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Mind Over Microbes: Could our Gut Microbiome Hold the Secret to Treating Alzheimer’s?

Three neurons - blue, purple, and pink - with the purple one in the center being distorted and its dendrites surrounded by orange plaques are positioned in a zig-zag in front of a background with different shades of blue. Dark blue text reads “Mind Over Microbes,” where each word is positioned above each neuron’s axon. Neurons of varying sizes and colours are placed further in the background.

Written by Saniya Gunputh
Illustrated by Sabrina Chen

“Gut microbiome”

“Gut healthy”

You’ve likely heard these, or similar terms being thrown around lately given the rise of “gut health” products on the market or of TikTok “health influencers”. Perhaps you’ve even watched Netflix’s Hack Your Health, a show exploring the science behind optimizing human health (would recommend it if you haven’t yet). As the name suggests, the microbiome refers to all the bacteria, viruses, and fungi (i.e. microorganisms) living on or inside of us humans, notably in the digestive system. As we get older, this microbiome diversifies and can be influenced by the food that we eat¹. Research on the human microbiome has gained much traction over the past few years, with researchers focusing on its interactions within different bodily systems and involvement in disease².

One interesting discovery has been the potential link between the gut microbiome and Alzheimer’s Disease (AD), the most common form of dementia. Cognitive impairments such as memory loss and difficulty making decisions or communicating are hallmarks of Alzheimer’s³.

Bacteria and the brain? How does that even make sense? Let’s break it down.

An illustration of the small and large intestines are placed in front of a light blue background. Three speech bubbles, containing 2D ball and stick models of the neurotransmitters serotonin, dopamine, and glutamate, are positioned in front of the intestines with dark blue text indicating each transmitter. Dark blue text in the bottom right corner reads “The gut microbiome plays a critical role in the production of these 3 major neurotransmitters.”

The Development of Alzheimer’s Disease

In patients with AD, we typically find amyloid-beta plaques, sticky deposits that form between neurons, and tau tangles, misfolded proteins that disrupt cell function³. Amyloid-beta proteins are made by neurons in the brain, and they are typically reduced to other peptides. The brain destroys the resulting amyloid-beta peptides on its own but has a back-up mechanism to transport the very few peptides left over out of the brain. When these both fail, the amyloid-beta peptides start to accumulate and form plaques between neurons⁴. Tau proteins help support cells and transport among neurons. When these proteins misfold and tangle, this eventually leads to improper function and cell death⁵.

Inflammation has also been uncovered as a third feature of AD. Typically, inflammation is a short-term effect caused by the immune system. The immune system’s cells are the body’s line of defence against varying pathogens, toxins, or injury. The buildup of amyloid plaques and tau tangles around neurons can lead to signalling errors which, in turn, lead to chronic inflammation in the brain. This creates a damaging and infinite feedback loop in the brain⁶.

The Gut-Brain Axis

There are now numerous studies outlining the connection between the gut and overall brain functions. The generation of neurotransmitters like glutamate, dopamine, and serotonin, which are essential for nervous system function, is greatly influenced by the gut microbiota. Cognitive deficits in Alzheimer’s disease have been connected to dysregulation of these circuits⁷. Additionally, the fermentation of dietary fibre by gut bacteria produces important bacterial metabolites influencing brain function. Changes in the gut bacteria (gut dysbiosis) have been linked to numerous mood and mental disorders due to the changes in neurotransmitter production³. Changes in the levels of different bacteria strains in the gut have also been observed in individuals diagnosed with AD⁷.

There are many interactions with the immune system, with over 1000 species of bacteria in a healthy gut. This diverse microbiome is important to maintain, especially as dysbiosis can lead to a compromised gut barrier. This means that harmful products of bacteria may be released into the bloodstream and make their way to the brain. If they manage to cross the blood-brain barrier, the brain’s defence mechanism controlling immune responses, this can lead to neuroinflammation (the villain in AD)³.

Ultimately, there are many complex interactions between the gut and the brain in the development of Alzheimer’s Disease. There is still so much more for scientists to learn in this area but so far, research has been promising. While there is no clear-cut way of preventing Alzheimer’s Disease, there are some small, but meaningful things that we can do today to decrease our risk, such as maintaining our gut microbiome. Easier said than done, but we should try to consume a range of whole foods. Getting some exercise and staying socially active can also be helpful⁸.

Lesson of the day: go for a walk, meet up with a friend, and don’t forget your fibre!

Sources:

  1. Robertson R. How Does Your Gut Microbiome Impact Your Overall Health? Healthline. 2023 April 03. https://www.healthline.com/nutrition/gut-microbiome-and-health
  2. Gao Y, Li D, Liu YX. Microbiome research outlook: past, present, and future. Protein & Cell. 2023 May 23; 14(10):709-712. doi: https://doi.org/10.1093/procel/pwad031
  3. Dissanayaka DM, Jayasena V, Rainey-Smith SR, Martins RN, Fernando WMAD. The Role of Diet and Gut Microbiota in Alzheimer’s Disease. Nutrients. 2024 January 31; 16(3): 412. doi: https://doi.org/10.3390/nu16030412
  4. Rukmandagachar LA, Bollu PC. Amyloid Beta Peptide. StatsPearls. 2023. https://www.ncbi.nlm.nih.gov/books/NBK459119/
  5. Findley, C. Tau Protein and Alzheimer’s Disease: What’s the Connection? BrightFocus Foundation. 2024 February 13. https://www.brightfocus.org/alzheimers/article/tau-protein-and-alzheimers-disease-whats-connection
  6. Kinney JW, Bemiller SM, Murthishaw AS, Leisgang AM, Salazar AM, Lamb BT. Inflammation as a central mechanism in Alzheimer’s disease. Alzheimers Dement. 2018 September 6; 4: 575-590. doi: https://doi.org/10.1016/j.trci.2018.06.014
  7. Chen Y, Xu J, Chen Y. Regulation of Neurotransmitters by the Gut Microbiota and Effects on Cognition in Neurological Disorders. Nutrients. 2021 June 19; 13(6): 2099. doi: https://doi.org/10.3390/nu13062099
  8. Risk factors for dementia. Alzheimer Society. 2023 October 17. https://alzheimer.ca/en/about-dementia/how-can-i-reduce-risk-dementia/risk-factors-dementia?gad_source=1&gclid=EAIaIQobChMI0LHIiozgiQMVY2dHAR0lNDF3EAAYASAAEgK2BPD_BwE

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