A major new review of human studies finds consistent differences in gut bacterial communities between people with mild cognitive impairment (MCI), Alzheimer’s disease (AD), and healthy older adults.
Instead of proving cause and effect, the research highlights unique “microbiome signatures” at different stages of cognitive decline that might relate to inflammation or immune activity.
The findings suggest potential for future gut-targeted strategies — like diet or probiotics — but researchers emphasize that clinical trials are still needed to test if altering the microbiome affects cognition.
Most of us think of gut bacteria in terms of digestion. However, scientists are increasingly interested in a two-way communication network called the microbiota-gut-brain axis, which connects the trillions of microbes in your intestine with your nervous system.
In a recent study, researchers focused on whether patterns of gut bacteria are different in people with mild cognitive impairment (MCI) or Alzheimer’s disease (AD), compared with older adults who are cognitively healthy.
This topic matters because Alzheimer’s affects millions globally, and understanding early biological changes might point to ways to spot or even slow cognitive decline.
How the study was done
Rather than collecting new data from individuals directly, the team performed a scoping review, which means they systematically gathered and summarized findings from many earlier human studies.
They searched three major scientific databases up to February 2023 and screened thousands of articles, narrowing them down to 58 that met strict criteria — focusing only on human research and excluding animals or non-English reports.
Across these studies, investigators looked at the composition and diversity of gut microbiomes in adults with MCI or AD compared with cognitively normal peers. The bacterial profiles were typically assessed using genetic sequencing techniques that identify which groups of microbes are present and in what proportions.
While methods varied widely (different populations, different lab techniques), the goal was to see repeating patterns across the literature rather than isolated findings.
What the review found
Across the human studies, certain trends emerged.
Individuals with Alzheimer’s tended to have more of some bacterial groups — especially members of Pseudomonadota and Actinomycetota — and in some cases, lower overall microbial diversity compared with healthy older adults.
Similar but not identical patterns appeared in people with MCI, suggesting that the microbiome changes might shift as cognitive symptoms progress.
The review also noted that beyond who’s there, what the bacteria are doing seems to differ: gut microbes in people with AD sometimes showed reduced activity in pathways linked to energy production and immune function. These functional clues don’t prove that gut changes cause cognitive decline — but they do suggest biological connections worth exploring.
Importantly, the authors emphasized that these findings don’t establish direct cause and effect. Instead, they highlight consistent associations across diverse human cohorts and point toward the potential for future studies to test whether interventions — from dietary changes to probiotics — could influence cognitive health through the gut.
“These microbial signatures offer an important starting point for understanding how gut health may influence neuroinflammation and cognitive decline,” Leigh Frame, lead researcher and associate professor at the George Washington University School of Medicine and Health Sciences, said in a news release.
“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.”