The Mouth-Brain Connection

The Mouth-Brain Connection: How Your Oral Microbiome Affects Your Mood, Memory, and More

95% of your serotonin is produced in your gut. Your gut is seeded by your mouth 1,000 times a day. The oral-gut-brain axis is the most important health connection most people have never heard of.

Nothing happening in your mouth stays in your mouth

The mouth is upstream of everything. Every time you swallow — approximately 1,000 times per day — you carry the living community of your oral microbiome into your gastrointestinal tract. Most of these organisms are destroyed by stomach acid. But some survive. They establish in the gut. They shape the gut microbial community over time.

In a person with a healthy oral microbiome — aerobic-dominant, diverse, coordinated — this daily seeding delivers protective organisms downstream. In a person with a disrupted oral ecosystem, it delivers the organisms of dysbiosis into the gut, continuously, all day.

The gut is informing the brain — not the other way around

The gut microbiome is connected to the brain through a pathway called the gut-brain axis. The key structures: the enteric nervous system (500 million neurons lining the gastrointestinal tract, producing and responding to the same neurotransmitters as the central nervous system) and the vagus nerve (the longest cranial nerve, connecting the brainstem to the abdomen).

95% of the body's serotonin — the neurotransmitter most associated with mood, sleep, and cognitive function — is produced in the gut. The vagus nerve carries signals from the gut to the brain at a ratio of nine upward for every one downward. The brain is not governing the gut. The gut is informing the brain. Your mood, your stress response, your cognitive clarity — all are shaped daily by the microbial community in your gut. And that community is seeded, every day, by what lives in your mouth.

The science · Dominy et al. · Science Advances · 2019

Researchers at Cortexyme (now Quince Therapeutics) examined brain tissue from Alzheimer's patients and found Porphyromonas gingivalis — the primary bacterium associated with periodontal disease — in 96% of brains examined. The bacterium's toxic enzymes, called gingipains, were present in 91% of cases and were found in the highest concentrations in the hippocampus and superior frontal cortex — the regions most damaged in Alzheimer's disease. In animal models, oral infection with P. gingivalis resulted in brain colonization and the production of amyloid beta peptides — one of the hallmarks of Alzheimer's pathology. The authors were careful to note that correlation is not causation: this research does not prove that gum disease causes Alzheimer's. It does document a real, biologically plausible pathway from the disrupted oral terrain, through the bloodstream, to brain tissue.

Dominy et al. · "Porphyromonas gingivalis in Alzheimer's disease brains" · Science Advances · 2019

The heart: how oral bacteria govern blood pressure

The connection between oral health and cardiovascular disease has been observed epidemiologically for decades. The mechanism has been harder to establish. The Bescos et al. study (Scientific Reports, 2020) provided one of the clearest demonstrations: the oral bacteria responsible for generating nitric oxide — the cardiovascular signaling molecule that regulates blood pressure and prevents clotting — are among the first casualties of antiseptic mouthwash use.

Nitrate from green vegetables is absorbed in the small intestine and concentrated in saliva. Specific aerobic oral bacteria — particularly Veillonella and Rothia species — convert that nitrate into nitrite. The nitrite is swallowed, reaches the stomach's acidic environment, and is converted into nitric oxide. Nitric oxide then enters the circulation and performs its cardiovascular functions.

When the aerobic oral bacteria are destroyed, the nitrate-nitrite-nitric oxide pathway stops at the mouth. The Bescos study measured the consequence directly: blood pressure increased measurably within one week in healthy volunteers using antiseptic mouthwash twice daily.

The oral microbiome is governing cardiovascular function. The most common oral care product was interrupting it.

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