Definition and Overview
The gut-brain axis is a bidirectional signaling system formed between the gastrointestinal tract and the central nervous system. Through this pathway, the brain regulates intestinal motility, secretion, and immune functions, and the intestine transmits information about its own state to the brain, affecting mood, cognition, and stress response.
The concept of the gut-brain axis originated in the 19th century when William Beaumont reported that emotions affect stomach function through observations of gastrostomy patients. Later, as the enteric nervous system was identified as an independent neural network, the concept of the “second brain” took root. There are approximately 500 million neurons in the enteric nervous system, which is similar to the number of neurons in the spinal cord.
Recently, as the role of gut microbiota has been discovered, the gut-brain axis has been expanded into the microbiota-gut-brain axis. About 100 trillion microorganisms live in the human intestine, and their total number of genes is 150 times that of human genes. Research has confirmed that these microbial communities are directly involved in brain function through neurotransmitter production, immune regulation, and maintenance of barrier function.
component
The gut-brain axis operates through four main pathways: Its components include the vagal pathway, hypothalamic-pituitary-adrenal (HPA) axis, immune system, and gut microbiota.
vagus nerve pathway
The vagus nerve is the most direct physical connection to the gut-brain axis. The 10th cranial nerve, the vagus nerve, runs from the brain stem to the abdomen and transmits intestinal mechanical stimulation, chemical signals, and microbial metabolite information to the brain.
Approximately 80% of the vagus nerve fibers are afferent fibers, an ascending pathway that sends information from the gut to the brain. The remaining approximately 20% are efferent fibers that transmit commands from the brain to the intestines. This means that in the gut-brain axis, the amount of information the gut sends to the brain is much greater than the amount of information the brain sends to the gut.
In a 2011 study by Bravo et al., anxiety behavior was reduced and brain GABA receptor expression was changed in mice administered Lactobacillus rhamnosus, but these effects were lost in mice whose vagus nerve was cut. This result is direct evidence that regulation of brain function by intestinal bacteria is achieved through the vagus nerve.
Hypothalamic-pituitary-adrenal (HPA) axis
The hypothalamic-pituitary-adrenal axis (HPA axis) is a key endocrine pathway in the stress response. When stressed, corticotropin-releasing hormone (CRH) is secreted from the hypothalamus, followed by adrenocorticotropic hormone (ACTH) from the pituitary gland, and cortisol from the adrenal gland.
Cortisol increases the permeability of the intestinal mucosa and changes the composition of the intestinal microflora. When the so-called “leaky gut” phenomenon occurs, endotoxin (lipopolysaccharide (LPS)) from gut bacteria can enter the bloodstream and trigger a systemic inflammatory response. In animal experiments, it was reported that germ-free mice showed an exaggerated HPA axis response compared to normal mice, and that this excessive response was normalized after colonization with Bifidobacterium.
immune system pathways
The intestine is the largest immune organ where approximately 70% of the body's immune cells reside. Gut-associated lymphoid tissue (GALT) continuously interacts with intestinal microorganisms and maintains immune homeostasis.
When intestinal dysbiosis occurs, the secretion of inflammatory cytokines (TNF-α, IL-6, IL-1β) increases. These inflammatory mediators can reach the brain through the bloodstream or activate afferent fibers of the vagus nerve, causing neuroinflammation in the brain. There is accumulating evidence that chronic low-grade inflammation contributes to the development of depression and anxiety disorders.
intestinal microorganisms
The community of approximately 100 trillion microorganisms living in the human intestine is called the gut microbiota, and the entire genetic information is called the microbiome. Adult intestinal microorganisms mainly consist of Firmicutes and Bacteroidetes, with these two phylums accounting for approximately 90% of the total.
Intestinal microorganisms go beyond simple digestive aids and participate in gut-brain axis communication through various pathways, including neurotransmitter synthesis, production of short-chain fatty acids, maintenance of barrier function, and immune regulation. Reduced diversity of the microbial community is known to be a risk factor for gut-brain axis dysfunction.
