The Gut-Brain Axis: A Clinical Primer for Therapists and RDs
By Brittany Adelman, RD LDN and Brittaney Wood, LPC Co-founders, allyd health | Published on allyd Insights
Your anxious client has irritable bowel syndrome. Your client with depression eats almost nothing and feels nauseated most of the time. Your client with a trauma history reports chronic bloating, constipation, and stomach pain that no gastroenterologist has been able to fully explain.
These are not separate problems wearing the same face. They are expressions of a single, bidirectional communication system- the gut-brain axis and understanding it is increasingly essential for any provider who works with mental health.
This post is a clinical primer. It covers the science you need to understand what is happening in the body of many of your clients, the clinical presentations where gut-brain knowledge changes what you do or refer for, and the psychoeducation framework you can use in both therapy and nutrition sessions without overclaiming or straying outside your scope.
What the gut-brain axis actually is
The gut-brain axis is the bidirectional communication network between the gastrointestinal system and the central nervous system. It is not a metaphor. It is a documented physiological system involving neural, hormonal, and immunological pathways.
The primary neural pathway is the vagus nerve, the longest cranial nerve in the body, running from the brainstem to the abdomen. Approximately 80 percent of the fibers in the vagus nerve run from the gut to the brain, not the other way around. This means the gut is continuously sending information upward to the brain, influencing mood, cognition, stress reactivity, and behavior.
The enteric nervous system sometimes called the second brain is the network of neurons lining the GI tract. It contains approximately 500 million neurons, more than the spinal cord. It can function independently of the central nervous system and directly influences digestion, immune activation, and neurotransmitter production.
The gut microbiome- the community of trillions of bacteria, fungi, and other microorganisms living in the GI tract is an active participant in this system, not a passive occupant. The microbiome produces neurotransmitters including serotonin, dopamine, and gamma-aminobutyric acid. It regulates the immune system, influences the permeability of the intestinal lining, and communicates with the brain via the vagus nerve and via systemic circulation.
Serotonin and the gut: what every therapist should know
Approximately 90 to 95 percent of the body's serotonin is produced in the gut, not the brain. Enterochromaffin cells in the intestinal lining produce serotonin in response to food, mechanical stimulation, and signals from the microbiome. Gut-produced serotonin does not cross the blood-brain barrier and does not directly contribute to brain serotonin levels but it regulates gut motility, and it communicates with the brain via the vagus nerve in ways that influence mood, appetite, and anxiety.
This is clinically relevant for several reasons. Clients taking selective serotonin reuptake inhibitors often report GI side effects- nausea, diarrhea, altered bowel patterns because SSRIs affect serotonin signaling in the gut as well as the brain. Clients with irritable bowel syndrome have serotonin signaling dysregulation that is not primarily psychological, even when psychological factors contribute to symptom severity. And the nutritional conditions that support gut serotonin production dietary- tryptophan, fiber diversity, a healthy microbiome are modifiable through nutrition in ways that are clinically meaningful.
The microbiome and mental health: the current research
Research on the microbiome-gut-brain connection has expanded substantially in the past decade. While the field is still developing and causality is complex, several findings are consistent enough to inform clinical understanding.
Microbiome diversity is associated with mental health outcomes. Studies consistently find that individuals with depression and anxiety have less diverse microbiomes than controls. Whether this is cause, effect, or bidirectional relationship is still being established but the association is robust across populations.
Specific bacterial strains produce neuroactive compounds. Lactobacillus and Bifidobacterium strains produce GABA, the primary inhibitory neurotransmitter associated with anxiety reduction. Research suggests that certain probiotic strains may reduce anxiety and depressive symptoms, though the clinical application of this research is not yet fully standardized.
Gut permeability and inflammation affect brain function. When the intestinal lining is permeable sometimes called leaky gut bacterial products can enter systemic circulation and activate the immune system, producing inflammatory cytokines that cross the blood-brain barrier and directly affect neurotransmitter function, mood, and cognition. Chronic low-grade inflammation is associated with depression, and dietary patterns that reduce gut permeability are an active area of clinical nutrition research.
Stress affects the microbiome. The relationship is bidirectional: the microbiome influences stress reactivity, and stress alters the composition and function of the microbiome. Clients with chronic stress, trauma histories, or chronic anxiety often have microbiome dysregulation that perpetuates the physiological stress response, a loop that neither therapy nor nutrition can fully interrupt alone.
