Graz/Bonn (dpa/tmn) – Stomach rumbling, diarrhea or even nausea – sometimes these symptoms do not reflect physical causes, but psychological ones. For example, stress, excessive emotional stress or depression. The so-called gut-brain axis is responsible for this connection.
It describes the complex interaction between our digestive system and the brain. It is now scientifically recognized that it is not a one-way street. Rather, signals flow in both directions – from the gut to the brain and vice versa. