WASHINGTON (January 22, 2026) — A comprehensive new review is shedding light on the growing evidence that gut microbiome imbalances—known as dysbiosis—may play a key role in the development and progression of mild cognitive impairment (MCI) and Alzheimer’s disease, which affects an estimated 6.7 million individuals in the United States and nearly 50 million worldwide.
The research team analyzed 58 human studies published through February 2023 across different countries, making this the most thorough review to date that focuses exclusively on human studies, which is important in this space because animal models do not typically translate well.
Researchers are working to understand what biological processes lead to memory loss and cognitive decline. One area they’re studying is the microbiota-gut-brain axis—the communication system between the gut, the immune system, and the brain. Research suggests that this system may contribute to brain inflammation and damage to nerve cells, which could play a role in Alzheimer’s disease.
Across studies, individuals with MCI and Alzheimer’s disease consistently showed different gut bacteria (a.k.a. microbiota) compared to older adults without cognitive decline.
Key findings include:
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People with Alzheimer’s disease tend to have higher levels of certain groups of gut bacteria—particularly the phyla Pseudomonadota and Actinomycetota.
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Some people with Alzheimer’s disease have lower microbial diversity in their gut—meaning they have fewer types of bacteria than usual.
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The types and amounts of gut bacteria are different in people with mild cognitive impairment, Alzheimer’s disease, and healthy individuals. This suggests different microbiome signatures at different stages of cognitive decline.
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In people with Alzheimer’s disease, functional changes in gut bacteria were also observed, including reduced activity in pathways linked to energy production and immune function.
“These microbial signatures offer an important starting point for understanding how gut health may influence neuroinflammation and cognitive decline,” said Leigh Frame, lead researcher and associate professor at the George Washington University School of Medicine and Health Sciences “While these early, largely observational results are encouraging, we need clinical trials to determine whether microbiome-targeted interventions can meaningfully alter disease trajectories. These trials are already underway in our lab and labs around the world.”
Although the review does not establish cause and effect, the findings align with broader evidence suggesting that diets rich in fiber-containing foods may support a healthier gut microbiome, which could have downstream implications for inflammation and brain health.
The review, The microbiota–gut–brain axis in mild cognitive impairment and Alzheimer's disease: a scoping review of human studies was published in Alzheimer's & Dementia: The Journal of the Alzheimer's Association on January 22nd.
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Journal
Alzheimer s & Dementia