyone who tells you gum disease causes Alzheimer’s is ahead of the evidence.

Does flossing help prevent dementia?

Nobody can promise that, and I’d still floss like my brain depends on it. It might, and the downside of being wrong is zero.

A woman in her sixties once brought that 2019 headline to her cleaning, printed out and folded into quarters. “My mom had Alzheimer’s,” she said. “Did her gums do that?” I told her nobody can trace her mom’s disease back to her gums, and that what we could do was look at her own gums that day. We went through them pocket by pocket.

The specific goal is gums that don’t bleed. Bleeding is the door P. gingivalis uses to get into your bloodstream, and closing it is the whole game.

That means daily flossing at the gumline and a soft brush angled into the gum. It also means a periodontal exam with pocket measurements, since periodontitis is painless. If you have pockets, treat them, and keep up with the maintenance cleanings.

Oral health and dementia share a lot of their other risk factors too, which is convenient. Nasal breathing at night keeps saliva flowing and the gum bacteria in check. A diet built on vegetables and fat-soluble vitamins feeds both the gums and the brain.

And skip the antiseptic mouthwash. It wipes out the beneficial bacteria along with the bad.

Keep this in proportion

This is a risk-factor story. That makes it a reason to act, and a poor reason to panic over one bleeding gum.

Most people with periodontitis do not develop Alzheimer’s, and plenty of people with pristine gums do, because genetics, sleep, vascular health and a dozen other inputs shape that disease. If dementia runs in your family, treat your gums as one modifiable risk among several, and talk to your doctor about the rest.

In one breath

Large population studies link gum disease and Alzheimer’s, and researchers have found P. gingivalis and its enzymes inside Alzheimer’s brains, with a plausible mechanism through amyloid’s antimicrobial role. The human drug trial did not succeed, so causation stays unproven. Keep your gums from bleeding anyway; it’s the cheapest brain insurance available.

Quick questions about your gums and your brain

Is it too late to fix my gums if dementia runs in my family? No. Gum inflammation responds to care at any age; bleeding usually settles within a week or two of consistent flossing. A periodontal exam tells you whether you have pockets that need treatment. Your gums are one of the few dementia risk factors you can measure and change this month.

What are gingipains? Gingipains are the toxic enzymes P. gingivalis uses to chew through tissue. In the 2019 Science Advances study, researchers found them in Alzheimer’s brain tissue, and higher gingipain levels tracked with more tau pathology. Blocking them reduced brain inflammation in mice, which is why they became a drug target.

Can a dentist test for P. gingivalis? Saliva tests that look for specific gum bacteria exist, and the GAIN trial used saliva to detect P. gingivalis. A periodontal exam with pocket measurements is still the most useful first step, because it tells you whether active gum disease is giving bacteria a way into your bloodstream.

Does mouthwash kill P. gingivalis? Antiseptic mouthwash kills a lot of bacteria, including the helpful ones, and I’d rather you skip it. P. gingivalis hides in gum pockets that rinses barely reach. Flossing, a soft brush angled at the gumline and professional treatment of any pockets do far more to close the door into your bloodstream.

Dr. Staci Whitman, DMD, FMCP, is a board-certified dentist and is certified in functional medicine. She is based in Portland, Oregon and is the author of SPIT (Crown, 2027).

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