Your Mouth is Talking To Your Brain—And You Should Be Listening

In this episode we discuss:

• The connection between periodontal disease and cognitive decline
• How oral bacteria may access and influence the brain
• The emerging evidence linking periodontal disease and Alzheimer’s disease
• The role of Porphyromonas gingivalis and gingipains in neurodegeneration
• The newly identified oral-gut-brain pathway and microbiome disruption
• What current research suggests about oral health and Parkinson’s disease
• The gap between medical and dental care when addressing systemic disease
• Practical prevention strategies to support oral and cognitive health

Show notes:

• “Oral frailty and neurodegeneration in Alzheimer’s disease” by Dibello, V., et al.
• “Porphyromonas gingivalis in Alzheimer’s disease brains: Evidence for disease causation and treatment with small-molecule inhibitors” by Dominy, S.S., et al.
• “Oral health’s inextricable connection to systemic health: Special populations bring to bear multimodal relationships and factors connecting periodontal disease to systemic diseases and conditions” by Kapila, Y. L.
• “Porphyromonas gingivalis and the pathogenesis of Alzheimer’s disease” by Liu, S., et al.
• “The link between oral health and neurodegeneration: A review of periodontitis in Alzheimer’s and Parkinson’s disease and dementia” by Wagaskar, P., et al.
• “An update on periodontal inflammation and bone loss” by Zhang, M., et al.
• “Porphyromonas gingivalis induces disturbance of kynurenine metabolism through the gut-brain axis: Implications for Alzheimer’s disease” by Zhu, H., et al.
• RHR: The Science Behind Oral Inflammation, with Dr. William Levine
• Learn more about Adapt Naturals Biome Protect, Bio-Avail Colostrum+, or take our quiz to see which products best suit your needs
• If you’d like to ask a question for Chris to answer in a future episode, submit it here
• Follow Chris on Instagram or Facebook

Hey everybody, Chris Kresser here. Welcome to another episode of Revolution Health Radio. A few months ago, I had a conversation on this show with Dr. William Levine, a board-certified periodontist with 35 years of clinical experience, about the connection between oral health and systemic disease. If you haven’t listened to that episode yet, I’d encourage you to go back and check it out, because it lays an important foundation for what I’m covering today.

We talked about how gum disease develops, how oral bacteria enter the bloodstream and drives systemic inflammation, and why periodontal health belongs in the same conversation as diet and lifestyle when it comes to preventing chronic illness. Today I want to go a little deeper on one specific thread from that discussion, the connection between oral health and brain health, specifically cognitive decline, Alzheimer’s disease, and Parkinson’s disease. This is an area where the research has accelerated significantly in the last several years, and the picture emerging is one that most people, including most physicians and most dentists, are not fully aware of.

I’ve been talking about the gut-brain connection on this show for years. There’s now a substantial body of research demonstrating that what happens in the gut does not stay in the gut – that the state of your gut microbiome, your intestinal barrier, and your gut immune system has far-reaching effects on brain function, mood, cognition, and the risk of neurological disease. In functional medicine, we sometimes say “fire in the gut, fire in the brain.” What the science is now making clear is that we need to extend that principle all the way back to the beginning of the digestive tract. Because the mouth is, technically, part of the gut. It’s where digestion begins, where roughly 700 different bacterial species reside, and where a chronic low-grade infection can, over years and decades, contribute to the very neurodegenerative diseases that most of us are working hard to prevent.

One of the most striking pieces of evidence for this connection comes from a study published in Science Advances in 2019. Researchers found Porphyromonas gingivalis, the keystone pathogen in chronic periodontitis, in the brains of Alzheimer’s disease patients. They also found toxic proteases from that bacterium, called gingipains, in Alzheimer’s brains, and the level of those gingipains correlated with both tau pathology and amyloid accumulation, the two hallmarks of Alzheimer’s disease. That’s not a correlation you can easily dismiss. And it raises a pressing question: if oral bacteria can reach the brain, and if gum disease affects more than half of adults over the age of 35, a number that climbs to somewhere between 70 and 80 percent by the time people reach their 60s and 70s, why isn’t this a standard part of the conversation about dementia prevention? By the end of this episode, you’ll understand how oral bacteria reach the brain and contribute to neurodegeneration, what the research shows on Alzheimer’s and Parkinson’s connections, where the medical and dental systems are falling short, and what concrete steps you can take right now to address it. Let’s dive in.

