The Gut-Brain Connection: What It Is, Why It Matters, and How Chiropractic Care Fits In

By Dr. Andrew Biggs, DC | Principled Chiropractic | Royal Palm Beach, FL

You have probably heard someone say they had a "gut feeling" about something. Or that they felt "sick to their stomach" with anxiety. Or that stress always seems to go straight to their digestion. These are not just figures of speech. They are descriptions of a real, physiological connection, one that science is now understanding in remarkable detail.

The relationship between the gut and the brain is one of the most exciting frontiers in modern health research. And what researchers are discovering is overturning many long-held assumptions about how the body works, about where mood comes from, why stress affects digestion, why digestive disorders and mental health conditions so frequently occur together, and what it actually takes to support the health of both systems simultaneously.

At Principled Chiropractic, this research connects directly to how we understand the role of the spine and nervous system in whole-body health. Because at the center of the gut-brain connection, governing the conversation between your digestive system and your brain, is a nerve whose health depends directly on the structural integrity of the upper cervical spine.

That nerve is the vagus nerve. And its story is worth telling.

The Gut-Brain Axis: A Two-Way Conversation

The gut-brain axis refers to the bidirectional communication network that exists between the gastrointestinal tract and the central nervous system. It is not a metaphor. It is an anatomical and neurochemical reality; a complex system of nerves, hormones, immune signals, and microbial metabolites that keeps the gut and brain in constant, continuous dialogue.

This dialogue flows in both directions. The brain sends signals to the gut that influence digestion, motility, enzyme secretion, immune activity, and the composition of the gut microbiome. The gut sends signals back to the brain that influence mood, stress responses, cognitive function, sleep, pain perception, and behavior.

At the heart of this communication is the vagus nerve, the longest cranial nerve that extends from the brainstem to the abdomen. About 80 to 90 percent of the nerve fibers in the vagus nerve are dedicated to carrying information from the gut to the brain, meaning the gut is not just a passive recipient of brain instructions. It is an active participant in a conversation, constantly sending information upward that shapes how the brain perceives and responds to the world.

This architecture has profound implications for health. When the gut-brain conversation is flowing clearly and without interference, both systems function well. When something disrupts that communication, whether at the level of the nervous system, the gut microbiome, or the structural pathway through which the signals travel, the effects are felt in both directions simultaneously.

The Enteric Nervous System: Your Body's Second Brain

To fully appreciate the gut-brain connection, you need to know about a part of your nervous system that most people have never heard of.

Your digestive system has its own nervous system, a vast, independent network of neurons embedded in the walls of the gastrointestinal tract from the esophagus to the rectum. It is called the enteric nervous system (ENS), and it contains over 100 million neurons that control digestive function (more neurons than are found in either the spinal cord or the peripheral nervous system).

This is why the enteric nervous system is called the "second brain." It is capable of operating independently of the brain, governing digestion, regulating gut motility, managing secretion and absorption, and coordinating the immune activity of the gut wall, all without waiting for instructions from above. With that said, it does not operate in isolation. It is in constant communication with the brain through the vagus nerve and the autonomic nervous system, and the health of one profoundly influences the other.

The Microbiome: The Third Partner in the Conversation

The gut-brain axis has a third key participant that has become one of the most intensively studied areas in all of modern medicine: the gut microbiome.

The gut microbiome consists of trillions of microorganisms, bacteria, fungi, viruses, and other microbes, that live in the gastrointestinal tract. Far from being passive residents, these microbes are active participants in the gut-brain conversation. They produce neurotransmitters, regulate immune function, influence hormone levels, and communicate directly with the enteric nervous system and the vagus nerve through chemical signals.

Perhaps the most striking finding in recent microbiome research involves serotonin, the neurotransmitter most commonly associated with mood, emotional stability, and the prevention of depression and anxiety. Most people assume serotonin is produced primarily in the brain. The research tells a very different story.

The enterochromaffin cells in the gastrointestinal system are responsible for the production of around 90% of the body's total serotonin. The brain only contains approximately 5% of the remaining serotonin.

Read that again. Ninety percent of the serotonin in your body is produced in your gut. Not your brain.

This single fact reframes the entire conversation about mood, mental health, and the body systems that support them. When we talk about serotonin deficiency as a driver of depression and anxiety, and when we talk about interventions that support serotonin availability, the health of the gut, and the nervous system that governs it, must be part of the discussion.

