The gut–brain axis: how the gut affects the mind

For a long time, the gut was primarily seen as a digestive organ. We now know that it contains a dense network of nerve cells and is closely connected to the brain. The two organs are constantly exchanging signals – via nerve pathways, hormones and the immune system. This two-way communication is referred to as the gut–brain axis. Current research suggests that the gut–brain connection influences our mood, our stress management and possibly our risk of mental illness.

The main conduit of the gut–brain axis: the vagus nerve

The most important connection between the brain and intestine’s nervous system is the vagus nerve. This is the longest cranial nerve in the body and reaches all the way from the brain stem to the abdomen. The intestine and brain communicate in both directions via the vagus nerve: approximately 80 to 90 percent of its fibres carry information from the gut to the brain, and 10 to 20 percent of the fibres transmit information in the opposite direction from the brain to the gut.

At the same time, the vagus nerve regulates the parasympathetic nervous system, our body’s system for rest and recovery. The parasympathetic nervous system slows down your heartbeat, aids digestion and helps your body to build up energy reserves and maintain internal balance.

Messengers and microbiome

The intestine and brain communicate not only via the vagus nerve’s neural pathways, but also via messengers such as hormones and neurotransmitters. Hormones are produced in certain glands or cells and transported through the blood, triggering different reactions, such as hunger, tiredness or stress. Neurotransmitters transmit signals directly from one nerve cell to the next.

The microbiome plays an important role. This includes all the microorganisms in the intestine – about 38 trillion bacteria from over a thousand species. These bacteria are not inactive inhabitants: they influence the production of transmitters such as serotonin and dopamine, form short-chain fatty acids, train the immune system and regulate inflammatory processes. 

The most important hormones and neurotransmitters for the gut–brain axis:

• Serotonin: around 90 to 95 percent of the body’s serotonin is produced in the intestines. This hormone mainly acts locally and controls digestion, among other processes.
• GABA: gamma aminobutyric acid (GABA) is a calming neurotransmitter produced by certain intestinal bacteria. It dampens anxiety responses and stress reactions.
• Cortisol: this stress hormone influences both the intestinal mucosa and the microbiome. It plays an important role in regulating stress and can also affect digestion.
• Satiety hormones: hormones such as GLP-1 (glucagon-like peptide-1) and CCK (cholecystokinin) control hunger and satiety and have an effect on our mood.

Studies show that the gut and the mind are in close communication. Stress and psychological distress place the intestines under strain. Conversely, the composition of the microbiome seems to influence our psyche.

From the gut to the mind

Chronic intestinal problems affect our well-being. It’s no surprise that good digestion has long been associated with overall health. But the influence of the intestines goes even further. When the microbiome is out of balance, this also changes the communication between the gut and the brain. For example, serotonin production suffers, which affects mood, sleep and how we sense pain.

The connection between the gut and mind is also evident in people with depression or anxiety disorders; they sometimes have altered intestinal flora.

From the mind to the gut 

And what influence, conversely, does the mind have on the gut? When stressed, the body reacts by releasing adrenaline and cortisol, two hormones that have a direct effect on the intestines. Our digestion changes, blood circulation decreases and the intestinal mucosa becomes more permeable. Bacterial components can enter the bloodstream through this weakened barrier and trigger inflammation. These inflammatory substances, in turn, reach the brain and can influence mood and concentration. 

In addition, persistent mental stress changes the microbiome: inflammatory bacteria increase, while the proportion of protective bacteria decreases. 

A disrupted gut–brain axis manifests itself in various ways. Many sufferers experience abdominal pain, fatigue and low mood at the same time. The fact that physical and psychological complaints often occur in conjunction with one another is an indication of the complex interplay between the brain and the gut, which science is only beginning to decipher.