The Scale of the Problem

Let’s start with some context on how widespread this issue actually is, because I think most people significantly underestimate it. Gum disease, or periodontal disease, is one of the most common chronic infections in the world. In the United States, studies estimate that half of adults over 35 have some form of active gum disease. That percentage rises sharply with age. By the time people are in their 60s and 70s, we’re looking at 70 to 80 percent with significant disease, meaning not just surface gum inflammation, but actual bone loss around the teeth. The reason this flies under the radar for most people is that gum disease is largely painless, especially in its early stages. The gums may bleed when you brush, they might be slightly swollen or tender, but these symptoms don’t typically rise to the level where someone feels compelled to act. Bleeding gums, in particular, tend to get normalized. People rinse, spit, and move on. But that bleeding is the body signaling an active inflammatory process, one that, if it’s allowed to persist for months or years without treatment, leads to progressive bone loss, deepening spaces between the gums and teeth, and a chronic low-level entry of oral bacteria into the bloodstream. The Dr. Levine episode goes deeper on the mechanics of how this develops, how to recognize it, and how it’s treated, so I’ll point you there if you want that level of detail. What I want to focus on here is what happens after those bacteria enter the bloodstream, specifically what happens when the inflammatory storm generated by gum disease starts affecting the brain.

The Two Pathways to the Brain

So, there are two distinct routes by which oral disease can affect brain health, and understanding both helps explain why the evidence for this connection has been building so consistently across different research methodologies. The first is a direct bloodstream route. When the gum tissue surrounding the tooth is chronically inflamed and ulcerated, the capillary bed beneath it becomes highly permeable. Porphyromonas gingivalis, the bacteria at the center of most of the brain research, is particularly good at exploiting that permeability. It produces small, highly mobile structures called outer membrane vesicles that can carry its virulence factors, including its toxic gingipain proteases, far beyond the site of infection. These vesicles are small enough and structurally equipped to cross the blood-brain barrier, the tightly regulated interface is supposed to keep the brain’s environment protected from what’s circulating in the rest of the body. A 2023 review in Critical Reviews in Microbiology showed exactly how this works. The outer membrane vesicles produced by P. gingivalis carry functional proteins that trigger neuroinflammation, disrupt the brain’s ion regulation, and accelerate the accumulation of amyloid plaques and tau tangles. That’s not theoretical, it’s a documented mechanism with supporting evidence from both human tissue studies and animal models. The bacteria have been found in Alzheimer’s brains. Their toxic enzymes have been found there, and the levels of those enzymes correlate with the severity of the disease.

The second pathway is one that connects directly to what we know about the gut-brain axis. It turns out that oral bacteria don’t only enter the bloodstream through the gum tissue, they also travel through the digestive tract. When you swallow, you’re swallowing a microbial community, and in the presence of gum disease, that community contains a disproportionate share of pathogenic species. A 2025 study in the Journal of Dental Research tracked what happens when P. gingivalis colonizes the oral cavity and found that it causes significant gut microbiome dysbiosis, disrupts intestinal barrier integrity, and alters a metabolic pathway called kynurenine metabolism. This pathway matters enormously for brain health, because kynurenine metabolites are directly involved in regulating neuroinflammation and neuronal survival. The study found elevated levels of a neurotoxic metabolite, 3-hydroxykynurenine, in the blood and the hippocampus of affected animals, and this metabolite suppressed a key neuroprotective gene, triggering cell death in the brain, and Alzheimer’s-like pathology. This means there’s a meaningful route from oral disease to brain damage that doesn’t even require the bacteria to cross the blood-brain barrier directly. They disrupt the gut, the gut dysbiosis disrupts the kynurenine pathway, and the disrupted kynurenine pathway contributes to neurodegeneration. This is, in a precise sense, an oral-gut-brain axis, and it’s the same logic that has governed our understanding of the gut-brain axis for the last decade, just extended upstream to where digestion actually begins.