Clinical presentations where gut-brain knowledge changes your approach
Several clinical presentations that therapists encounter routinely are significantly informed by gut-brain axis understanding.
Anxiety with significant GI symptoms. Irritable bowel syndrome co-occurs with anxiety and depression at rates of 40 to 60 percent. IBS is not "just stress." It involves documented dysregulation of gut motility, gut-brain signaling, and often microbiome composition. Clients with anxiety and IBS benefit from a coordinated approach in which the therapist addresses the psychological dimensions of anxiety and the dietitian addresses the gut health dimensions of GI dysregulation. These are not the same intervention wearing different clothes they address different mechanisms.
Depression with appetite and GI symptoms. Loss of appetite, nausea, constipation, and early satiety are common somatic features of depression. They are also nutritionally consequential: a client who is barely eating is not getting the dietary tryptophan, B vitamins, omega-3 fatty acids, and mineral diversity that support neurotransmitter production and mood regulation. The depression affects nutrition, and the nutritional insufficiency worsens the depression. Recognizing this loop and involving a dietitian is clinically important.
Trauma and chronic GI complaints. Trauma is stored in the body in ways that include chronic GI dysregulation. Research on adverse childhood experiences and IBS, chronic constipation, and functional GI disorders is substantial. Clients with significant trauma histories and unexplained GI symptoms deserve a clinical approach that acknowledges the physiological underpinning of their GI complaints not one that interprets everything through a purely psychological lens. Trauma-informed care in this context means understanding that the gut symptoms are real, physiologically grounded, and require attention to both the neurological and nutritional dimensions.
Clients who are taking or considering psychotropic medications. The gut-brain axis is clinically relevant to medication discussions in ways most therapists have not been trained to address but can acknowledge. SSRIs, SNRIs, antipsychotics, and mood stabilizers all affect GI function and are in turn affected by gut health. This is a specific area where involvement of both a psychiatrist or prescriber and a dietitian who understands psychotropic medications can significantly improve client outcomes.
What this means for psychoeducation in session
Gut-brain axis psychoeducation is appropriate in both therapy and nutrition sessions, and it can be a clinically powerful tool particularly for clients who feel ashamed of their GI symptoms, who dismiss their somatic experience as "just anxiety," or who have been told by medical providers that their GI complaints are psychological without being offered anything constructive to do about it.
A therapist delivering gut-brain psychoeducation might frame it this way: "What you are experiencing in your gut is real and it is connected to your anxiety but the connection goes both directions. Your gut is actually sending more signals to your brain than your brain sends to your gut. What is happening in your stomach when you feel anxious is part of the anxiety, not just a symptom of it. There are things we can do on the mental health side, and there are things a dietitian can address on the nutrition side, and both matter for the full picture."
A dietitian delivering the same psychoeducation in a nutrition session might frame it this way: "The foods you eat directly affect the bacteria in your gut, which are producing some of the same neurotransmitters involved in mood and anxiety regulation. When we work on your gut health, we are not just addressing the IBS we are also addressing one of the physiological systems that contributes to how you feel emotionally."
Neither framing overclaims. Both are clinically accurate and clinically useful.
The referral pattern for gut-brain presentations
Therapists who encounter clients with significant GI symptoms alongside mental health conditions should refer to a dietitian with gut health experience specifically one who is familiar with the low-FODMAP approach for IBS, with microbiome support strategies, and with the nutritional dimensions of chronic inflammation. This is a specialty area within dietetics, and not all registered dietitians have this training.
Dietitians who encounter clients with significant anxiety, depression, trauma history, or chronic stress affecting their gut health should refer to a therapist who is comfortable with the somatic dimensions of mental health. A therapist who dismisses GI symptoms as psychological will not serve these clients well; a therapist who can integrate the somatic experience into the clinical picture will.
When both providers are working with the same client and communicating with each other, the client is receiving care that addresses the full mechanism. That is the standard of integrative care not a luxury, but what the gut-brain research shows these clients actually need.
Brittany Adelman, RD LDN is a registered dietitian specializing in mental health nutrition, gut health, and functional medicine. She co-founded allyd health and practices through Function Forward Nutrition. Brittaney Wood, LPC is a licensed professional counselor specializing in anxiety, trauma, eating disorders, and integrative mental health. She co-founded allyd health and practices through Through the Woods Mental Health Services.
allyd health provides cross-disciplinary training for therapists and registered dietitians, including a clinical module on the gut-brain axis in integrative care. Learn more at joinallyd.com.