If you’ve ever had a big decision to make, you’ve probably been told to go with your gut. It may just be a figure of speech, but that figure of speech is more scientifically accurate than you might realize.
It turns out, there’s a pretty significant communication network between the gastrointestinal tract and the brain.
What does gut-brain axis mean? The gut-brain axis refers to the system responsible for coordinating activity between the gut and the brain. This system regulates many of the body’s functions, including digestion, metabolism, immunity, and mood. It’s a hot topic in modern gastroenterology.
The gut-brain axis is closely linked to the body’s stress response system, and the gut microbiome can influence mood and behavior. Growing evidence shows that this system plays a vital role in overall wellbeing. Maintaining a healthy gut is essential for optimal health and wellbeing.
There are a lot of moving parts to this connection. Let’s take a look at all the elements involved.
Also called the microbiota-gut-brain axis, the gut-brain axis is a two-way street. You already know the brain influences gut function. But we’re interested in how the gut impacts cognitive function and brain development. The GI tract is home to trillions of bacteria, which can influence the brain by producing neurotransmitters and other signaling molecules.
The 100,000 trillion microorganisms that occupy the human gastrointestinal system are known as intestinal microbiota, or the gut microbiome. Many of these bacterial species are beneficial and help to:
- Digest food
- Absorb nutrients
- Synthesize vitamins
- Produce building blocks for hormones and neurotransmitters
- Fight harmful bacteria
- Regulate immune function
- Maintain homeostasis
- Aid metabolic pathways
Some conditions linked to imbalanced gut bacteria include:
- Type 2 diabetes
- Inflammatory bowel disease
- Cardiovascular disease
- Mood disorders
Nerves: Your Body’s Communication System
The gut-brain axis involves two-way communication between the central nervous system (CNS) and the enteric nervous system (ENS). We won’t get into all the different types of nervous system activity, but here’s what you need to know for our purposes:
The CNS is the brain and spinal cord, and you probably already know what they do. The ENS is a network of neurons extending from the esophagus to the rectum that controls the motility and secretions of the gastrointestinal (GI) tract.
These two systems operate independently. In fact, the ENS is often referred to as the “brain in the gut” because it can sense, process, and respond to stimuli without any CNS involvement.
The vagus nerve is the longest cranial nerve in the body, extending from the brainstem to the abdomen. It’s a key player in the autonomic nervous system — the part of the ENS that controls the body’s involuntary functions like:
- Heart rate
- Respiratory rate
- Vasomotor activity (sending blood to various parts of the body)
- Reflex actions like coughing, sneezing, swallowing, & vomiting
The vagus nerve is the main communication pathway between the brain and the gut. It is a major component of the parasympathetic nervous system, which regulates the body’s relaxation response.
The 100 billion or so neurons in the vagal connection are more important to brain function than most people realize. The vagus nerve plays a role in mental health and may be involved in conditions like anxiety and depression. In fact, experts in psychiatry have been conducting research stimulating the vagus nerve to treat some mental health conditions.
Neurotransmitters Start in Your Gut
Neurotransmitters are chemicals that help relay messages between nerve cells. They are essential for proper brain function. What you may not know is that many of these neurotransmitters are produced by your gut microflora. Some of these include:
- Serotonin: responsible for regulating mood, sleep, and appetite. Serotonin is the neurotransmitter that makes you happy. Approximately 90% of the body’s serotonin is produced in the gut.
- Dopamine: responsible for regulating movement and motivation. Dopamine is a neuroendocrine hormone. About 50% of the body’s dopamine is produced in the gut.
- GABA (gamma-aminobutyric acid): responsible for regulating anxiety and stress. GABA is responsible for reducing anxiety and promoting relaxation. It has a calming effect.
- Norepinepherine: responsible for increasing blood pressure and heart rate. Norepinepherine is a neuroendocrine hormone involved in the fight-or-flight response, and is released in response to stress.
Gut Microbe Byproducts
Gut microbe byproducts, such as microbial metabolites, are small molecules produced by gut bacteria and play an important role in health. They are involved in various physiological processes, including metabolism, immunity, and gut-brain communication.