The Alzheimer’s Evidence

The Science Advances study I mentioned in the intro deserves a closer look, because it’s arguably the most significant piece of direct evidence for causality in this research area. Researchers examined postmortem brain tissue from Alzheimer’s patients and found P. gingivalis DNA and gingipain proteins in the majority of samples. Critically, these weren’t just incidental findings. The levels of gingipains in the brain tissue correlated with the extent of tau pathology and the density of amyloid plaques. The more gingipain, the worse the Alzheimer’s pathology. The researchers then took the next step of infecting mice with P. gingivalis and observed what happened over time. The oral infection led to brain colonization, increased production of amyloid-beta, neuroinflammation, and damage to neurons in the hippocampus, which is the brain region most critical for memory consolidation and the region that shows some of the earliest degeneration in Alzheimer’s disease. When the researchers treated the infected mice with small-molecule inhibitors designed to block gingipain activity, they found a reduction in bacterial load in the brain, decreased amyloid production, reduced neuroinflammation, and rescued hippocampal neurons. The bacteria can get there, they cause the specific type of damage we see in Alzheimer’s disease, and blocking them protects the brain.

Now, this doesn’t mean that gum disease is the only cause of Alzheimer’s, or even the primary one in most cases. Alzheimer’s is a complex, multifactorial disease, and the research is clear that genetic predisposition, metabolic health, sleep quality, cardiovascular risk, exposure to environmental toxins, and many other factors play meaningful roles. But the evidence that P. gingivalis is a significant contributing factor, particularly in susceptible individuals, is strong and increasingly difficult to dismiss. A 2021 review in Neural Regeneration Research made the case that oral microbiota may influence Alzheimer’s disease risk through both circulatory and neural access to the brain, and that the connection between periodontal disease and cognitive decline is consistent enough across the literature to warrant serious attention to oral health as part of any comprehensive dementia prevention strategy. The researchers noted that a bacterial protease inhibitor targeting P. gingivalis was at that point in Phase II/III clinical trials for mild to moderate Alzheimer’s disease, which is a meaningful indicator of how seriously the scientific community is beginning to take this pathway.

Your mouth may be telling you more about your future brain health than you realize. In this episode, Chris Kresser explores the emerging science connecting gum disease, inflammation, the microbiome, and cognitive decline—and explains why oral health could be a missing piece of long-term wellness. #ChrisKresser #BrainHealth #OralHealth #PeriodontalDisease #GutBrainAxis

The 2025 Journal of Dental Research study adds another layer to the picture by demonstrating the gut-brain route I described earlier. If you take those together, the lines of evidence, direct brain colonization, gingipain-amyloid correlations, outer membrane vesicle transport across the blood brain barrier, and gut-mediated kynurenine disruption, converge on a plausible and increasingly well-supported model for how a chronic oral infection ignored for years can contribute to the erosion of cognitive function over time.

Beyond Alzheimer’s – Parkinson’s, Systemic Consequences, and the Inflammation Thread

The Alzheimer’s data is the most developed, but the neurological connections don’t stop there. A 2025 review in Inflammopharmacology examined the evidence for periodontitis as a risk factor across three conditions: Alzheimer’s, Parkinson’s, and vascular dementia. The researchers found overlapping biological mechanisms across all three: systemic inflammation, immune dysregulation, disrupted protein homeostasis, and they concluded that the oral-neurodegeneration connection is broad enough that treating periodontal disease may represent a meaningful preventive strategy, not just for Alzheimer’s, but for neurodegenerative disease more generally.

The Parkinson’s connection is less well-established than the Alzheimer’s one, but it makes biological sense. Parkinson’s disease is characterized by neuroinflammation, misfolded proteins, and progressive loss of dopaminergic neurons. Chronic systemic inflammation, of the kind generated by ongoing periodontal infection, is a recognized contributor to neuroinflammatory cascades, and there’s emerging evidence that gut dysbiosis, which, as we’ve seen, oral bacteria can trigger, plays a significant role in Parkinson’s pathogenesis through the gut-brain axis. And I saw this in my practice all the time. Every single patient that I treated with Parkinson’s or Parkinsonian syndromes had significant GI issues, and when we addressed the GI issues, we often saw meaningful improvements in their symptoms.

