A specific molecule produced by bacteria living within the human digestive system may significantly increase the risk of developing Alzheimer’s disease and accelerate cognitive decline in individuals already experiencing dementia, according to compelling new research. The scientific breakthrough, which illuminates a complex biochemical pathway connecting the gut to the brain, could ultimately guide researchers toward innovative and targeted treatment strategies aimed at lowering Alzheimer’s risk and altering the trajectory of neurodegenerative disease.
Nearly a decade ago, a dedicated research team led by University of Wisconsin-Madison professors Barbara Bendlin and Federico Rey made a fundamental observation: the diverse ecosystem of microorganisms residing in the human intestine—collectively known as the gut microbiome—differs noticeably between people diagnosed with Alzheimer’s disease and healthy individuals. That early discovery opened up an entirely new avenue of investigation into how gastrointestinal health might influence neurological function.
"Since then, we’ve been trying to figure out how this difference in the gut perhaps leads to changes in the brain," says Bendlin, a prominent professor of medicine within the UW School of Medicine and Public Health.
A Gut Molecule With Effects Beyond the Intestines
In a recent study published in the prestigious scientific journal Nature Communications, Bendlin, Rey, and their multidisciplinary team of collaborators narrowed their focus onto a specific compound called imidazole propionate, commonly referred to as ImP. This organic molecule is produced naturally by certain types of bacteria residing in the human gut as they metabolize nutrients. The findings of the new study strongly suggest that ImP may actively contribute to the detrimental brain changes that characterize Alzheimer’s disease and related forms of dementia.
The production of ImP varies considerably from one person to another, reflecting the immense individuality of the human microbiome. While some individuals appear to generate relatively large quantities of the compound, others produce very little or trace amounts.
"ImP-producing bacteria are present in a large fraction of people, but they’re not very abundant in most people," explains Rey, who serves as a professor of bacteriology at UW-Madison. "Pero something we have learned over the years is that a microbe doesn’t have to be abundant to have an impact on the host."
Once synthesized by bacteria in the gut, ImP is capable of crossing the intestinal barrier, entering systemic circulation, and traveling through the bloodstream to reach distant organs and tissues throughout the body. Previous scientific investigations have already linked the presence of this circulating compound to metabolic and cardiovascular issues, including type 2 diabetes and coronary artery disease.
The new study, however, demonstrates that the biological reach of ImP extends even further, penetrating the central nervous system. In controlled studies involving laboratory mice, the researchers discovered that ImP reaching the brain actively promoted the accumulation of abnormal beta-amyloid and tau proteins. These two misfolded proteins represent the major pathological hallmarks of Alzheimer’s disease, forming plaques and tangles that disrupt normal cellular communication.
"That process eventually results in the death of neurons, and in humans is a key feature of Alzheimer’s disease," Rey notes.
Higher ImP Levels Linked to Faster Cognitive Decline
To determine whether these laboratory findings translated to humans, the research team expanded their investigation by examining blood plasma samples obtained from nearly 1,200 participants. These volunteers were enrolled in the prominent Wisconsin Registry for Alzheimer’s Prevention as well as ongoing studies conducted through the Wisconsin Alzheimer’s Disease Research Center, providing a robust and well-characterized human cohort.
The analysis revealed a striking correlation: participants who exhibited higher concentrations of ImP in their blood were significantly more likely to display established biological markers associated with abnormal protein accumulation and impaired neuronal function, both of which are central to the progression of dementia.
Because these study volunteers had also completed rigorous cognitive testing repeatedly over many years, the researchers possessed a unique longitudinal window into their neurological health. This allowed the team to directly compare individual blood levels of ImP with actual changes in thinking, memory, and executive function over time.
"And because we have the results of cognitive tests these volunteers took over time, we can see that the people with the highest ImP levels also experienced much faster cognitive decline," Rey says.
Further deepening their genetic analysis, the team also uncovered a specific genetic variation that is associated with substantially higher levels of ImP circulating in the bloodstream. Approximately 43 percent of the individuals participating in the study were found to carry this particular genetic variant.
The researchers suspect that this genetic difference may alter how efficiently the kidneys filter and remove ImP from the blood, thereby influencing the overall concentration and duration of the compound remaining in circulation.
"This genetic variation has been associated with increased Alzheimer’s risk in large genetic studies before, and now we may understand why it’s connected," Rey points out.
A Potential New Target for Alzheimer’s Prevention
Perhaps the most clinically promising implication raised by these new findings is that ImP itself could eventually serve as a viable target for preventing or slowing down the onset of Alzheimer’s disease and related dementias. By identifying a specific biochemical culprit and connecting it to a known genetic variation, scientists now have a much more precise target to investigate for therapeutic intervention.
However, researchers caution that reducing ImP levels through dietary modifications alone is unlikely to be a simple or straightforward solution. Gut bacteria produce the compound while breaking down and generating energy from histidine, an essential amino acid that is crucial for human health. Histidine is naturally abundant in many common foods, particularly those rich in protein.
"Generally improving your diet would probably help," Bendlin observes. "But it’s not as easy as saying, ‘Stop eating eggs’ or ‘Don’t eat so much red meat.’ Because you need histidine, and it’s all over the place."
Rather than attempting to eliminate essential nutrients like histidine from the human diet, the research team believes that the discovery will ultimately pave the way for pharmaceutical treatments designed to selectively lower ImP levels in the bloodstream.
Drawing a parallel to modern cardiovascular medicine, Bendlin compares the potential strategy to managing other well-known health risks.
"It could be just like cholesterol, where people with elevated cholesterol take a drug, a statin, that reduces their risk for heart disease," Bendlin says. "If we can find an inhibitor that can help decrease the levels of ImP in the blood, that could hopefully reduce the risk of Alzheimer’s and the speed of cognitive decline for a significant number of people."
In addition to researchers at the University of Wisconsin-Madison, scientists from the University of California, Los Angeles, and the University of Gothenburg also contributed significantly to the study.
The research was supported in part by grants from the Wisconsin Partnership Program, the National Institutes of Health under award numbers R01AG070973, R01AG083883, R01AG092220, R21AG089348, R01HL168493, R01DK143650, and U54HL170326, as well as funding from the U.S. Department of Agriculture under grant WIS03073.
