Written by: Devin Juros ‘23
Edited by: Jason Mero ‘22
Alzheimer’s Disease is a slow progressive disease that is painful for both patients and their loved ones. Alzheimer’s is the most common form of dementia, which refers to a group of symptoms characterized by impairment of memory and reasoning. This brutal course of brain degeneration is also the sixth leading cause of death in the United States . Despite the focus on Alzheimer’s research amid its rising prevalence, treatments thus far have been largely unsuccessful, leading scientists to look in more unexpected places for answers . One such accelerating field focuses on the relationship between the gut microbiome, which is all of the bacteria and microorganisms living in the gastrointestinal tract, and Alzheimer’s pathology. Besides influencing the healthy functioning of your heart, liver, and muscles, could what you eat affect your brain and memories?
The gut microbiome consists of bacteria, viruses, fungi, and other microorganisms that live in your lower gastrointestinal tract and play various roles in normal human physiology. These roles include promoting the metabolism of various nutrients, supporting the immune response to pathogens in the gastrointestinal tract, and communicating with the nervous system to regulate digestion and metabolism . In recent years, the gut microbiome has been the spotlight of medical research for numerous human diseases in the gastrointestinal tract and beyond. Some disease contexts in which the gut microbiome has been studied include Inflammatory Bowel Disease, obesity, cardiovascular disease, cancer, and arthritis . More recently, the communication of the gut microbiome with the nervous system has spurred the investigation of the relation of the gut microbiome to different neurodegenerative conditions, including schizophrenia, autism, depression, and Alzheimer’s .
Alzheimer’s Disease presents with an obvious brain pathology: plaques of amyloid beta protein and tangles of tau protein. Early researchers of the disease noticed these plaques and tangles in the brain cells of Alzheimer’s patients and focused on these pathological signs in attempts to develop treatments for Alzheimer’s, with little success. Many researchers have started to conjecture that the plaques and tangles might be the result of an underlying process actually incurring the observed progressive neurodegeneration of Alzheimer’s . More novel research on Alzheimer’s has pivoted toward immune system dysregulation, vascular changes, and changes in the gut microbiome .
But could changes to the gut microbiome really lead to the breakdown of memory and reasoning which is characteristic of Alzheimer’s? As there are tens of millions of neurons that innervate the lining of the gastrointestinal tract, this scientific jump might be shorter than it seems, especially as evidence is building to support this claim. For instance, the Bendlin Lab found that patients with Alzheimer’s had reduced microbial diversity in their gut microbiome . Another study by the Morita Lab replicated the gut microbiome of humans with and without Alzheimer’s by implanting fecal matter from these groups into different mice. The researchers found that mice with the gut microbiome implanted from Alzheimer’s patients had faster cognitive decline . Researchers have hypothesized that abnormal brain states can disrupt the gut microbiome, which can in turn increase neuroinflammation, creating a positive feedback loop of exacerbating neurodegeneration that would help to explain the progressive decline of memory and reasoning of Alzheimer’s .
So, what does this all mean for how we develop treatments for Alzheimer’s? If the dysregulation of the gut microbiome is contributing to the breakdown of memory and cognitive function in Alzheimer’s patients, the door opens to food-based therapy or probiotic supplements as methods to potentially prevent or treat Alzheimer’s . Diet can affect the composition and activity of the microbiome, so it is certainly possible that what we choose to eat could factor into our risk for developing Alzheimer’s. More research will have to focus on the specific pathways linking certain gut microorganisms to the degenerating brain, and how these pathways can be altered clinically to prevent or treat Alzheimer’s. It may be that there are "brain foods" with a more profound impact than we thought possible, working to protect our memories and brain over time.
 Bull, M. J., & Plummer, N. T. (2014). Part 1: The Human Gut Microbiome in Health and Disease. Intregrative Medicine,13(6), 17-22.
 Ding, R., Goh, W., Wu, R., Yue, X., Luo, X., Khine, W. W., . . . Lee, Y. (2019). Revisit gut microbiota and its impact on human health and disease. Journal of Food and Drug Analysis, 27(3), 623-631. doi:10.1016/j.jfda.2018.12.012
 Facts and Figures. (n.d.). Retrieved November 28, 2020, from https://www.alz.org/alzheimers-dementia/facts-figures
 Fujii, Y., Nguyen, T. T., Fujimura, Y., Kameya, N., Nakamura, S., Arakawa, K., & Morita, H. (2019). Fecal metabolite of a gnotobiotic mouse transplanted with gut microbiota from a patient with Alzheimer’s disease. Bioscience, Biotechnology, and Biochemistry, 83(11), 2144-2152. doi:10.1080/09168451.2019.1644149
 Interaction of the Microbiome. (2018, February 12). Retrieved November 28, 2020, from https://curealz.org/the-research/areas-of-focus/the-microbiome/
 Kowalski, K., & Mulak, A. (2019). Brain-Gut-Microbiota Axis in Alzheimer’s Disease. Journal of Neurogastroenterology and Motility, 25(1), 48-60. doi:10.5056/jnm18087
 Needham, B. D., Kaddurah-Daouk, R., & Mazmanian, S. K. (2020). Gut microbial molecules in behavioural and neurodegenerative conditions. Nature Reviews Neuroscience, 21(12), 717-731. doi:10.1038/s41583-020-00381-0
 Rocca, W. A., Petersen, R. C., Knopman, D. S., Hebert, L. E., Evans, D. A., Hall, K. S., . . . White, L. R. (2011). Trends in the incidence and prevalence of Alzheimer's disease, dementia, and cognitive impairment in the United States. Alzheimer's & Dementia, 7(1), 80-93. doi:10.1016/j.jalz.2010.11.002
 Vogt, N. M., Kerby, R. L., Dill-Mcfarland, K. A., Harding, S. J., Merluzzi, A. P., Johnson, S. C., . . . Rey, F. E. (2017). Gut microbiome alterations in Alzheimer’s disease. Scientific Reports, 7(1). doi:10.1038/s41598-017-13601-y
 Wallis, C. (n.d.). It's Time to Shift Tactics on Alzheimer's Disease. Retrieved November 28, 2020, from https://www.scientificamerican.com/article/its-time-to-shift-tactics-on-alzheimers-disease/
[Image Citation] Edwards, K. (2020, April 15). The Belly-Brain Connection: The Gut Microbiome and Alzheimer's Disease. Retrieved November 28, 2020, from https://www.adrc.wisc.edu/news/belly-brain-connection-gut-microbiome-and-alzheimers-disease