A 2021 review in Periodontology 2000 cataloged the full scope of systemic consequences of periodontal disease: diabetes, metabolic syndrome, cardiovascular disease, rheumatoid arthritis, adverse pregnancy outcomes, cancer, and Alzheimer’s disease. And they weren’t fringe associations from small studies; they were patterns replicated across population research, clinical trials, and mechanistic studies in animals and human tissue. A 2024 update in Frontiers in Immunology confirmed that the immune pathways driving periodontal inflammation –microbial dysbiosis, activation of neutrophils and macrophages, systemic release of pro-inflammatory cytokines, etc.– are the same pathways implicated in systemic chronic disease across the board. It makes sense, right? You have chronic inflammation, which we know is the root of virtually all modern disease, which is caused by these periodontal conditions. So this is where the functional medicine principle of total inflammatory burden becomes important. In functional medicine, we don’t think about sources of inflammation in isolation. We think about the cumulative load on the immune system – how many fires are burning simultaneously, and what that means for tissues and organ systems that are exquisitely sensitive to inflammatory signaling, especially the brain. An ongoing periodontal infection is one source of that burden. But if it’s combined with gut dysbiosis, poor sleep, metabolic dysfunction, or chronic psychological stress, or some combination of all of those, the compounding effects of the brain can be substantial. “Fire in the gut, fire in the brain” has always been an expression of this principle, and the mouth is where that fire can start.

The Diagnosis Gap – Why Nobody is Talking About This

If the evidence for this connection is as strong as I have described, the obvious question is why isn’t it a standard part of the conversation about cognitive health and dementia prevention? The honest answer is that it comes down to the structural silos between medicine and dentistry, and to a broader failure of both systems to translate emerging research into clinical practice. When you see your primary care physician for an annual physical, the focus is probably going to be on cardiovascular risk factors: blood pressure, cholesterol, maybe blood sugar, and body weight. Your mouth is not typically on that checklist. There’s no standard protocol that prompts a physician to ask about your gum health, inquire about whether you’ve had bone loss, or flag periodontal disease as a contributing factor to your inflammatory markers. When you see your dentist, the focus is usually on cavities, cleanings, and cosmetics. The systemic implications of what’s happening in your gum tissue and what it might be doing to your brain in 20 years are simply not part of the routine conversation. These two systems operate in separate lanes, and the patient navigating both of them is the one who falls through the gap.

When I was practicing, I routinely asked patients about their oral health as part of a functional medicine intake, particularly if they were presenting with autoimmune disease, cognitive symptoms, or unexplained chronic inflammation. It was a standard piece of my root cause workup, and I found consistently that many patients had significant periodontal issues that had never been identified as relevant to their broader health picture. They’d been to their dentist, been told they had some gum sensitivity or should floss more, and that was often the end of the conversation. Nobody connected the dots. Their internist didn’t know about the gum disease, and their dentist didn’t know about the neurological symptoms. The practical implication of this is you may need to be the person who initiates this conversation. At your next dental appointment, ask specifically about your periodontal health. Ask whether there are any signs of active gum disease, whether you’ve had measurable bone loss, and what the depth of your periodontal pockets is. Those measurements, which should be taken during a standard dental exam, tell you a great deal about the state of the gum tissue and whether pathogenic bacteria have the kind of access to the capillary bed that creates systemic risk. If there are concerns, ask for a referral to a periodontist, not because the issue is necessarily advanced, but because a specialist will give you a more thorough assessment and, importantly, will understand the systemic stakes. And if your physician has never asked you about your oral health as part of a broader discussion of inflammation or cognitive risk, bring it up. It definitely belongs in that conversation.

