Gut-Brain Axis

It does not seem very sophisticated to connect blissful happiness with processed food and waste. Yet, scientists have become more and more aware in recent years of the strong connection between the digestive system and mood balance.The truth is that happiness depends on gut health and vice versa. 

Our nervous system, responsible for our behaviour, is formed by two interconnected parts – the Central Nervous System (brain and spinal cord) and the Enteric Nervous System (ENS). ENS is located along the entire digestive tract – from the oesophagus through the stomach and intestines and down to the anus. ENS can perform tasks independently of the brain such as coordinating reflexes and secreting. It is built by the same neurones and neurotransmitters as the Central Nervous System and often is called “the second brain”. 

The two systems talk to each other and the communication system between them is called gut-brain axis.  The largest nerve in the body, the vague nerve, which runs from the brain to the colon is part of it. When a person is in danger, in “fight or flight” mode, the ENS helps by slowing down or stopping the digestion so more energy can be used to deal with the threatening situation. Another example is when the person is afraid of public speaking. Their digestive system again reacts by slowing down or speeding up and that can cause abdominal pain or diarrhoea. My personal experience of this brain-gut axis is that before my uni exams I was frequently visiting the  bathroom. Emotions like love or excitement can lead to feelings of “butterflies” in the stomach. 

The opposite also happens – poor gut health can influence how we behave and our mood. Problematic intestines, or a troubled stomach may signal to the brain and cause anxiety, stress and depression. 

Communication between the brain and the guts (particularly stimulation of the vague nerve)  is the reason for feeling so good after dropping a poo, so called “poo-phoria”. Bowel movement stimulates the vague nerve, the heart rate and blood pressure drop and you feel relaxed and pleased. 


Serotonin (remember, the happy chemical?) impacts every part of the body from emotions to motor skills. Serotonin is critical for the functioning of the digestive system. It is primarily produced in the body’s intestines, and affects many sides of the gut functioning: how fast the food goes through the system, how much mucus is secreted in the intestines and how sensitive intestines are to pain and fullness from eating. Changes in the serotonin level affect the guts as well as the brain. If you deal with chronic digestive issues your serotonin level is being impaired. Serotonin rises when you eat something toxic or have food poisoning in order to quickly remove the food from the body by vomiting and diarrhoea. Having a low level of serotonin creates a savage circle as serotonin is fundamental for ensuring proper nutrition digestion and absorption.

People with Irritable bowel syndrome (IBS) who experience constipation often have lower levels of serotonin  – the muscles in their rectum are less reactive to serotonin and they are more likely to have hard stools. 

Those with IBS with higher levels of serotonin may have diarrhoea – their rectums are more reactive, with loose watery stools.


90-95% of the body’s serotonin is produced in the gut cells. Production of serotonin is affected by the microbiome in the guts – millions of viruses and bacterias. The microbiome diversity is important for gut health but unfortunately, it decreases with age. Also high stress can cause changes in the microbiome.

Research shows that healthy gut bacteria plays a role in maintaining healthy serotonin levels. Serotonin is produced in the digestive tract by enterochromaffin cells and particular types of immune cells and neurones. Studies have found that several species of gut bacteria are missing in people with depression and, furthermore, imbalance in gut flora can lead to imbalance in mood. 

Nowadays it is widely accepted that a better understanding of the serotonin production in the guts may inform the treatment of certain mental health conditions in the brain. 

If this subject resonates with you there is a ton of information on YOUTUBE, various podcasts, many books and articles, including recipe books for clever and happy guts. 

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