The Microbiota–Brain Connection: What Your Gut May Be Telling Your Nervous System
For many of us, the connection between emotional experience and the body is familiar. Anxiety can feel like a knot in the stomach. Stress can change digestion. A difficult conversation can leave us nauseated, while feeling safe and settled can bring a sense of ease throughout the body.
Modern microbiome research is giving us a deeper understanding of this relationship. The trillions of microorganisms living in and on our bodies, collectively known as the microbiota, are increasingly understood as active participants in the communication between the gut, immune system, nervous system, and brain.
For a somatic psychotherapy practice, this research offers an intriguing expansion of the mind-body perspective: our psychological experience is embodied, and the body itself is an ecosystem.
Your gut is part of a two-way communication network
The relationship between the gut and brain is not a one-way pathway. Scientists describe it as the microbiota–gut–brain axis, a complex communication network involving the nervous, endocrine, immune, and metabolic systems.
The International Scientific Association for Probiotics and Prebiotics (ISAPP) describes the gut–brain axis as one of the important dimensions of gut health. Its recent expert consensus on gut health identifies six interconnected domains, including the gut microbiome, gut barrier, gastrointestinal physiology, immune function, metabolism, and the gut–brain axis.
This helps explain something many people discover through therapy: the body does not simply “carry” psychological experience. It continuously participates in creating, interpreting, and responding to that experience.
And the communication runs in both directions.
Psychological stress can influence intestinal motility and the microbial environment. At the same time, signals generated in the gut can influence immune activity, metabolism, neural signaling, and brain function. Stanford microbiologist Justin Sonnenburg has described this relationship as a “two-way street” between gut microbes and the brain.
What does your microbiota have to do with mood?
Research has found associations between differences in gut microbial communities and conditions such as depression and anxiety. Some studies, for example, have observed differences in microbial diversity or in the abundance of particular groups of bacteria among people experiencing these conditions.
But there is an important scientific caveat: an association is not the same as a cause.
A 2025 review in Nature Mental Health emphasizes that the relationship between the gut microbiota and mental health may involve a combination of causal, correlational, and bidirectional processes. Diet, medication, stress, sleep, physical health, socioeconomic factors, and many other variables can affect both the microbiome and psychological well-being.
In other words, it would be premature to say that a particular microbial profile “causes” depression or anxiety, or that changing your microbiome will necessarily resolve a psychological condition.
What we can say is that the microbiota appears to be one component of a much larger biological system involved in the regulation of stress, inflammation, metabolism, and brain–body communication.
The nervous system and the microbiome are constantly listening to one another
One reason this research is particularly relevant to somatic psychotherapy is that the gut contains an extensive nervous system of its own (the enteric nervous system) and communicates with the brain through pathways that include the vagus nerve, immune signaling, hormones, microbial metabolites, and other chemical messengers.
This gives us another way of understanding familiar embodied experiences.
When someone says:
“I feel it in my stomach.”
“My gut is telling me something.”
“I can't digest what happened.”
“My whole body is on alert.”
these statements need not be interpreted as merely metaphorical.
They reflect the remarkable degree to which emotional and physiological processes are intertwined.
Somatic psychotherapy does not require us to reduce emotional experience to biology. Rather, microbiome science can reinforce a more integrated understanding of human beings: thoughts, emotions, sensations, physiology, relationships, environment, and behavior continually influence one another.
Why diet matters, but not in the simplistic way wellness culture sometimes suggests…
One of the most interesting contributions of Justin and Erica Sonnenburg's research has been showing how strongly the gut microbial ecosystem responds to its environment, particularly to diet.
Their research has highlighted the importance of dietary fiber and what they call microbiota-accessible carbohydrates, complex carbohydrates that gut microbes can use as fuel. Their work suggests that diets low in these substrates can alter the microbial ecosystem and, in experimental models, contribute to losses of particular microbial populations over generations.
The implication isn't that there is one perfect “microbiome diet.”
Rather, it points toward a broader ecological principle:
What we repeatedly provide to an ecosystem influences the ecosystem that can thrive there.
Our gut microbes are not separate from our lifestyle. They respond to what we eat, medications we take, environmental exposures, and other aspects of our lives.
This is one reason Sonnenburg and colleagues have investigated dietary approaches such as increasing fiber and consuming fermented foods. In a randomized human trial, a diet high in fermented foods increased microbiota diversity and reduced several measures of inflammation, although the study was relatively small and was not designed to demonstrate improvements in mental health.
That last distinction matters.
A healthier microbial ecosystem and lower inflammation are biologically interesting findings. They do not automatically mean that eating fermented foods treats depression, anxiety, trauma, or other mental health conditions.
What are probiotics and prebiotics, and why does terminology matter?
Microbiome language can quickly become confusing.
ISAPP has emphasized the importance of precise definitions because many foods and supplements marketed as “probiotics” do not necessarily meet the scientific definition.
A probiotic is a live microorganism that, when administered in adequate amounts, has been demonstrated to confer a health benefit on the host. The benefit is therefore specific to the organism, preparation, dose, and context, not simply to the fact that something contains bacteria.
