Alzheimer’s Disease and the Gut Microbiome

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Research has demonstrated that the relative abundances of the gut microbiota differ between individuals with a clinical diagnosis of Alzheimer’s disease (AD) and healthy individuals. Scientists have been able to gain insights into this by comparing the taxa and abundance of organisms present in the gut microbiota of individuals with AD and healthy individuals. These findings can help to understand how a neurodegenerative disease, such as AD, may be linked to the composition of the gut microbiome. Given these findings, scientists are now trying to understand if gut microbial alterations are linked to AD. In a recent 2023 study, researchers compared the gut microbiome of asymptomatic pre-clinical AD patients who have biomarkers indicating a pre-clinical state of AD with that of a control group of individuals who have a healthy gut microbiome and normal cognitive state. Their findings suggest that the gut microbiomes between the pre-clinical AD group and the control group differ in gut microbial composition. Do poor diet habits that lead to poor gut health have a detrimental effect on our brains and contribute to the onset of Alzheimer’s disease?

“Given these findings, scientists are now seeking to understand the complete picture of how gut health is linked to Alzheimer’s disease.”

In the recent study performed by researchers at the Washington University School of Medicine in St. Louis and published in the prestigious journal Science Translational Medicine, they found that the gut microbiome could provide a useful view into understanding the risk an individual develop AD. The researchers studied a group of 164 human participants to draw a comparison between those with biomarkers pointing to pre-clinical AD and those with no sign of pre-clinical AD. From the selected participants, 49 of them were in the pre-clinical AD group. Pre-clinical AD patients are asymptomatic with normal cognitive function, but showing brain abnormalities of altered brain amyloid and tau proteins, established biomarkers of early AD. The researchers performed a cross-sectional study that considered clinical covariates and dietary intake to fully understand the relationship between the brain and the gut microbiome. Despite eating the same diet and controlling for any clinical covariates, researchers found that both the composition and function of the gut microbiome differed between the two groups. These results suggested that the observed gut microbiome changes occur early in the disease process, preceding any cognitive impairment. Researchers were able to make this conclusion based on their findings of the gut microbiome composition correlating with β-amyloid (Aβ) and tau pathological biomarkers, but not with biomarkers of neurodegeneration.

“The results of the differences in the gut microbiome of each group indicated that these changes may take place early in the disease process.”

This research does not mean that every individual who has poor gut health is bound to develop Alzheimer’s disease, but that there is a link between cognitive health and gut health. From this new research, we now know that the state of the gut microbiome can affect the onset of this disease. While further research is needed to draw more direct conclusions., this study provides insight into a possible causal link between the condition of the gut microbiome and the risk of AD. Therefore, individuals who are concerned with their cognitive health, have a family history of early cognitive decline and have positive biomarkers for AD should be strongly advised to take steps to avoid or to slow progression towards dementia by focusing on their diet and the health of their gut microbiome.

“While further research is needed to draw more direct conclusions, this study provides insight into a causal link between the condition of the gut microbiome and the risk of AD.”

The way we eat has a major effect on the health of our gut, the brain and the interactions between the two. Eating an array of fruits and vegetables will increase the diversity and richness of the gut microbiome. Fruits, vegetables, whole grains, and nuts are great options for optimizing gut health due to their rich nutritional profile and high amounts of fiber and polyphenols. Another way to promote good gut health is by regularly adding fermented foods to our diet. Fermented foods, like kefir, Sauerkraut, cheese and kombucha contain live microbes which can help to increase the abundance “good bacteria” in your gut. While the multiple microorganisms contained in naturally fermented foods are not officially classified as probiotics, research also shows that these organisms are beneficial for reducing symptoms in patients with an AD diagnosis. Gut health can be affected by many factors aside from diet alone such as our environment, genetics, lifestyle choices, stress levels and emotions. Researchers have found the amount of stress we perceive and how we respond to it can play an important role in gut health. Finding ways to manage stress effectively, such as mindfulness based stress reduction or abdominal breathing is important for an optimal gut microbiome. Talking to a trusted individual, creating community, exercising, and good sleep are all ways that have been shown to work well for individuals to decrease their stress levels.

Future research will hopefully be able to draw a clearer conclusion between AD and gut health, but for now scientists hypothesize that the link between neurodegeneration and gut microbial health is related to a state of dysbiosis, increased intestinal permeability, leading to metabolic endotoxemia, and ultimately to inflammation in the brain. While these gut symptoms can start years before any neurological and psychiatric symptoms, AD symptoms don’t begin to manifest until the disease process has progressed and the damage to the brain is irreversible. Therefore, the optimal time for therapeutic intervention is during the preclinical phase, often years before any clinical symptoms are detectable. Identifying and validating the specific characteristics of a gut microbiome that increases the risk of AD in patients with a family history of AD and positive biomarkers should be a major research priority.

Amanda Johnson Amanda is a recent graduate from the University of Southern California where she received her degree in Psychology. In addition to her university studies, she earned her Integrative Nutrition Health Coach certification from the Institute of Integrative Nutrition (IIN).

This article was reviewed and approved by Emeran Mayer, MD