Understanding the Oral-Gut-Brain Axis and Implications for Brain Health - Food for the Brain

The mouth is a hub of activity, housing around 50–100 billion bacteria from 200 different bacterial species. The role of these resident bacteria in the mouth, also known as the oral microbiome, is an emerging area of research. Alterations in the oral microbiome may occur as a result of factors including consuming high amounts of sugar, smoking tobacco and experiencing chronic stress. Drinking large amounts of alcohol can also negatively impact the oral microbiome. Disruptions to the oral microbiome can lead to gut dysbiosis, which has been associated with increased permeability of the Blood Brain Barrier (BBB). 

Findings to date suggest that the oral microbiome, via interactions with the gut and brain (a network called the oral-gut-brain axis), may be a key consideration for brain health, and multiple associated conditions. This post will focus on three key areas where there is present research: autism, Down’s syndrome, and Alzheimer’s disease. 

Autism

Individuals with autism have been indicated to have alterations in their oral microbiome, as well as gut dysbiosis and related disruptions to the gut-brain axis. A study investigating the oral microbiome indicated that children with autism have a higher incidence of gastrointestinal disturbance and food allergies. Moreover, children with autism were observed to have a disruption to the ratio of Firmicutes: Bacteroidetes bacteria, in favour of Firmicutes. Balance of the Firmicutes: Bacteroidetes ratio is key for integrity of the gut, and disruptions to this ratio are indicative of gut dysbiosis.  

Moreover, two specific groups of bacteria, Brucella and Enterococcus faecalis were observed to be elevated in autistic children, whilst Flavobacterium sp. levels were demonstrated to be decreased. Research has suggested that individuals with autism have a higher risk of developing Alzheimer’s disease earlier in life. One potential mechanism for this could be due to alterations to the Firmicutes: Bacterodetes ratio.

Firmicutes and Bacteroidetes both utilise amyloids in the gut as structural material. Recognition of amyloids may become impaired by increased synthesis of inflammatory T cells by the innate immune system, as the result of intestinal permeability. Translocation of amyloids across the BBB has been hypothesised to be associated with enhanced Aβ deposit in the brain, although this requires further research.

Down’s Syndrome

Individuals with Down’s syndrome have been demonstrated to be more susceptible to periodontitis, or gum disease. One potential explanation for these findings could be due to alterations in oral microbiome composition. One study observed that individuals with Down’s syndrome have higher levels of Streptococcus mutans in their saliva. A further study observed increased levels of the pathogenic bacterial strains Aggregatibacter actinomycetemcomitans and Porphyromonas gingivalis.  Individuals with Down’s syndrome have an increased risk of developing Alzheimer’s disease later in life, with 50% of individuals >60 years of age meeting diagnostic criteria for dementia. One hypothesised mechanism for this is because of altered expression of inflammation and immune system modulating genes in periodontitis.

Alzheimer’s Disease

Individuals with Alzheimer’s disease have been observed to have higher levels of the oral bacteria, Treponema, in the brain. Moreover, disruptions to the oral-gut-brain axis has been associated with increased accumulation of beta amyloid and Tau, two key markers of Alzheimer’s disease.

Supporting the Oral-Gut-Brain Axis 

Supporting the oral-gut-brain axis is an area of research that is undeveloped, however, it seems logical that many of the measures employed for supporting gut and brain health would also be salient. 

Increase Fibre & Polyphenols

Consuming a wide array of colourful vegetables, fruits, herbs and spices is a great way of increasing prebiotic fibres, which help to support gut health via increasing production of SCFAs (short chain fatty acids), and polyphenols, plant compounds that have antioxidant properties and have been demonstrated to support the oral-gut-brain axis

Increase Omega-3 Fats

Omega-3 fats exert anti-inflammatory effects in the body, whilst increasing microbiome diversity via balancing the Firmicutes: Bacteroidetes ratio, which is essential for gut health and gut barrier integrity. Additionally, increased levels of omega-3 have been associated with reduced incidence of periodontitis. Ways to increase omega-3 include increasing consumption of oily fish such as salmon, mackerel and sardines, and also flaxseeds, walnuts and algae. 

Increase Fermented, Probiotic Foods

Probiotics have been associated with improved oral health due to decreased presence of pathogenic bacteria in the mouth. Examples of probiotic foods include fermented foods such as kimchi, kombucha, kefir, sauerkraut and sourdough bread.