Typical symptoms of a disrupted gut–brain axis:

• Persistent abdominal pain or flatulence with no evidence of a physical cause
• Irritable bowel symptoms or irregular bowel movements (constipation, diarrhoea or both)
• A persistent feeling of exhaustion• Concentration problems and reduced mental performance
• Mood swings and irritability
• Sleep disorders
• Increased sensitivity to stress

Depression and sleep problems 

Around 90% of the body’s serotonin – the “happiness hormone” – is produced in the intestines. Bacteria such as Lactobacillus and Bifidobacterium are particularly responsible for this. If these bacterial strains are reduced, there is less serotonin available. At the same time, serotonin is the precursor of melatonin, the key sleep hormone. Less serotonin therefore also means less melatonin. As a result, researchers suspect that sleep disorders and depression are related to the microbiome via the gut–brain axis.

Anxiety disorders

Many people with anxiety disorders also suffer from gastrointestinal complaints. One possible explanation is the hypothalamic–pituitary–adrenal axis (HPA axis), which controls the release of cortisol during stress. If the intestinal flora is disrupted, this axis reacts more sensitively to stress and may release more cortisol. This excessive stress signal in turn affects the brain via the gut–brain axis. 

Irritable bowel, small intestinal bacterial overgrowth and migraines

The gut–brain axis may also play a role in the following diseases:

• Irritable bowel syndrome (link to the new blog post on IBS)): in this condition, stress and intestinal complaints exacerbate each other.
• Small intestinal bacterial overgrowth (SIBO): sufferers report psychological complaints such as anxiety, irritability or depressive moods.
• Migraines: there are indications that increased intestinal permeability and inflammation can lead to migraines.

No prescription medication is required for a healthy gut–brain axis: if you eat an anti-inflammatory diet, exercise regularly and reduce stress, you can strengthen your intestinal barrier every day. 

Nutrition as a key lever

It may sound trivial, but a balanced diet lays the groundwork for a healthy gut, a strong gut–brain axis and mental well-being. Eat lots of fruit and vegetables, wholegrains, nuts and fermented dairy products, and consume and fish and meat in moderation. The Mediterranean diet, for example, serves as a guide.

Make sure you eat the following foods:

• Fibre and prebiotics: fibre and prebiotic foods help to build a diverse and balanced microbiome that is in balance with the immune system. Prebiotics are indigestible plant fibres. They promote the growth of beneficial bacteria in the intestine and are found in foods such as chicory, Jerusalem artichokes, leeks, garlic and oats.
• Probiotics: fermented foods such as yogurt, kefir, raw sauerkraut, kimchi, miso and kombucha are rich in probiotics. These living microorganisms promote the balance of intestinal flora, strengthen the intestinal barrier and can reduce inflammatory markers. 

Experts also recommend avoiding sugar and heavily processed foods wherever possible: these promote inflammatory processes in the body and put the intestines under strain.

Calming the gut–brain axis

Alongside nutrition, lifestyle – such as exercise and sleep – also influences the gut–brain axis. Breathing or mindfulness exercises can also help you strengthen the gut–brain axis. Regularity is more important than duration: short training sessions that you incorporate into your daily routine promote physical health and mental resilience to a greater extent than long training sessions that only take place occasionally. This doesn’t have to involve a lot of effort. Several of the following recommendations can be combined with a post-meal walk to support your digestion:

• Breathing exercises: breathing slowly deliberately into your abdomen activates the vagus nerve. Keep an eye on your breathing during your day-to-day life, for example when you’re waiting for the bus or standing in a lift.
• Mindfulness: yoga and meditation can reduce our stress levels.
• Exercise: regular physical activity is good for you and promotes the diversity of your intestinal flora.
• Sleep: getting enough sleep has a positive effect on the microbiome and supports a stable intestinal barrier.
• Nature: spending time in nature not only stimulates your senses, but also brings you into contact with a wide range of environmental bacteria. This also promotes the diversity of the microbiome.

Research is increasingly revealing how closely the gut and brain work together. If you look after your intestinal health, this doesn’t just benefit your digestive system, but also your mind. A balanced diet, exercise and targeted stress reduction support the gut–brain axis – and have a positive effect on general well-being.