The role of the vagus nerve
The vagus nerve is the main information highway of the gut-brain axis. The afferent fiber terminals of the vagus nerve are widely distributed in the lower layer of the intestinal mucosa, and transmit information such as intestinal nutrients, pH, osmotic pressure, and microbial metabolites in real time to the nucleus tractus solitarius of the brain stem.
Information arriving at the nucleus tractus solitarius is projected to higher brain centers such as the hypothalamus, amygdala, and prefrontal cortex. The hypothalamus is responsible for appetite and energy homeostasis, the amygdala is responsible for anxiety and fear responses, and the prefrontal cortex is responsible for decision-making and emotional regulation. Through this pathway, the state of your gut affects your mood, appetite, and stress response.
A 2018 study by Bonaz et al. found that individuals with low vagal tone have impaired gut-brain axis communication and are at increased risk of developing irritable bowel syndrome, inflammatory bowel disease, and functional dyspepsia. Vagal tone can be measured indirectly through the high-frequency component of heart rate variability (HRV), and decreased HRV is an indicator of gut-brain axis dysfunction.
Efferent fibers of the vagus nerve form the cholinergic anti-inflammatory pathway. Through this pathway, the vagus nerve suppresses the intestinal inflammatory response, and vagus nerve stimulation significantly reduces the secretion of inflammatory cytokines.
Gut microbiota and the brain
Serotonin production
Serotonin (5-HT) is a key neurotransmitter in mood regulation. About 95% of the body's serotonin is produced by enterochromaffin cells in the intestines, not the brain. In a 2015 Cell paper by Yano et al., it was confirmed that intestinal spore-forming bacteria promote serotonin synthesis in enterochromaffin cells, and that in germ-free mice, blood serotonin concentration decreases to about 60% of that in normal mice.
Serotonin produced in the intestines does not directly pass through the blood-brain barrier, but serotonin signals from the enteric nervous system are transmitted to the brain through the vagus nerve, and intestinal microorganisms are also involved in the metabolism of tryptophan, a serotonin precursor, indirectly affecting serotonin synthesis in the brain.
short chain fatty acids
Intestinal bacteria ferment dietary fiber and produce short-chain fatty acids (SCFAs). Representative examples include acetate, propionate, and butyrate.
Butyrate is the main energy source for intestinal epithelial cells and prevents increased intestinal permeability by strengthening the tight junctions of the intestinal wall. Propionate and acetate can cross the blood-brain barrier and reach the brain, and are involved in the maturation and function of brain microglia. In animal experiments, the effect of short-chain fatty acid administration was observed to reduce anxiety behavior and normalize HPA axis responses.
Other neuroactive substances
In addition to serotonin, intestinal microorganisms produce or are involved in the synthesis of neurotransmitters such as GABA, dopamine, norepinephrine, and acetylcholine. Certain Lactobacillus species produce GABA directly, and Bifidobacterium species contribute to serotonin synthesis by regulating tryptophan metabolism.
related diseases
Irritable Bowel Syndrome (IBS)
Irritable bowel syndrome (IBS) is a representative disease of gut-brain axis dysfunction. Approximately 11% of the world's population suffers from IBS, and reduced intestinal microbial diversity, increased visceral hypersensitivity, and decreased vagal tone are commonly observed in these patients. Approximately 60% of IBS patients have anxiety disorders or depression, reflecting bidirectional abnormalities in the gut-brain axis.
depression and anxiety disorders
In an analysis of the intestinal microorganisms of depressed patients, a decrease in beneficial bacteria such as Lactobacillus and Bifidobacterium and an increase in certain harmful bacteria were reported. A 2013 study by Foster & Neufeld found that exaggerated stress responses and anxious behavior were observed in germ-free mice, and transplantation of normal gut bacteria corrected these behavioral changes. Randomized controlled trials using probiotics reported significant improvements in anxiety and depression scores after 4 to 8 weeks of use.
Parkinson's disease
The "Braak hypothesis", which states that alpha-synuclein aggregates, which are a characteristic pathological finding of Parkinson's disease, are first discovered in the enteric nervous system and then spread to the brain stem through the vagus nerve, is being studied. In approximately 80% of Parkinson's disease patients, gastrointestinal symptoms such as constipation and delayed gastric emptying precede the onset of motor symptoms by 10 to 20 years. Epidemiological studies have reported that the incidence of Parkinson's disease in patients who underwent vagotomy is lower than that in the control group, but additional large-scale studies are needed.
