The human gut houses trillions of bacteria and other microorganisms that are part of your immune system and are collectively known as the gut microbiome. Similarly, your mouth teems with microscopic life, collectively known as the oral microbiome.
Just like bacteria in the gut, some are friendly and helpful, while others are downright hostile and harmful.
Around 700 species of bacteria make your mouth their home. Around a dozen or so species are considered “bad”, or pathogenic. These rogues are responsible for more than just gum disease; they are also involved in the development of Alzheimer’s.
Before dementia takes hold, there is usually a stage called mild cognitive impairment, when minor memory problems occur with increasing frequency.
In July 2020, the journal Neurology published a study that assessed the oral health of over 4,500 people, for an average period of 18 years.
According to study author Ryan T. Demmer of the University of Minnesota School of Public Health in Minneapolis, “…people with the most severe gum disease at the start of our study had about twice the risk for mild cognitive impairment or dementia by the end.”
Bugs in the brain
On the face of it, the connection between mouth and brain is not immediately obvious. You could argue that people with severe gum disease quite possibly consume the worst diets, and therefore have the worst health outcomes. However, the evidence suggests a direct link.
“It is clear from longitudinal monitoring of patients that chronic periodontitis contributes to declining cognition.”
The journey from mouth to brain begins with the development of periodontitis, an inflammatory gum disease, which if left untreated can destroy bone and cause tooth loss.
In the same way that an imbalance of bacteria in the gut creates a state of dysbiosis that can lead to disease, an imbalance of bacteria in the mouth can lead to oral dysbiosis. Of all the species of bacteria known to cause and progress periodontitis, none is more aggressive than Porphyromonas gingivalis, a major periodontal pathogen and a “master immune evader”.
Dysbiosis creates inflammation. Pathogenic bacteria such as P. gingivalis in the mouth are most at home in an inflammatory environment where they can get to work causing periodontal disease. Once they have created the right conditions for themselves, these oral pathogens are able to invade gum tissue and from there gain access to circulation. Once circulating in blood, they have access to all areas, including the brain.
Most bacteria in the healthy mouth are gram-positive. But as inflammation sets in, there is a shift towards gram-negative bacteria. When periodontitis arises, there is a vast presence — approximately 85% — of gram-negative bacteria. P. gingivalis is a gram-negative bacterium that attacks the gums. Just one periodontal pocket may harbour up to 300 million organisms.
“Loss of more than 16 teeth in early to mid-life is significantly associated with the development of dementia”
Some scientists believe that the Alzheimer’s brain “harbors its own microbiome”. Indeed, P. gingivalis has been found in the post-mortem brain tissue of people who died with Alzheimer’s, but not in the brains of people who died without the disease. Animal models infected with P. gingivalis also demonstrate hippocampal damage. The hippocampus is the region of the brain most associated with memory. Hence the suggestion that there is a “strong” causal link between chronic periodontitis and Alzheimer’s disease, a link that is underpinned by inflammation.
Chronic periodontitis is also associated with other inflammatory diseases, including cardiovascular disease, diabetes type 2 and Parkinson’s disease.
The sugar-Alzheimer’s connection
One of the most effective ways to increase inflammation in your gums is by eating sugar. When you consume sugary food and drinks, acid is produced by bacteria, and this acid dissolves the tooth surface - the first stage of decay that leads to cavity formation. The higher the sugar intake, the greater the prevalence of periodontal disease.
Sugar is pro-inflammatory and triggers insulin resistance, a risk factor for periodontitis. Insulin resistance arises when the body is no longer able to produce sufficient insulin, or respond to insulin, so blood sugar levels remain high. If the situation continues, diabetes type 2 may follow.
Over 80% of people with Alzheimer’s disease have either type 2 diabetes or abnormal blood sugar levels. So close is the link between the two conditions that scientists have taken to referring to Alzheimer’s as “type 3 diabetes”.
To read more about how sugar is directly linked to Alzheimer’s (even without gum disease) see the following article:
Alzheimer’s disease and the sugar connection.
They call it type 3 diabetes — with good reason.
Break the link
“It is estimated that 35% of dentate adults (those with teeth) in the United States between 30 and 90 years of age have periodontitis, and the prevalence of periodontitis increases with age to afflict approximately 50% of people older than the age of 55” .
Worryingly, gum disease is becomingly increasingly common, if not “normal”. It has been described as “nearly pandemic in children and young adults”. Yet the FDI World Dental Federation states that it is entirely preventable. According to the FDI:
- Severe periodontal disease, which may result in tooth loss, is found in 15–20% of middle-aged (35–44 years) adults.
- Almost 100% of adults and 60%-90% of schoolchildren in the world have dental caries.
- Risk factors for oral disease include an unhealthy diet — particularly one rich in sugars — tobacco use, harmful alcohol use and poor oral hygiene.
- Over the past 50 years, worldwide sugar consumption has tripled, an increase which is expected to grow.
- The World Health Organization recommends that the daily intake of free sugars be limited to less than 10% (or 50 g = around 12 teaspoons) of total energy intake in both adults and children.
Chronic periodontitis is a preventable risk factor for dementia. Several studies have supported links between lifestyle choices, oral health and development of Alzheimer’s disease.
According to Ryan T. Demmer,
“.. the good news was that people with minimal tooth loss and mild gum disease were no more likely to develop thinking problems or dementia than people with no dental problems.”
When it comes to oral hygiene, you know the drill: regular brushing and flossing. Your dentist or hygienist can provide further, personalised advice.
But what you eat is crucial too — or rather, what you avoid.