Approximately 90% of the body's serotonin is produced by enterochromaffin cells, the primary source of serotonin in the body, and microbial metabolites are required for its production. Serotonin is very important in the regulation of mood, anxiety, and emotional balance, and low serotonin levels have been linked to people with depression and anxiety disorders.

The gut microbiome also produces dopamine, the neurotransmitter associated with motivation, pleasure, and reward, as well as GABA, a primary inhibitory neurotransmitter whose disruption is associated with anxiety disorders. Evidence indicates that gut microbiota modulate neurochemical pathways involving serotonin, dopamine, GABA, and glutamate. The bidirectional nature of this system means that when the microbiome is disrupted, a state called dysbiosis, the neurochemical balance of the brain can be directly affected.

Increasing evidence highlights a strong, bidirectional relationship between the nervous system and the gastrointestinal tract. Disruptions, commonly referred to as dysbiosis, have been linked to various health conditions. These include not only digestive disorders like IBS and inflammatory bowel disease, but also anxiety, depression, autism spectrum disorders, ADHD, cognitive decline, and neurodegenerative diseases.

The Nervous System as the Governing Pathway

With this understanding of the gut-brain axis established, we can now look at where the spine fits and why structural health is inseparable from gut-brain health.

The gut-brain conversation does not happen spontaneously. It travels through specific anatomical pathways, and those pathways run through or adjacent to the spine. There are three primary neurological routes through which the brain and gut communicate, and each of them has a direct relationship to spinal structural integrity.

The Vagus Nerve

The vagus nerve is the primary highway of the gut-brain axis. Approximately 75% of parasympathetic nervous system signals travel along this nerve to organs such as the stomach and intestines. It governs gastric acid production, gut motility, intestinal immune function, the anti-inflammatory response, and, through its relationship with the enteric nervous system, the coordination of the entire digestive process.

The vagus nerve originates in the brainstem and travels downward through the neck, directly adjacent to the upper cervical spine. This anatomical proximity is critical. When there is a NeuroStructural Shift in the upper cervical region, a misalignment of the atlas (C1) or axis (C2) that creates tension or mechanical pressure on the surrounding structures, the vagus nerve is among the most vulnerable neurological structures in the vicinity.

Compromised vagal tone, the term for reduced vagus nerve function, has direct consequences for gut-brain communication. Fight-or-flight mode (sympathetic dominance) can slow digestion, change motility, and increase gut sensitivity. Rest-and-digest mode (parasympathetic activity, including vagus nerve signaling) supports more normal digestion.

When upper cervical NeuroStructural Shifts suppress vagal tone and push the nervous system into chronic sympathetic dominance, the digestive system bears the consequences directly. Motility slows. Gastric acid secretion becomes dysregulated. Intestinal immune function is compromised. The gut's ability to communicate clearly with the brain is diminished. And the microbiome, which depends on a well-regulated nervous system environment to maintain its healthy composition, begins to shift toward dysbiosis.

The Sympathetic Nerve Roots of the Thoracic and Lumbar Spine

The vagus nerve is not the only neurological pathway involved in gut-brain communication. The thoracic spine, specifically from T5 to T12, gives rise to the sympathetic nerve roots that innervate the stomach, small intestine, large intestine, liver, gallbladder, and pancreas. These nerves govern the sympathetic (fight-or-flight) regulation of digestive function, working in counterbalance to the parasympathetic signals delivered by the vagus nerve.

Nerve roots emerging from specific spinal levels innervate different aspects of the digestive system. When vertebral subluxations interfere with these neural pathways, digestive dysfunction can result.

When NeuroStructural Shifts in the thoracic spine create neurological interference in these sympathetic pathways, the result can be altered gastric function, impaired enzyme secretion, abnormal gut motility, and chronic digestive discomfort, even in the absence of any disease process in the gastrointestinal tract itself. The gut is functioning exactly as its nervous system is instructing it to. The problem is that its nervous system is receiving a disrupted signal.

The HPA Axis

The hypothalamic-pituitary-adrenal (HPA) axis, the body's primary stress response system, is a third critical pathway in the gut-brain conversation. The HPA axis is the major connection that exists between the gastrointestinal tract and the brain. The bacteria in the gut can control the levels of stress hormones such as cortisol through this axis.

