The Belly-Brain Connection: The Gut Microbiome and Alzheimer’s Disease

graphic of person with vagus nerve highlighted from gut to brain

In research, it is understood that neurodegenerative diseases like Alzheimer’s disease are a result of more than just genetics. Even amongst genetically identical twins, the age of onset of Alzheimer’s disease can vary greatly. Clinically, there’s been up to a documented 18-year difference of onset between twins.

Barabara Bendlin, PhD, an Alzheimer's disease researcher at UW-Madison, is interested in the environmental aspects of risk factors for Alzheimer’s disease. One area Bendlin is studying is the role of people’s gut microbiota and Alzheimer’s disease. 

Microbes, small organisms that are everywhere in the environment, live in and on everyone’s body. People are composed of about 30 trillion human cells, and have about 38 trillion microbe cells living on or in them. Microbes include bacteria, viruses, and fungi, and most of them are beneficial — they play an important role in health. The majority of the microbes cohabiting with humans live in the gut.

Recently in neuroscience research, scientists have identified the connection of these microbes in the context of the brain. The gut is well connected to the brain via the vagus nerve, which runs from the brain stem to part of the colon. Through the vagus nerve, the brain can communicate to the gut and the gut can communicate to the brain. Microbes in the gut produce signaling molecules that send messages up to the brain. Differences in gut microbiotas have been identified in many disorders, including anxiety, depression, addiction, anorexia, schizophrenia, attention deficit, autism, stroke, ALS, epilepsy, Multiple Sclerosis, and Parkinson’s disease.

Bendlin’s laboratory looks at gut microbiota differences between people with and without Alzheimer’s disease. From stool samples, Bendlin uses genetic sequencing and is able to determine what microbes are in the gut, how many microbes are there, and how many different kinds are there. Some participants also provide lumbar punches for more detailed brain insights. Bendlin’s lab has found that people with Alzheimer’s disease have less diversity in their microbiomes. In people without Alzheimer’s disease, looking at the cerebrospinal fluid from the lumbar punches, they found the bacteria in a participant’s gut matches the pathology in their brain.

photo of vegetables
Try to incorporate 30 plant-based foods in your diet each week. The world’s largest microbiome study found participants who ate more than 30 different plant types per week had gut microbiomes that were more diverse than those who ate 10 or fewer types of plants per week./Photo by Magda Ehlers from Pexels

Many things factor into people’s microbiome diversity, and it is constantly being modified. A lot of the microbiome is introduced to people from their mothers at birth. Who people live with, what they eat, and different medications they take also alter microbiome composition. One radical way to change the gut microbiota are fecal microbiota transplants. In fecal microbiota transplants, stool from a healthy donor is given to increase diversity in people with reduced microbiota diversity.

While the exact link and role between gut microbes and Alzheimer’s disease is still unknown, it’s promising to know that microbes play a key role in health. They offer immense potential opportunities for treatments and intervention for Alzheimer’s disease in the future. Bendlin’s lab continues to study this relationship, and recently launched the Alzheimer’s Microbiome Intervention Trial (AMBITION) to learn more.

Story by Kaitlin Edwards