May 18, 2022 6 min read
Alzheimer’s Disease is a common and progressive form of dementia that causes serious cognitive decline. It’s not a normal part of aging, and we don’t know much about what causes it.
There is no cure for Alzheimer’s, and we’re only beginning to identify treatments. Surprisingly the microbiota may play a role in its development and even prevention. It turns out that the composition of gut microbiome impacts inflammation and the formation of plaques that are thought to be the main culprit of Alzheimer’s.
In this post we’ll explore how Alzheimer’s works. Then we’ll discuss the relationships between the microbiome and the development of diseases like Alzheimer’s, and how these gut bacteria may hold the key to the cure to diseases like Alzheimer’s.
Alzheimer’s Disease (AD) is the most common form of dementia, and effects mostly adults over 65. Alzheimer’s causes serious declines in cognitive function that manifest as profound memory loss and behavior changes which progress with time1.
We don’t really know what causes AD, but it’s thought to be mutated, pro-inflammatory proteins that build up in the brain and impair communication between brain cells. As these proteins accumulate, they form plaques that inhibit major brain functions and lead to tissue loss and shrinkage of brain matter.
Our risk of Alzheimer’s increases with age, but it’s not a normal part of aging. It’s thought that genetics and environmental factors contribute to the onset of Alzheimer’s. Dementia and AD have been linked to a number of other diseases and conditions, including diet. Some studies have even shown a relationship between the microbiota and AD.
There is reason to believe that the gut-brain-axis influences cognitive function, and therefore the development of diseases like Alzheimer’s, and this is what we’ll explore.
The human microbiome can be thought of as its own organ or organ system. This organ supports, and is supported by, other organs and tissues, like the liver or kidneys. And like other organs, the microbiome can be diseased or fail. This is called dysbiosis.
And with organ failure, dysbiosis has serious implications for health. Gut dysbiosis directly causes, results from, or is strongly linked to several diseases including colitis and Crohn’s, obesity and diabetes, and organ failure itself2. The microbiota is even thought to regulate, or be involved in, brain function, particularly dopamine and serotonin production and responses to hunger hormones.
The major factor that shapes the gut microbiota is diet – our gut bacteria help to break down certain foods. They then produce things like short chain fatty acids (SFCA) that feed other bacteria. This relationship keeps the gut bacteria thriving. The composition of the gut microbiome differs between health and obese people, and even changes with our diets.
These dramatic changes can cause dysbiosis. We can fix dysbiosis by simply changing our diets. But long term dysbiosis has serious implications: the gut immune system starts to fail which leads to chronic gut inflammation; the intestinal cells start to die off causing even more inflammation. This causes serious infections and changes the production of certain hormones from the liver and other organs.
This process keeps evolving with bacteria producing toxic ammonia and other compounds. This eventually compromises how neurotransmitters in the brain are produced and how their target organs respond3,4. In fact, we’ve known about the gut-brain connection for some time now, and it seems this is exactly the way the microbiota influences brain health and even the onset of diseases like AD or Parkinson’s.
The microbiota-gut-brain-axis (MGBA) is the term that describes the interaction between the brain and central nervous system and the gut microbiome. We’ve known that the gut can directly influence the brain to produce neurotransmitters like serotonin to that can regulate water balance, glucose levels, and overall gut function. But we’ve recently seen that specific bacteria can directly influence the brain, and vice versa.
For instance, our gut bacteria are constantly sampling the inside of the gut for pathogenic bacteria. These samples are sent to local immune factories that then manufacture and direct immune cells and traffic around the gut. But these bacteria can help to reduce inflammation in the nervous system by showing the brain where to look.
What’s more, the gut bacteria can influence the hypothalamus to produce certain hormones like cortisol, which is involved in stress adaptations and energy metabolism. Dysbiosis can disrupt how the hypothalamus functions, which can lead to impaired metabolism and gut function, even alterations in how the body responds to stress.
This is important to note because the hypothalamus is the ‘master gland’ in the body; it regulates the production of hormones that are vital to maintaining metabolic homeostasis. When it fails, the body isn’t able to adjust to stressors appropriately, and this can impair bodily and neural functioning.
There’s a lot more to the MGBA, much we know very little about. But the point is that it is intimately involved in overall function and health.
When it comes to the microbiome and Alzheimer’s, scientists have demonstrated a relationship, but we’re not entirely sure of the details.