Chronic activation of the HPA axis, which is directly influenced by the state of the nervous system, produces sustained elevation of cortisol and other stress hormones that suppress immune function, alter gut microbiome composition, increase intestinal permeability (the phenomenon sometimes called "leaky gut"), and disrupt the production of the neurotransmitters that regulate mood and behavior.

Chiropractic adjustments appear to influence hypothalamic-pituitary-adrenal axis function, potentially normalizing cortisol patterns often disrupted in depression and anxiety disorders.

By reducing the chronic sympathetic dominance and neurological dysregulation associated with NeuroStructural Shifts, chiropractic care supports a more balanced HPA axis response, with downstream benefits for the gut microbiome, serotonin production, immune regulation, and the full spectrum of gut-brain communication.

What This Means Clinically: Conditions Connected to the Gut-Brain Axis

The clinical implications of the gut-brain connection are broad and, for many patients, personally relevant. Here are some of the conditions in which gut-brain axis dysfunction, and its relationship to spinal neurological function, is particularly significant.

Irritable Bowel Syndrome (IBS)

IBS is one of the most common gastrointestinal conditions in the world, affecting an estimated 10 to 15 percent of the global population. It is characterized by chronic abdominal pain, bloating, altered bowel habits, and unpredictable digestive distress, all without any identifiable structural disease in the gut.

The gut-brain axis is central to IBS pathophysiology. IBS involves dysregulated communication between the enteric nervous system and the central nervous system, a state of visceral hypersensitivity in which the gut's sensory signals are amplified and misinterpreted by the brain. The vagus nerve, the sympathetic nerve roots, and the HPA axis all play documented roles.

Pain modulation: spinal care can reduce visceral hypersensitivity associated with IBS. By restoring structural integrity to the spine and reducing neurological interference in the pathways governing gut-brain communication, NeuroStructural chiropractic care addresses a fundamental component of IBS that no medication targeting the gut alone can reach.

Acid Reflux and GERD

Gastroesophageal reflux disease involves dysfunction of the lower esophageal sphincter, the valve that prevents stomach acid from refluxing into the esophagus. This sphincter is governed by the vagus nerve and the thoracic sympathetic nerve roots. The thoracic spine's influence on vagus nerve function makes chiropractic care particularly relevant for these conditions.

Many patients who have been managing acid reflux with proton pump inhibitors for years have never had the neurological supply to the lower esophageal sphincter evaluated from a structural standpoint. For some, the structural component is a significant contributor that has never been identified or addressed.

Anxiety, Depression, and Mood Disorders

Given that 90 percent of the body's serotonin is produced in the gut, and given the documented relationship between gut microbiome dysbiosis and mood disorders, the connection between gut health and mental health is no longer speculative, it is well-established science.

Multiple clinical studies demonstrate that spinal manipulation influences autonomic nervous system function. Vagal parasympathetic stimulation is also considered an effective therapy for major depression as it releases neurotrophins, including brain-derived neurotrophic factor and nerve growth factor.

Preliminary research suggests improved nervous system function following chiropractic care may positively affect gut microbiome composition, increasingly recognized as a key factor in mental health.

We are not claiming that chiropractic care treats depression or anxiety. We are noting, accurately, and based on peer-reviewed research, that the nervous system pathways chiropractic care supports are directly involved in the neurochemical processes that underlie mood regulation. A nervous system operating with less structural interference is a nervous system better positioned to support the gut-brain communication on which emotional wellbeing depends.

Infant Colic and Digestive Distress

The gut-brain axis is not exclusively an adult concern. Infants whose upper cervical spines have been stressed during birth often show signs of vagal tone dysregulation that manifest directly in digestive and behavioral symptoms, such as excessive crying, poor feeding, difficulty settling, and the constellation of symptoms we call colic.

As we discuss in detail in our post on infant colic, the vagus nerve plays a central role in the regulation of the infant's gut-brain connection from birth. Upper cervical structural correction that restores vagal tone in infants frequently produces rapid improvement in digestive symptoms, not because we treated the gut, but because we removed the structural interference that was preventing the nervous system from governing it properly.

Constipation and Motility Disorders

Bowel motility, the coordinated muscular contractions that move food through the digestive tract, is governed by the enteric nervous system in coordination with the vagus nerve and the sacral parasympathetic nerve roots. When any of these pathways are compromised by structural dysfunction, motility can slow, producing constipation, bloating, and the discomfort of a digestive system that is not moving as it should.

