Gut feelings: How stress may reshape the microbiome and influence brain health

For years, scientists have understood that the effects of brain disease and injuries can extend far beyond the skull, triggering inflammation, immune responses and lasting neurological effects. But emerging research suggests the relationship between the brain and the body may be even more intertwined than once thought, extending deep into the gut.

New findings from University of Georgia researchers show that even subtle, everyday experiences, like feeling stressed during exams or losing sleep, may leave measurable fingerprints in the gut microbiome. And that discovery could reshape how scientists think about mental health, disease detection and even treatment.

A two-way conversation between brain and gut

The study builds on a growing body of research focused on the “gut-brain axis,” a network of communication pathways linking the brain and digestive system. Signals travel along the vagus nerve — which carries signals between the brain, heart and digestive system — through immune responses and via hormones like cortisol, creating a constant feedback loop between what we feel and what our bodies experience.

“When something happens in the brain, like stress or injury, it can trigger inflammation and changes in the gut,” explained Franklin West, professor and researcher at the Regenerative Bioscience Center in UGA’s College of Agricultural and Environmental Sciences. “But it also works in the other direction. Changes in the gut can influence how the brain functions.”

That two-way relationship is at the heart of the new research. Previous work using animal models showed that traumatic brain injury can disrupt the gut microbiome and increase inflammation throughout the body, creating a harmful cycle that worsens brain damage. In those studies, restoring the microbiome helped reduce injury severity.

While West’s work has focused on how brain injury affects the microbiome, Hea Jin Park, co-investigator on the study and an associate professor of nutritional sciences in the College of Family and Consumer Sciences, has spent years studying how gut microbes influence brain development and function.

“It’s a bi-directional communication,” Park said. “Whoever has an interest in the brain should look at the changes in these microbes, or what is the healthy population of these microbes in our gut.”

Looking at “healthy” people in a new way

To determine the influence of the gut microbiome in people without major injuries or diagnosed disease, the research team analyzed the gut microbiomes of 44 adults who were otherwise considered healthy, with no major chronic cosnditions, average body weight and typical daily lifestyles.

Participants reported whether they experienced stress, anxiety, depression or sleep problems, even at mild or occasional levels. Researchers then compared their gut microbiomes to those of participants who reported no symptoms.

What they found was striking: even among healthy individuals, those who reported mental health symptoms had distinctly different gut microbial communities.

The differences showed up in both the diversity of microbes and the relative abundance of specific bacterial groups, suggesting that the microbiome is highly sensitive to subtle shifts in mental well-being.  

“For people who are otherwise normal and healthy, you wouldn’t expect to find anything,” West said. “But we did. It shows just how sensitive this system is.”

The findings were especially noteworthy because they emerged in people without diagnosed disease or injury.

“It was a very interesting study because we found this very similar concept and communication — gut-brain communication — in a relatively healthy population,” Park said.

Microbes, mood and metabolism

Digging deeper, researchers identified patterns that connect gut bacteria to biological processes linked with mental health.

Park said the connection between nutrition, the microbiome and brain health is one of the reasons researchers are increasingly looking beyond traditional neurological pathways.

“The gut is directly related to our food, and that is directly affecting the enteric nervous system that can communicate with the brain,” Park said.

Many of the microbial changes centered on bacteria that produce short-chain fatty acids, compounds that serve as energy sources for the brain and help regulate inflammation.

When those beneficial microbes were reduced, participants were more likely to report symptoms such as stress or anxiety. Scientists have previously linked these compounds to mood regulation, brain signaling and immune balance.

Other pathways involved tryptophan metabolism, a key process for producing serotonin and melatonin, chemicals critical for mood, sleep and memory.

Taken together, the findings point to a biological explanation for a familiar experience: When stress disrupts sleep or mood, it may also be reshaping the gut in ways that reinforce those same feelings. Researchers caution that the study does not determine whether changes in the microbiome cause mental health symptoms or are caused by them.

“We don’t know which one is the cause and what is the result,” Park said. “However, we have confidence that by altering the gut microbiome, it can affect mental health.”

A potential new kind of biomarker

One of the most promising implications of the research is the idea of using the microbiome as a biomarker, a measurable signal that can indicate health status or disease risk.

Currently, diagnosing conditions like mild traumatic brain injury can be difficult, especially when symptoms are subtle. Existing blood tests rely on proteins that only appear after more severe damage.

The microbiome may offer a more sensitive alternative.

“This could allow us to detect small changes, whether it’s mild brain injury, stress or early disease, before they become severe,” West said.

That could have real-world applications for athletes, military personnel and others exposed to repeated low-level impacts or stressors, potentially guiding decisions about recovery time or treatment.

Moving beyond a “pill for everything”

In addition to detection, the research suggests new ways to improve health by directly targeting the gut.

Diet, fiber intake and microbial therapies like probiotics or prebiotics can all influence the composition of the microbiome. Because of that, researchers see opportunities to develop more natural interventions that complement or perhaps reduce reliance on traditional medications.

“If we can identify the pathways and the bacteria involved, we might be able to change outcomes by changing the microbiome,” West said.

However, he cautions that not all probiotics are created equal. In fact, the team’s initial attempts to track certain probiotic strains showed that some products may not even reach the gut effectively. That highlights just how complex the microbiome is, and one-size-fits-all solutions are unlikely to work.

Park said dietary habits may ultimately prove just as important as supplements in supporting a healthy microbiome.

“Fiber intake is directly related to a healthy microbiome,” she said, noting that improving overall diet quality through adequate fiber intake and attention to sodium and fat consumption may help support gut health, though specific approaches will likely need to be tailored to individuals. “We have to provide the environment where beneficial microbes can grow healthy.”

A rapidly evolving field

While the study offers new insights, it also raises important questions. The sample size was small, and mental health symptoms were self-reported rather than clinically diagnosed. Researchers emphasize that more work is needed to confirm the findings and understand cause and effect.

Still, the results align with broader scientific trends showing that the microbiome plays a role in a wide range of conditions, from neurological disorders to metabolic disease. They also affirm that the boundary between mind and body is far less distinct than once believed.

“We’re starting to see the microbiome the way we once saw the genome,” West said. “It’s another layer of information that helps explain who we are and how we feel.”