What You Can Actually Do

The good news in all of this, and I want to spend some real time here, because the research is genuinely encouraging, is that gum disease is highly treatable, particularly when caught early, and treating it has measurable downstream effects on systemic inflammation. Studies have shown that successful periodontal treatment reduces circulating inflammatory markers, including C-reactive protein and interleukin-6, the same markers associated with cardiovascular disease, metabolic dysfunction, and cognitive decline. Treating the source matters. Getting the infection under control, reducing the bacterial load, and restoring the integrity of the gum tissue are all achievable goals, and the systemic anti-inflammatory benefit follows. The treatment options have also expanded considerably. The Dr. Levine episode covers this in much more detail, but beyond standard deep cleaning protocols, practitioners are now using laser therapy, PRP, or platelet-rich plasma, and photobiomodulation to address periodontal disease in ways that are less invasive and, in many cases, more effective than traditional surgical approaches. If you’ve been told you have gum disease and haven’t explored what the current treatment landscape looks like, it’s worth having a conversation with a periodontist. And make sure to go back and listen to that Dr. Levine episode.

On the prevention side, the fundamentals are unsexy but genuinely important. Brushing twice daily is the baseline, but teeth have four surfaces, and brushing only reaches two of them. The spaces between the teeth, where pathogenic bacteria like P. gingivalis preferentially colonize because the environment there is warm, nutrient-rich, and protected from immune surveillance, require flossing or interdental cleaning to clear. This is not optional if you’re taking the brain health implications of oral disease seriously. The frequency of professional cleaning should also be calibrated to your individual risk level. If you have a genetic predisposition to gum disease, a history of bone loss, or other systemic risk factors, twice yearly cleanings may not be sufficient. A periodontist can help you determine what interval makes sense for your specific situation.

Beyond the mechanics of oral hygiene, everything that reduces your overall inflammatory burden also reduces your susceptibility to periodontal disease, and vice versa. A diet that supports a healthy gut microbiome, rich in diverse plant fibers, fermented foods, and anti-inflammatory fats creates a better bacterial environment throughout the entire digestive tract, including oral cavity. The research suggests that a bi-directional relationship is real. Gut dysbiosis can worsen oral health, and oral dysbiosis can worsen gut health. Addressing one without the other is leaving half the job unfinished. So, this is where attending to gut health through probiotics like Biome Protect or colostrum like Bio-Avail Colostrum+ can be really helpful. There are also some oral probiotics that can address the oral microbiome that have shown promise in some studies, but more research is needed to see what the exact impact of this is on periodontal health and gum disease.

Chronic stress, sleep deprivation, and blood sugar dysregulation all impair immune function, and a compromised immune system is less able to contain and reset a periodontal infection. This is why I keep coming back to the integrated picture. Oral health is not a separate domain from gut health, metabolic health, or stress management. It’s part of the same system, subject to the same upstream drivers and downstream consequences. So, if you have any symptoms of gum disease:  bleeding when you brush, swollen or receding gums, persistent bad breath, sensitivity in the gum tissue, or any looseness in the teeth, don’t wait. Get evaluated. If it turns out to be mild, you’ll feel better knowing that and knowing what to do about it. If it’s more advanced, earlier intervention is always more effective, and the research we’ve covered today makes a compelling case for why acting now has implications well beyond the health of your teeth.

Functional medicine has long held that the body is an integrated whole – that what happens in one system affects every other system, and that chronic disease rarely has a single cause. The gut-brain connection has become one of the most powerful demonstrations of that principle, and it has fundamentally changed how we think about brain health, mood, and neurological disease risk. The oral-brain connection is the same principle operating at the very beginning of the digestive tract, and the evidence for it is building in exactly the same way. The mouth is where the immune system first encounters the microbial world. What lives there, and whether it’s in the state of chronic dysbiosis and inflammation, sets the tone for everything downstream. That’s a lever most people haven’t considered, and it’s one of the more actionable ones available to us.

Thanks for listening, everybody. You can find show notes and all the studies I mentioned at ChrisKresser.com. If you have any questions about this episode or suggestions for future topics, head over to ChrisKresser.com/podcastquestion and leave me a message. I read all of them, and your questions help shape the content I create. Until next time, be well.