Inside your gut lives an enormous community of microbes, trillions of bacteria, viruses, and fungi–far more microbes than human cells in the body. These organisms help digest food, regulate the immune system, and produce molecules that circulate throughout the body. Over the past decade, researchers have also discovered that these microbes communicate with the brain, through what scientists call the “gut–brain axis.” They do so in several ways, including releasing small molecules, interacting with immune cells, and influencing hormone signals. Together, these signals may even affect how the brain functions.
A new study published in the Journal of Alzheimer's Disease examined the gut microbiome and cognitive function in middle-aged and older adults. The study included participants from the Hispanic Community Health Study/Study of Latinos, one of the largest long-term health studies of Latino populations in the United States.
To do this, the team analyzed data from 2,471 participants who had both provided stool samples and completed detailed cognitive testing–both collected during the same study visit, which allowed the researchers to examine the two together. The stool samples revealed which bacteria were living in each person's gut and what kinds of chemical processes those microbes were capable of carrying out. The cognitive testing covered a range of standardized assessments measuring memory, language fluency, attention, and processing speed.
When the researchers examined the microbiome as a whole, they found that the overall structure of the gut microbiome was not strongly associated with cognitive scores. But when they looked at a more detailed level, several bacterial species were associated with better cognitive performance. Among them were members of the genus Eubacterium, including Eubacterium eligens and Eubacterium siraeum, as well as the bacterium Clostridium phoceensis. These microbes are known to be strictly anaerobic and often linked to anti-inflammatory processes in the gut. Other bacteria, including certain Alistipes species, were also associated with better cognition. These microbes can produce compounds derived from the amino acid tryptophan, including indole-related metabolites that may have neuroprotective effects and influence communication between the gut and the nervous system. At the same time, the researchers identified several bacterial species that were associated with lower cognitive scores. These included certain members of the genus Prevotella as well as the bacterium Desulfovibrio piger. Some of these microbes have previously been linked to inflammatory processes or metabolic disorders that may contribute to cognitive decline.
Importantly, the study focused not on dementia itself, but on earlier stages of cognitive change. As senior author Dr. Kaplan explains, “while it’s been shown that patients with an Alzheimer’s disease diagnosis have altered gut microbiome composition, our study addressed subtle cognitive changes in persons who did not have dementia.” He adds, “we hope this focus on early stages of the disease process will offer prevention opportunities.”
The team also asked a deeper question: what are these microbes actually doing at the chemical level? Several microbial pathways were associated with worse cognitive performance, including those involved in ornithine biosynthesis, the urea cycle, and serine metabolism. These pathways play a role in how the body processes amino acids and produces neuroactive molecules–the chemical messengers that help regulate brain activity. When microbial metabolism disrupts these processes, it may interfere with the brain's chemical environment in ways that gradually affect cognition. In other words, what microbes do–not just which ones are present–may matter for brain health.
Like any study, this one comes with an important caveat. Because the gut microbiome and cognitive tests were measured at the same point in time, the researchers can't say which came first. As Dr. Kaplan explains, “we still don’t know whether age-related changes to the central nervous system might influence the gut’s function, rather than the other way around.”
Despite this limitation, the study represents the first and largest investigation to date examining the gut microbiome and cognitive function in a Hispanic and Latino population. Looking ahead, several important questions remain. Future work will also explore whether these findings are consistent across populations: “we’re hoping to learn whether the gut microbes associated with brain health are the same worldwide, or whether they differ across race-ethnic groups or global regions,” Dr. Kaplan explains. Ultimately, this line of research may open new possibilities for intervention. As Dr. Kaplan notes, “once harmful and health-promoting gut bacteria are identified, we can begin to explore strategies to modify the gut microbiome to prevent or delay the progression of dementia.”
This work was supported by funding from the National Institutes of Health.
Palacios, N., Gordon, S., Wang, T., Burk, R., Qi, Q., Huttenhower, C., Gonzalez, H. M., Knight, R., De Carli, C., Daviglus, M., Lamar, M., Telavera, G., Tarraf, W., Kosciolek, T., Cai, J., & Kaplan, R. C. (2025). Gut microbiome and cognitive function in the Hispanic Community Health Study/Study of Latinos. Journal of Alzheimer's disease : JAD, 108(1), 84–97.