We’ve all experienced it before: the nervous flutter of butterflies in your stomach before an important event, the power of love robbing you of your appetite and tying your stomach in knots, or making a decision based on a feeling deep down inside your belly...

Much of our emotional and intellectual experiences are located at waist-level: evidence of the delicate and yet palpable link between the belly and the  brain. This mind-midriff connection is hardly surprising; after all, the human brain and gut are cut from the same cloth and share the same developmental background. And there is still much more to learn about this special relationship, which is why research teams the world over are working hard to unlock the secrets of the brain, the gut, and the axis between them.

Our gut has a mind of its own – literally

The early digestive tract was the first system to be equipped with nerve cells – long before the first creatures developed a brain. This primordial structure evolved into our present-day “second brain”, which is located in the intestines. This intestinal brain comprises around 100 to 200 million neurons (nerve cells), making it larger than the neural system of our spinal cord. For comparison: the cerebral cortex of the average dog, which is generally considered to be a pretty smart animal, comprises only 160 million neurons. Clearly, our gut is pretty smart as well. It doesn’t require any input from central control for its primary activity, digestion, but independently sets to work as soon as you swallow your food. This arrangement is ideal, as it frees up your brain and allows it to concentrate on other matters.

In short: What is the abdominal brain – and what are its functions? 

The abdominal brain is a complex nerve network that extends from the esophagus to the intestinal outlet. As the lower command centre, the abdominal brain makes all the important decisions for the intestines on its own. It regulates intestinal perfusion and motor function, analyses food composition and coordinates what is absorbed and what is excreted. In addition, the abdominal brain communicates with both the immune system and its microbial inhabitants (microbiota). 

Gut/brain axis: How do the intestines and the brain relate to each other?

These two wise old birds – the gut and the brain – love to put their heads together on a frequent basis. Both control centres are in active exchange with each other via the gut-brain axis, using the nerve pathways (especially the vagus nerve), the messenger substances and microbial metabolites for this. Surprisingly, the gut is far more eloquent than the head, with 90% of the information being directed from the bottom up. Most of the time, we aren’t even aware of what our gut is telling our head. Only in certain cases – for example, when we eat rotten food and brain and belly brain together trigger physical discomfort such as diarrhoea or vomiting – do we experience the communication of both consciously and up close.

Listening to your gut: your microbiome is part of the conversation

At first glance, the special relationship between the bowel and the brain sounds like an intimate connection between two partners. But in actual fact, the two control centers are part of a three-way relationship: the key third player is your intestinal flora and it plays an indispensible role. For this community of billions of microbes colonizing your intestine can manipulate the link between the brain in your head and the one in your gut in a number of ways – both positive and negative. Extensive research in the past few years has led to a growing understanding of the secret power exercised by these tiny colonists. For example, the individual blend of microbial strains in your intestine can influence not only your emotional wellbeing, but also your eating habits: while some bacteria stimulate our appetite, there are others that can make you feel prematurely full. Even our stress coping skills (stress resilience) are, to a certain extent, determined in our bellies rather than our heads.

What is the intestinal brain barrier involved in?

Bowel and mind: How the gut affects our feelings and thoughts

Our society today is predominantly top-heavy, i.e. rational thinking and acting are paramount. Despite this, we still like to use our gut feeling for important decisions. Stress and worries hit us in the stomach and feelings of love trigger butterflies, but even more happens unconsciously. Neurogastroenterology is a relatively new field of research that intensively explores the “hot wire” between the gut and the brain. Many researchers now agree that our lives – our emotional and thought world, our decisions and our health – are more strongly influenced by the gut than we currently know or even suspect.

This thesis is supported by the fact that both “brains” in their embryonic development are made of the same material. In addition, both control centres speak the same language by communicating via the same messenger substances. These include the happiness hormones serotonin and dopamine as well as the neurotransmitters GABA and acetylcholine. Interestingly, the former is not primarily formed in the brain, but is produced to over 90% in our gut, where it controls our bowel activity. Although serotonin cannot reach the brain tissue from the centre of our body, the “feel-good hormone” can influence our emotional centre in the brain (the limbic system) via the vagus nerve and thus interfere with our mood.

Bowel and stress: Stress detector and manager at the same time

How does stress affect the intestine?

Chronic stress – whether at work or in private life – can cause intestinal complaints that have no pathological causes, but can severely restrict the quality of life and the social life of the individual. Because a stressed bowel has repercussions on the mind and reduces well-being.

Stress causes bowel problems, bowel problems cause stress. In many cases, this vicious circle prevents bothersome bowel problems from disappearing by themselves. In such cases, the stressed intestine can even become an irritable bowel.

But it is not only our digestive organs, the stomach and the intestines, that react, our intestinal inhabitants also suffer from the constant pressure. On the one hand, the increased release of stress hormones leads to a decrease in useful bacteria (lactobacilli and bifidobacteria) and bacterial biodiversity, and on the other hand, the reduced digestive activity can lead to shifts in the bacterial balance.

What role does the intestinal flora play in stress? 

Conversely, studies suggest that our intestinal flora has more than a word to say in our individual stress management. Whether we deal with stress well or are easily overwhelmed by it determines to some extent the state of our intestinal flora. Due to the potential stress-induced vicious cycle, looking after and supporting your intestinal health is to be highly recommended, especially during periods of stress.

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Did you know that the intestine and its bacterial flora are an important part of our immune system?

Their vital role within the immune network is also reflected in the numbers, with 70% of all immune cells located in the small and large intestine, and almost 80% of all immune defense reactions taking place there. Our intestinal flora weighs 1–2 kg and forms an important barrier against pathogenic germs. These microscopic residents also interact with our intestinal immune system, training the immune cells and stimulating the formation of antibodies that benefit not only the intestine but also other areas such as the nasal mucous membranes.

Happiness begins in the gut – but what makes your gut happy?

Although there is still much we do not know, modern research is shedding an increasing amount of light on the unexpected connections between the head and the gut. As our gut and its ecosystem are very sensitive, it is especially important that we take good care of their health. The blog post "Intestinal Remediation" explains active steps that we can take to make our feel-good organ and its microbial roommates more comfortable.

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