This is a particularly common finding in children with upper cervical dysfunction and in adults with lumbar or sacral NeuroStructural Shifts. Restoring structural integrity to these regions frequently produces improvements in bowel regularity that patients often mention as a secondary benefit they were not expecting.

The Gut-Brain-Spine Axis: A More Complete Picture of Health

What the research on the gut-brain axis ultimately describes is a picture of the body that is radically more integrated than the conventional medical model acknowledges.

The gut is not simply a digestive organ. It is a neurological system, one that produces the majority of the body's most important mood-regulating neurotransmitters, houses an independent nervous system containing more neurons than the spinal cord, and maintains a continuous two-way conversation with the brain that shapes everything from digestion to immune function to emotional stability.

The brain is not the sole governor of health. It is one participant in a partnership, a partnership that depends on clear, uninterrupted communication through the vagus nerve, the spinal nerve roots, and the autonomic nervous system pathways housed in and protected by the spine.

The spine is not simply a structural column. It is the architectural housing of the body's most critical communication network, a network whose integrity determines the quality of the gut-brain conversation, the health of the microbiome, and the neurochemical environment in which mood, cognition, immunity, and digestion all unfold.

This is what we mean when we say that NeuroStructural chiropractic care supports whole-body health. We are not claiming to treat your digestion or your mood directly. We are restoring the structural integrity of the system through which your body governs both (and allowing your Innate Intelligence to do what it has always been designed to do).

Practical Steps to Support Your Gut-Brain Axis

While NeuroStructural chiropractic care addresses the structural and neurological foundation of gut-brain health, there are complementary lifestyle practices that support the full system.

Eat to support the microbiome. A diet rich in diverse, whole plant foods feeds the beneficial microorganisms that produce serotonin, dopamine, and other neuroactive compounds. Fermented foods, yogurt, kefir, sauerkraut, kimchi, kombucha, etc., introduce beneficial bacteria that support microbiome diversity. Ultra-processed foods, excessive sugar, and antibiotic overuse are among the most damaging influences on microbiome health.

Prioritize sleep. Sleep quality improvement, better spinal health often correlates with improved sleep, which affects gut health. Sleep is the period during which the brain clears metabolic waste, the gut performs its overnight repair processes, and the nervous system shifts into the deep parasympathetic states that support recovery. Poor sleep disrupts microbiome composition and gut-brain signaling in measurable ways.

Manage chronic stress. Chronic psychological stress activates the HPA axis and drives sympathetic dominance, directly suppressing the parasympathetic gut-brain communication that healthy digestion requires. Practices that support nervous system regulation, including regular chiropractic care, movement, time in nature, and adequate rest, all contribute to a calmer, more balanced nervous system environment for the gut-brain axis to operate in.

Move regularly. Physical activity supports gut motility, microbiome diversity, vagal tone, and the parasympathetic nervous system states that enable healthy digestion. Even moderate regular walking has documented benefits for gut-brain health.

Consider your relationship with antibiotics. Antibiotics are sometimes necessary and lifesaving. They also broadly suppress the gut microbiome, not just the pathogens they target, but the beneficial bacteria that produce neurotransmitters and support the gut-brain axis. When antibiotics are necessary, probiotic support during and after the course can help protect microbiome diversity.

A Different Way of Thinking About Your Health

The gut-brain connection invites us to think about health differently, not as a collection of separate systems to be managed in isolation, but as an integrated whole whose parts are in continuous, dynamic communication.

When the spine is structurally sound, the nervous system can govern that communication clearly. The vagus nerve can regulate the gut with its full parasympathetic tone. The enteric nervous system can operate in a well-regulated state. The microbiome can thrive. Serotonin can be produced in the quantities the brain needs to maintain mood, emotional resilience, and cognitive clarity. The immune system can regulate inflammation effectively, and the whole elegant system can do what it was designed to do; maintain health, adapt to challenge, and express the full vitality of a body functioning without interference.

This is what we are working toward with every patient at Principled Chiropractic. Not the elimination of symptoms, though that is often a welcome result, but the restoration of a structurally sound foundation from which the body's own intelligence can govern health as completely as it is designed to.

If you have been living with chronic digestive symptoms, mood challenges, or the sense that your body simply is not functioning as well as it should, and if you have never had your spine and nervous system evaluated from a structural standpoint, we invite you to start that conversation with us.