The Gut-Brain Axis

Updated: Mar 16



Written by health writer Gerda Venter
Medically reviewed by Dr. Daniela Steyn


The gut can also be referred to as the body’s “second brain.” The reason for this is that the gut creates so many neuro-transmitting chemicals responsible for the way we feel. Connecting the gut and the brain might sound a little dramatic, but if you consider that we often refer to a feeling as having a “gut feel,” research has shown a definite relationship.


The gut-brain axis describes the two-way connection and communication between the gut and the brain. The study of the gut-brain axis is one of the most cutting-edge areas of research to date. We now better understand how the connection can impact physical and digestive issues and mental health conditions like depression and anxiety.


How does the gut-brain axis work?

In short, the gut and the brain are connected through the largest nerve in the body, called the Vagus nerve. The gut and the brain communicate through this nerve connection that runs from your brainstem to your intestines. All the microbes in the gut have access to that enteric nervous system via the Vagus nerve. This nerve also gives them direct access to the brain and vice versa. All-day long, they talk to each other and relay messages back and forth.

In addition to the Vagus nerve, other pathways involved in the complex functioning of the gut-brain axis exist. These pathways include communication through various chemical messengers, the endocrine system, gut hormones, and neurotransmitters. Many of our hormones and neurotransmitters are made in the gut. Some examples are gamma-aminobutyric acid (GABA), dopamine and serotonin. Therefore, what is happening in the gut can directly influence our brain function and behavior.


Together, the gut-brain axis is a complex interconnected circuit. This means that when an issue arises at any point within these communication loops, it can affect the whole system. Several health conditions, such as IBS, depression, anxiety, obesity, autism, and more, may arise when this happens. Keeping the gut microbiota healthy will lead to a healthy brain and subsequently a healthy mind and body.



The Gut-Brain Axis and Digestive Disorders

Did you know that only 10% of your body is controlled by your cells and DNA? The other 90% of your functioning comes from the trillions of microbes living in and on you. This microbial ecosystem is called the microbiome. The microbiome includes viruses, bacteria, protozoa, and all kinds of living organisms. It encompasses all the microorganisms and their genetic elements in and on their host (us). The gut has the highest concentration of microbes in the entire body. Microbes have genetic factors that regulate all sorts of functions, including hormone production, metabolic activity, appetite, mood, and sleep.


Whenever changes in the communication between the gut, brain and gut microbiota occur, you are at risk of developing digestive disorders like Irritable Bowel Syndrome (IBS). Changes in communication might happen when you go through a period of high stress, too little sleep, or bad eating habits.



When under stress, the body’s in-built fight-or-flight response is activated. The fight-or-flight response is the body’s natural survival reaction that occurs in response to a perceived threat. When activated, digestion slows as the body uses its energy resources on the danger at hand, increasing gut sensitivity. As a result, the gut function is altered, and IBS gut symptoms of bloating, stomach pain, wind and altered bowel habits are worsened. Having a healthy gut is therefore crucial for a healthy life.


The gut-brain axis and sleep

If you are not sleeping well at night, even though you are desperately tired, the bacteria in your gut could be to blame. Many neurotransmitters that influence sleep are made and released by the microbiome, including dopamine, serotonin, and the well-known sleep-inducing hormone melatonin. While the brain also makes some of these neurotransmitters, 400 times more melatonin are produced in the gut than in the brain. If you have low levels of certain types of bacteria in your gut, your body won’t be able to produce enough melatonin to help you fall asleep at night.

An interesting factor about sleep and the microbiome is that 70% of your immune system is in your gut. This immune tissue controls the inflammatory response in the body. It is essential to mention that not all inflammation is harmful. Certain types of inflammatory markers are crucial for getting you to sleep. One example would be when you are feeling sick. In this instance, the inflammatory markers Interleukin 1 Beta and Interleukin 6 will increase, and in turn, you will feel tired and want to rest. These same markers are released in higher concentrations when the sun goes down. Microbes in your gut increase these markers to make you sleepy at night and have the benefit of activating your immune system so that it goes into the repair and patrolling mode while you are asleep.

These inflammatory markers are supposed to decrease after sleep. When the microbiome is dysfunctional, however, your inflammatory response will increase when you start getting tired, but then your gut won’t have the right microbes to bring it back down throughout the night. The result is a net accumulation of chronic inflammation over time. Adults need seven to eight hours of sleep a night to allow this entire rise and fall of inflammation to occur.