A prebiotic, by contrast, is a substrate selectively utilized by microorganisms in a way that confers a health benefit. Many prebiotics are dietary fibers.
And fermented foods are not automatically probiotics. ISAPP specifically distinguishes fermented foods from probiotic products because fermented foods are not required to contain a characterized microorganism that has demonstrated a particular health benefit.
These distinctions are more than semantics. They help protect us from the temptation to turn promising microbiome research into overly broad health claims.
What about depression and anxiety?
This is an area of active research.
Several recent systematic reviews and meta-analyses have reported that some probiotic or other microbiome-targeted interventions may reduce symptoms of depression and, in some studies, anxiety. However, the studies differ considerably in the strains used, intervention duration, populations studied, outcome measures, and overall quality of evidence.
A 2025 systematic review of gut microbiota in depression and anxiety likewise found recurring differences in microbial composition, but noted substantial inconsistency across studies and potential confounding from factors such as diet and psychiatric medications.
So the most responsible conclusion at present is not that “probiotics treat depression.”
It is that the microbiota–gut–brain axis is a promising area of research, and microbiome-targeted interventions may eventually become part of more individualized approaches to mental health care.
Psychotherapy remains psychotherapy. Nutrition remains nutrition. Medical treatment remains medical treatment. These approaches can potentially complement one another without being collapsed into a single explanation.
A somatic perspective: caring for the ecosystem within
There is also something philosophically resonant about microbiome research for somatic psychotherapy.
Somatic approaches invite us to become curious about the body rather than treating bodily sensations as obstacles to psychological work. We learn to notice activation, contraction, warmth, pressure, movement, breath, impulses, and shifts in nervous-system state.
The microbiome offers another layer of this embodied picture.
We are not isolated organisms. We are ecosystems living within ecosystems.
Our internal microbial communities respond to the food we provide, the medications we take, our physiology, and our environment. At the same time, their activity can influence aspects of our physiology and immune and metabolic functioning.
This does not mean we need to monitor our microbiome obsessively or interpret every emotional experience through the lens of gut bacteria.
In fact, doing so could become another form of disconnection from the body; turning embodied awareness into another project of self-optimization.
Instead, microbiome research can invite a gentler question:
What conditions help the whole system flourish?
For some people, that may include eating a varied, fiber-rich diet. For others, it may mean attending to sleep, movement, stress, relationships, medical care, or digestive symptoms. It may also mean cultivating a less adversarial relationship with the body. Research shows that balance and diversity, with whatever microbiome we uniquely play host to, are keys to a “healthy” microbiota profile. There is no “standard” healthy microbiome.
And in psychotherapy, it may mean recognizing that psychological healing happens within a living body, not in a mind floating separately above it.
The microbiome is not destiny
Perhaps the most important message is that your microbiota is dynamic rather than deterministic.
The scientific community is still learning which microbial characteristics matter, which are consequences rather than causes of illness, how individual microbiomes respond to interventions, and whether changes in the microbiome can reliably translate into improvements in mental health.
Justin Sonnenburg has similarly cautioned that we are not yet at a point where we can simply manipulate the microbiome to specifically improve brain function. At the same time, he points to substantial evidence that diet and other factors can influence the health of the gut ecosystem.
That combination of curiosity and humility is useful for psychotherapy, too.
We can take the body seriously without reducing a person to biology.
We can recognize biological influences without assuming that biology determines our psychological future.
And we can appreciate the gut–brain connection without forgetting that human well-being also emerges from relationships, meaning, attachment, environment, culture, memory, behavior, and lived experience.
The microbiome is one part of the story.
And increasingly, it appears to be a fascinating part of the story of how body and mind communicate.
A note on clinical care
Microbiome research is evolving rapidly. Microbiome testing and commercially available probiotic products do not currently provide a simple way to diagnose the psychological health of an individual or prescribe a personalized mental-health treatment. If you are experiencing depression, anxiety, trauma-related symptoms, significant digestive symptoms, or another health concern, microbiome-focused approaches should be considered complementary to, not a replacement for, appropriate psychological and medical care.
Selected sources
International Scientific Association for Probiotics and Prebiotics (ISAPP), The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of gut health.
Marco ML, et al. ISAPP consensus statement on fermented foods, Nature Reviews Gastroenterology & Hepatology.
Sonnenburg JL & Bäckhed F. “Diet–microbiota interactions as moderators of human metabolism,” Nature (2016).
Sonnenburg ED, Smits SA, et al. “Diet-induced extinctions in the gut microbiota compound over generations,” Nature (2016).
Sonnenburg ED & Sonnenburg JL. “The ancestral and industrialized gut microbiota and implications for human health,” Nature Reviews Microbiology (2019).
Wastyk HC, et al. Research on fermented-food and high-fiber dietary interventions and the human microbiome, led by Stanford researchers including Justin Sonnenburg.
Kamath S, et al. “Distinguishing the causative, correlative and bidirectional roles of the gut microbiota in mental health,” Nature Mental Health (2025).
Asad A, et al. Systematic review and meta-analysis of prebiotics and probiotics for depression and anxiety in clinically diagnosed samples (2025).