Here’s what we know for sure4,5:
To be fair, AD is a very complex disease and the contributing factors are so poorly understood, including the microbiota. That said, we know the microbiota in involved in the pathology of Alzheimer’s. Dysbiosis of the gut results in decreased production of SFCA’s like butyrate; SFCA’s promote the growth of commensal bacteria but also modulate immune function and even the production of neurotransmitters, which is impaired in AD5.
In fact, one of the key indicators of AD is brain inflammation. Dysbiosis dramatically compromises immune system functioning, meaning inflammation persists, even in the brain. There are even some who believe that the overuse of antibiotics has altered our gut microbial landscape permanently, which has led to changes in brain function that cause Alzheimer’s. This is called the ‘Hygiene Hypothesis’ – it’s a stretch, but it highlights the marriage of our guts and the bacteria that live inside!
Gut dysbiosis can contribute greatly to inflammation, even in the brain. And since neuro-inflammation is a huge part of Alzheimer’s, it stands to reason that we can heal dysbiosis to prevent or reduce the symptoms of AD.
Sadly, it’s not so easy as there’s no cure and only limited treatments for AD. But, some studies have demonstrated that when you ‘correct’ the microbiome you may see reductions in inflammation in the brain5. There is even evidence that fecal transplants have the power to delay or prevent the onset of Alzheimer’s, but this is highly experimental.
The idea of correcting the microbiome to reduce inflammation, or Alzheimer’s is gaining traction. It seems probiotics like yogurt or prebiotics, like human milk oligosaccharides, can help restore the gut microbiota which reduces inflammation. Even more promising – prebiotics can actually decrease the severity of cognitive deficits resulting from Alzheimer’s!
The role of the microbiota in the evolution of our health is immense and at times elusive. Alzheimer’s disease is also elusive – we aren’t sure of its exact causes, nor do we understand how to treat it.
But we’ve shown that the microbiota, as usual, may play a distinct role in the onset and severity of AD. Dysbiosis can cause chronic inflammation in the brain, and bacteria can even produce toxic plaques that build up in the brain. But correcting dysbiosis through probiotics and prebiotics can reduce brain inflammation and even reverse cognitive decline in Alzheimer’s patients.
This is pretty remarkable, but we have a long way to go before we truly understand this relationship. What’s clear is the importance of maintaining good gut health. Probiotics and prebiotics are a great way to start. If you can promote the growth of good bacteria, you can help reduce gut inflammation and reduce symptoms of more sever diseases. It’s not too late to start!
Masters in Nutritional Biology (with emphasis in Immunology)
Certified Personal Trainer
(1) What is Alzheimer’s? Alzheimer’s Disease and Dementia. https://alz.org/alzheimers-dementia/what-is-alzheimers (accessed 2022-05-11).
(2) Sekirov, I.; Russell, S. L.; Antunes, L. C. M.; Finlay, B. B. Gut Microbiota in Health and Disease. Physiol. Rev. 2010, 90 (3), 859–904. https://doi.org/10.1152/physrev.00045.2009.
(3) Zhuang, Z.-Q.; Shen, L.-L.; Li, W.-W.; Fu, X.; Zeng, F.; Gui, L.; Lü, Y.; Cai, M.; Zhu, C.; Tan, Y.-L.; Zheng, P.; Li, H.-Y.; Zhu, J.; Zhou, H.-D.; Bu, X.-L.; Wang, Y.-J. Gut Microbiota Is Altered in Patients with Alzheimer’s Disease. J. Alzheimers Dis. 2018, 63 (4), 1337–1346. https://doi.org/10.3233/JAD-180176.
(4) Giau, V. V.; Wu, S. Y.; Jamerlan, A.; An, S. S. A.; Kim, S.; Hulme, J. Gut Microbiota and Their Neuroinflammatory Implications in Alzheimer’s Disease. Nutrients 2018, 10 (11), 1765. https://doi.org/10.3390/nu10111765.
(5) Varesi, A.; Pierella, E.; Romeo, M.; Piccini, G. B.; Alfano, C.; Bjørklund, G.; Oppong, A.; Ricevuti, G.; Esposito, C.; Chirumbolo, S.; Pascale, A. The Potential Role of Gut Microbiota in Alzheimer’s Disease: From Diagnosis to Treatment. Nutrients 2022, 14 (3), 668. https://doi.org/10.3390/nu14030668.
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