Your Guide to the Gut-Brain Axis

Published on: Aug 27 2018

Long before we knew about a gut-brain axis, people have experienced events that impact both their brain and their bowels. For me, one I recall quite vividly is waiting for my name to be announced at a public speaking event. While I waited, all that I could think of was “should I fight it out in the green room or flight to the wash room”. The competing feelings of pressure to perform or or the physical pressure that urges us to run to the wash room has intrigued many researchers to investigate if stress affects the stomach or vice versa. Today we know that there is a connection. So, can we alter the brain through the bowel?

Key takeaways:

-The gut is your “second brain”

-A dysfunctional gut microbiome can be the root cause of anxiety

-Highly processed, low fiber foods disrupt microbiome balance and consuming psychobiotics and fermented products such as yoghurt and kombucha can confer protection against mild anxiety

Gut and brain connection illustration

We have “two” brains!

Situated in your head is the seat of the Central Nervous system (CNS) made up of the brain and spinal cord. It is rightly called the “first brain” due to its primary role in controlling body functions.  Also, situated in the bowel is the Enteric Nervous System (ENS), a nine meter network from your esophagus to the anus. The ENS and CNS are formed from the same embryonic tissue, share similar neuronal networks, and hormones. The gut brain produces the same amount of dopamine (‘high’ hormone) and 90% of serotonin (‘happy’ hormone) compared to the first brain. The gut brain can work independently, and is responsible for 70% of the body’s immune system which earns it the right to be called the “second brain”.

Brains chat!

Not only do we have two ‘brains’, but there is also a bidirectional communication line between them  through the vagus nerve. The vagus nerve is the main nerve in our body linking the brain with organs like the digestive tract, heart, and lungs.

Let us go back to the green room situation and learn how the brain talks to the gut. The anxiety ridden thought of speaking in front of an audience caused the CNS to release the stress hormone ‘cortisol’. The primitive function of cortisol is to prepare us to flee in the face of danger, thus increasing the blood flow to the limb muscles. In turn, the blood flow to the ENS reduces, the colon contracts in protest, and the digested waste moves along the colon quickly, leading to the quest for the nearest washroom.

Now, the gut brain also talks to the head brain chiefly to regulate the ‘eat or starve” equation. The hormone ghrelin is released in the stomach, signals hunger. After a meal, the satiety hormone leptin indicates to stop eating. When we are under stress the gut releases dopamine. No wonder the glazed donut feels so tempting when we have a deadline to meet!

Man with microbe illustrations

The Second brain is bugged!

We harbor a large microbial ecosystem on and in our body that outnumber the human genes by factor of 100. An average person has approximately 1.5 kilograms of gut bacteria called the microbiome. This begs the question “Are we less human and more microbial?” There is now a significant body of evidence that indicate that physiological and psychological disorders (anxiety and depression) are influenced by the gut microbiota.


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“Feel good” hormones are in the gut!

40–60% of patients with functional gastrointestinal disorders experience psychiatric symptoms and up to 50% of psychiatric patients are diagnosed with irritable bowel syndrome (IBS). Patients suffering from IBS have spells of anxiety, and are treated with antidepressants that increases serotonin availability. Serotonin is a hormone implicated in regulating mood in addition to gut motility and is produced in the gut enterochromaffin cells (EC cells). Alterations in serotonin content and EC cell numbers have been observed in IBD patients. Surprisingly, patients treated with antidepressants showed improved IBD relapse rates and reduced use of medications.  Antidepressants modulate the serotonin content by slowing down the secreted serotonin reuptake in the brain. The finding that gut microbes can modulate serotonin levels raises the interesting prospect of using them to drive changes in mood and alleviate anxiety.

Manipulate the mind with microbes!

Woman looking forlorn out of windowIn healthy conditions, the tight junctions in the gut inner layer and mucus layer form a physical barrier to bacteria and antigens. Factors like antibiotic treatment, poor diet, infection as well as stress can disrupt the intestinal barrier (leaky gut) and affect mucosal permeability. Pathogens that had a restricted entry before can now easily pass across the mucosal lining to access immune cells. Metabolites of these pathogens like toxins and inflammatory molecules enter the blood stream and cause altered moods and anxiety in the host. Campylobacter jejuni administered orally to rats in subclinical doses led to anxiety-like behavior, while the depression in germ free rats (delivered surgically and raised in sterile isolators) could be reversed by administering Bifidobacterium infantis that exclude pathogens and strengthen the intestinal barrier to improve the mental states (psychobiotics).


Probiotic and other gut commensal bacteria produceadequate amounts of metabolites (serotonin, GABA (gamma-Aminobutyric acid)  essential for feeling of wellbeing.

There is some convincing evidence for the antidepressant properties associated with targeting the gut microbiota. For example, reduction in anxiety and depression in healthy women after administration of probiotic formula (containing Lactobacillus helveticus and Bifidobacerium longum) for 30 days;  significant decrease in anxiety symptoms in  patients with chronic fatigue syndrome after 2 months therapy with Lactobacillus casei ; improvements in brain activity in areas that control emotion processing in women who were given probiotic fermented milk product. In the future, we might find that psychobiotics could restructure the gut microbiota to achieve society-wide control on mental illnesses. Although we are still a long way away from this level of understanding, it can be interesting to think about possibilities the future holds as we begin to learn more about the gut-brain axis.

Mouse sitting on cat's head

If you are still sceptical about the benefits of psychobiotics, let me tell you about the Toxoplasma infected mice who sought out cats and got eaten because the parasite churned out GABA (gamma-Aminobutyric acid), which reduces anxiety. Probiotic-fed mice showed increased GABA levels, too, and were fearless to stand up to the cat. That is what I call a gutsy mouse.


  • Suja Senan, PhD

    Suja Senan holds a PhD in Dairy Microbiology and is passionate about probiotics and probiogenomics. She did her postdoc at South Dakota State University in nanotechnology and culturomics. After serving in academia for 9 years she recently pivoted to the food industry. She is an IRCA approved Food Safety Auditor and has authored chapters and manuscripts in international journals.

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