Other related diseases
Gut-brain axis dysfunction is being studied for its relationship with autism spectrum disorder, chronic fatigue syndrome, fibromyalgia, and multiple sclerosis. The prevalence of gastrointestinal symptoms in children with autism spectrum disorder is approximately four times that of general children, and differences in the composition of intestinal microorganisms have been reported.
Diagnosis and Evaluation
No single test method has yet been established to directly identify gut-brain axis dysfunction. Currently, indirect evaluation is performed by combining several tests.
- Heart rate variability (HRV) analysis: a surrogate indicator of vagal tone. Decreased high frequency (HF) components suggest decreased vagal function and impaired gut-brain axis communication.
- Autonomic nerve function test: Autonomic nerve balance is assessed through deep breathing tests, Valsalva maneuvers, and tilt-table tests.
- Intestinal microbiome analysis: Confirm the composition and diversity of intestinal microorganisms through 16S rRNA gene analysis or metagenomics analysis using stool samples.
- Intestinal permeability test: The lactulose-mannitol test can be used to evaluate changes in the permeability of the intestinal wall.
- Inflammatory markers in the blood: C-reactive protein (CRP), cytokine levels, etc. are used to identify systemic inflammation.
- Quantitative electroencephalography (QEEG): Objectively evaluates changes in brain function and analyzes the correlation with autonomic dysfunction.
Treatment and Management
probiotics
Research is accumulating that certain probiotic strains have positive effects on brain function through the gut-brain axis. These strains are also called “psychobiotics.”
In a study by Bravo et al., Lactobacillus rhamnosus (L. rhamnosus JB-1) administration changed brain GABA receptor expression and decreased corticosterone levels. In a randomized controlled trial on humans, there was a report that depression and anxiety scores were significantly reduced compared to the placebo group after 4 weeks of taking a Lactobacillus and Bifidobacterium combination preparation. However, since the effect is strain-specific, it is advisable to select an appropriate strain after consulting with an expert rather than taking probiotics indiscriminately.
Dietary Management
Dietary fiber is the main source of nutrients for beneficial intestinal bacteria and promotes the production of short-chain fatty acids. It is recommended to consume at least 25 to 30 g of various dietary fiber sources, such as vegetables, fruits, whole grains, and beans, per day. Fermented foods (kimchi, soybean paste, yogurt, kefir) are natural sources of probiotics.
The Mediterranean diet is associated with increased gut microbial diversity and anti-inflammatory effects, and in an observational study, the risk of depression was approximately 33% lower in the group with high adherence to the Mediterranean diet. On the other hand, Western-style high-fat diets, artificial sweeteners, and processed foods can cause intestinal microbial imbalance.
Vagus nerve activation
Here are ways to improve gut-brain axis communication by increasing vagus nerve tone:
- Slow abdominal breathing: Breathing 6 times per minute (4 seconds for inhalation, 6 seconds for exhalation) directly increases vagus nerve activity. There is a study showing that HRV significantly increased after 4 weeks of slow breathing training.
- Regular exercise: Moderate aerobic exercise for more than 30 minutes, 3 to 5 times a week, increases vagal tone and increases intestinal microbial diversity.
- Sufficient sleep: Regular sleep of 7 to 8 hours is essential for parasympathetic nerve recovery and intestinal mucosa regeneration.
- Stress Management: Chronic stress continues to activate the HPA axis, increasing barrier permeability. Meditation, relaxation training, yoga, etc. help activate the vagus nerve.
Drugs and Neuromodulation Treatment
- Gut-targeted antidepressants: Low-dose tricyclic antidepressants (such as amitriptyline) reduce visceral irritability and pain in IBS and act in both directions of the gut-brain axis.
- Vagus Nerve Stimulation (VNS): An approved treatment for refractory epilepsy and drug-resistant depression, research is also underway to improve gut-brain axis function.
- Transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS): Non-invasive brain stimulation treatments are being studied for their effectiveness in indirectly improving gut-brain axis function by restoring autonomic balance.