A good night’s rest and gut health go together. When you don’t get enough sleep, your microbiome suffers. Likewise, when your microbiome is compromised, you become more susceptible to gut disorders like leaky gut and IBS. You also produce less serotonin, your immune system suffers, and the cycle continues as it becomes harder to get enough sleep.


Gut-brain axis and mental wellness

Healthy gut microbiota does not only influence digestive disorders and sleep, but it also regulates the brain and behavior. It influences normal brain function and is involved in many psychiatric conditions, including depression and anxiety. There is, for example, a reduction in both the diversity and total amount of healthy microbes in the gut of a person who suffers from depression.


Traditionally depression was thought to be just a chemical imbalance in the brain, or something strictly emotionally rooted. We now know the cause is much more complex and is likely a combination of genetic, psychological, physiological, and inflammatory factors.


Because 90% of the chemical serotonin (our happy hormone) is made in the gut and only 10% in the brain, scientists now believe that gut health should be the primary focus of depression treatment in addition to prescribed antidepressants.


Treatment Options

Several treatment options have successfully addressed the dysfunction of the gut-brain (and microbiome) axis. Treatment includes:


1. Psychological Therapies

Psychological therapies mainly focus on strategies to manage and reduce the stress response. Examples of this would be:

  • Deep breathing techniques

  • Cognitive behavioral therapy

  • Gut-directed hypnotherapy

These therapies focus on calming the mind and eliciting the body’s relaxed state, which reduces the activation of the fight-or-flight response, and in turn, improves mood and gut function.


2. Diet Therapy

Diet is also used successfully towards improving both mood and gut conditions. For example, a Mediterranean diet characterized by a high fruit and vegetable intake, healthy fats such as olive oil and fish, whole grains, legumes, and nuts, has been shown to increase the diversity of gut microbiota, which has a direct impact on the gut-brain-microbiome axis.


Another diet, called the low FODMAP diet, restricts fermentable carbohydrates to reduce gut symptoms.



3. Probiotics

Probiotics are live microorganisms that confer a health benefit on the host. We can consume probiotics through foods like yogurt, kefir, and sauerkraut, to name a few. Probiotic supplements are also a great way to ensure you have enough healthy bacteria working in your gut.


4. Prebiotics

Prebiotics are substrate that is selectively utilized by host microorganisms conferring a health benefit. Essentially, they are food or fuel for the healthy gut microbiota to help them grow and thrive in the gut. Improving the numbers and function of the gut microbiota through a high intake of prebiotics is a way that directly influences the gut-brain-microbiome axis and consequently gut health, behavior, and mood. Examples of foods with a high prebiotic concentration are garlic, legumes, onions, dandelion, asparagus, oats, apples and many more.


5. Improve your overall gut health

Improving your overall gut health might be the best treatment option as it will ultimately restore your microbiota and eventually lead to a healthy body and mind. Improve your gut health by:

  • Removing all food toxins from your diet

  • Maximizing your digestive capacity using supplemental acid and enzymes

  • Eating plenty of fermentable fibers (starches like sweet potato, yam, yucca, etc.)

  • Eating fermented foods like kefir, yogurt, sauerkraut, kimchi, etc., and taking a high quality, multi-species probiotic

  • Treating any intestinal pathogens (such as parasites) that may be present

  • Taking steps to manage your stress



A healthy gut and brain are crucial if you want to live a happy, healthy life. If the bacteria in your gut are healthy, it can help relay messages about when you’re full, when you need to take care of yourself, and when it’s time for bed to your brain. If it is not healthy, it can make you crave sugar, feel anxious and depressed, or keep you awake when you are desperate for some rest.



Reference List

1. Probiotics and Sleep: Ways Your Gut Health is Keeping You Up. https://www.sleepadvisor.org/sleep-and-microbiome/

2. How to get the most out of OT. https://www.adaptableoccupationaltherapy.com/post/how-to-get-the-most-out-of-ot

3. The Gut-Brain Axis Explained in Plain English | Diet vs .... https://www.dietvsdisease.org/gut-brain-axis/

4. Aktas, Busra, et al. “The Impact of Lactobacillus Casei on the Composition of the Cecal Microbiota and Innate Immune System Is Strain Specific.” PLoS One, vol. 11, no. 5, Public Library of Science, May 2016, p. e0156374.

5. Pathogens | Free Full-Text | Gut Microbiota Manipulation .... https://www.mdpi.com/2076-0817/10/9/1137/html

6. Leaky gut eating disorders Archives - Eating Disorder Pro. https://www.eatingdisorderpro.com/tag/leaky-gut-eating-disorders/



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