These molecules include:
- Short-chain fatty acids (SCFAs)
- Amino acids
Gut microbes produce a variety of metabolites that can be either beneficial or harmful to their host. The most well-known beneficial metabolites are SCFAs. These are produced by fermentation of dietary fiber and play an important role in gut health. SCFAs have been shown to promote gut barrier function, reduce inflammation, and help immune response modulation.
Note that healthy byproducts require healthy microbiota composition. Harmful gut microbes produce metabolites that can be harmful to their host. Some microbes have virulent factors that can damage the gut barrier and lead to inflammation. Some microbes can also produce toxins that can cause infections or other health problems.
Your Gut Affects Your Hormones
The gut microbiome significantly impacts the endocrine system, which is responsible for hormones. According to recent studies, the gut microbiome can influence the levels of several hormones, including:
- Ghrelin: the hunger hormone Ghrelin regulates appetite and energy balance.
- Leptin: the satiety hormone Leptin regulates energy intake and expenditure. In addition to its role in regulating energy balance, leptin also modulates a number of other physiological processes, including appetite, body weight, metabolism, and immune function.
- Insulin: the fat-storage hormone Insulin is a hormone released by the pancreas in response to high blood sugar levels. It helps move glucose from blood into cells, where it is used for energy.
- Glucagon-like peptide 1 (GLP-1) GLP-1 helps regulate blood sugar levels. It is released in response to food intake and stimulates insulin release from the pancreas. It also inhibits glucagon, a hormone that raises blood sugar levels.
Additionally, the gut microbiome may impact stress hormones (such as cortisol) and sex hormones (such as testosterone and estrogen).
When the gut microbiome is unhealthy, it can lead to hormone imbalance. This can lead to impairment of your body’s ability to absorb nutrients, regulate blood sugar, and store fat.
The Immune System Depends on a Healthy Gut
The gut microbiome has a huge impact on pathogenesis. It helps to train the immune system and to protect against pathogens. Gut bacteria can influence the development and function of immune cells, affecting how the body fights infection.
The gut microbiome also produces short-chain fatty acids that have anti-inflammatory effects. It may also be involved in regulating hypothalamic inflammation by impacting cytokine levels.
Cytokines are a type of protein secreted by cells in the immune system. They help to regulate immune response and fight neuroinflammation.
Microbial dysbiosis (an imbalance in the microbiome) has been linked to a number of autoimmune diseases, including:
- Inflammatory bowel disease
- Ulcerative colitis
- Crohn’s disease
- Rheumatoid arthritis
- Multiple sclerosis
Other conditions linked to dysbiosis include:
- Type 2 diabetes
- Alzheimer’s disease
- Parkinson’s disease
- Autism spectrum disorder
Dietary factors greatly impact your gut. Dysbiosis can occur if your diet includes:
- High fats
- Lots of processed foods
- Too much sugar
- Not enough fiber
- No probiotics
As you can see, it is important to maintain a healthy diet to promote gut and overall health.
What is “Leaky Gut?”
Intestinal permeability, or “leaky gut,” is a condition in which the barrier that separates the inside of the gut from the rest of the body becomes damaged, allowing bacteria and other toxins to pass through into the bloodstream. This can lead to a variety of health problems, including excess inflammation, autoimmune disease, and malnutrition.
Leaky gut is implicated with gastrointestinal dysfunction, including:
- Irritable bowel syndrome (IBS)
- Abdominal pain
Much like the immune system, the health of your gut flora can contribute to intestinal permeability. A diet high in processed foods and sugar can promote the growth of harmful bacteria and increase inflammation. Meanwhile, a diet rich in fiber and fermentable carbohydrates can help to maintain a healthy gut microbiota.
Other factors that may impact your intestinal permeability include stress and certain medications. Chronic stress can lead to the overgrowth of harmful bacteria and the depletion of beneficial microbes.
Certain medications, such as antibiotics and birth control pills, can also disrupt the gut microbiota and lead to increased intestinal permeability.
How Prebiotics and Probiotics Help
Gut health depends on how much and which types of gut bacteria are present. Your diet is a huge determining factor in this.
A diet high in fiber, fermentable carbs, and probiotic-rich foods, can help to maintain a healthy gut microbiota and even help heal an unhealthy gut.
Probiotics, such as bifidobacterium and lactobacillus, are live microorganisms that can help to restore balance. They live in the digestive tract and help to break down food, absorb nutrients, and keep the gut free of harmful bacteria such as firmicutes.
Examples of probiotic foods include:
Prebiotics are non-digestible food ingredients that promote the growth of beneficial bacteria in the digestive system. Prebiotic foods generally contain high amounts of dietary fiber and fermentable carbs.
Examples of prebiotic foods include:
- Chicory root
- Dandelion greens
Other foods to consider for a healthy gut microbiome include:
- Foods rich in tryptophan. Tryptophan is an amino acid precursor to serotonin. There is some evidence to suggest that tryptophan can help improve gut motility and reduce inflammation, both of which are important for maintaining a healthy gut microbiome.
- Foods rich in polyphenols. Polyphenols are a class of phytochemicals found in a variety of plant-based foods. They are known for their antioxidant and anti-inflammatory properties. Polyphenols modulate the composition and activity of the gut microbiota and reduce inflammation and oxidative stress.
- Whole foods over processed foods. Processed foods tend to include refined carbs and processed fats, which aren’t great for gut health. Eating whole foods, like fruit, vegetables, whole grains and healthy fats, can help promote healthy gut bacteria.
Have more questions?
The gut-brain axis is a complex system, and we’re still learning. What we do know is our gut can influence our mood, anxiety levels, immune system, and overall well-being. This is why it is so important to take care of our guts.
If you’re struggling with any of the conditions listed and want to learn about your options, we’re here to help. We’ll partner with you to find treatment plans and interventions to improve your health.
- Carabotti, M., Scirocco, A., Maselli, M. A., & Severi, C. (2015). The gut-brain axis: interactions between enteric microbiota, central and enteric nervous systems. Annals of gastroenterology: quarterly publication of the Hellenic Society of Gastroenterology, 28(2), 203. Full text:
- Clapp, M., Aurora, N., Herrera, L., Bhatia, M., Wilen, E., & Wakefield, S. (2017). Gut microbiota’s effect on mental health: The gut-brain axis. Clinics and practice, 7(4), 131-136. Full text:
- Galland, L. (2014). The gut microbiome and the brain. Journal of medicinal food, 17(12), 1261-1272. Full text:
Zheng, D., Liwinski, T., & Elinav, E. (2020). Interaction between microbiota and immunity in health and disease. Cell research, 30(6), 492-506. Full text:
- Rueckert, H., & Ganz, J. (2022). How to Heal the Gut’s Brain: Regeneration of the Enteric Nervous System. International journal of molecular sciences, 23(9), 4799.
- Breit, S., Kupferberg, A., Rogler, G., & Hasler, G. (2018). Vagus nerve as modulator of the brain–gut axis in psychiatric and inflammatory disorders. Frontiers in psychiatry, 44. Full text:
- Martin, A. M., Sun, E. W., Rogers, G. B., & Keating, D. J. (2019). The influence of the gut microbiome on host metabolism through the regulation of gut hormone release. Frontiers in Physiology, 10, 428.
- Prakash, S., Rodes, L., Coussa-Charley, M., & Tomaro-Duchesneau, C. (2011). Gut microbiota: next frontier in understanding human health and development of biotherapeutics. Biologics: targets & therapy, 5, 71.
- Belizário, J. E., & Faintuch, J. (2018). Microbiome and gut dysbiosis. In Metabolic interaction in infection (pp. 459-476).
- Camilleri, M. (2019). Leaky gut: mechanisms, measurement and clinical implications in humans. Gut, 68(8), 1516-1526. Full text:
- Sanders, M. E., Merenstein, D. J., Reid, G., Gibson, G. R., & Rastall, R. A. (2019). Probiotics and prebiotics in intestinal health and disease: from biology to the clinic. Nature reviews Gastroenterology & hepatology, 16(10), 605-616. Abstract: