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The oral-gut-brain connection: Microbiome influences on cognitive function

Anatomical structure of the human brain

Could your gut and oral health, or lack of, be affecting your brain and cognitive function?

According to studies, the answer is YES.

The oral cavity is the second-largest colonizer of the microbiome after the gut.[1]

Oral microbial dysbiosis is a chronic condition affecting more than 50% of older adults[2] and, considering most conditions are due, in part, to an alteration in the gut microbiome (IBD, IBS, type 2 diabetes, Crohn's, allergies, weight issues, depression, acne, eczema, CVD, PCOS, candida, poor immunity), I am going to go out on a limb and say gut dysbiosis affects… well… everyone.

You may have heard of the gut-brain connection, but the mouth and its microbiome is also a major player in this trio that influences health and impacts cognition.

The oral-gut-brain axis

It is well-known that gut microbiota and their metabolites can affect the brain by both direct and indirect means. The interaction between intestinal microbiota and brain cells has been recognised as influencing the development of AD as well as other neurodegenerative diseases.[3]

Microbial strains communicate through the vagus nerve, connecting the brain and the digestive tract, and microbial-derived metabolites interact with the immune system to maintain communication between the microbes and the brain.[3]

The impact of this is that the health of the gut affects the health of the brain. And if the gut is compromised, damaged or “leaky”, so too is the brain. Therefore, a leaky gut may directly contribute to the progression of AD via neuro-inflammation.

In addition to this, oral bacteria, such as the keystone pathogen Porphyromonas gingivalis in chronic periodontitis, have been identified in the brains of AD patients.[4]

Dysbiosis of oral microbiota has also been reported to induce and accelerate the formation of amyloid-beta plaques and neurofibrillary tangles. For instance, some oral microbes can spread to the brain through cranial nerves or cellular infections, spreading slowly over many years, which has been suggested to increase the risk of developing amyloid-beta plaques and AD.[3,4]

Oral-Gut-Brain axis diagram

What affects the health of your mouth and gut?

Alterations in the oral and gut microbiome may occur as a result of:

  • a diet low in fibre (fruits and vegetables)

  • consuming high amounts of sugar

  • drinking large amounts of alcohol

  • smoking tobacco

  • experiencing chronic stress

  • medications, such as antibiotics, that affect your microbiome.

After reading this article, you may now be motivated to attend your next (long overdue?) dental appointment, as well as make time to visit to your healthcare practitioner for a gut tune-up.

The importance of maintaining healthy oral and gut microbiome not only affects your cognitive function but, pretty much, the health of your whole body.


[1] Paudel D, et al. Effect of psychological stress on the oral-gut microbiota and the potential oral-gut-brain axis. Japanese Dental Sci Rev 2022;58:365-375.

[2] Wan J, Fan H. Oral microbiome and Alzheimer’s disease. Microorganisms 2023;11:2550.

[3] Narengaowa, et al. The oral-gut-brain axis: The influence of microbes in Alzheimer’s disease. Front Cell Neurosci 2021;15:633735.

[4] Dominy SS, et al. Porphyromonas gingivalis in Alzheimer’s disease brains: Evidence for disease causation and treatment with small-molecule inhibitors. Sci Adv 2019;5:eaau3333.


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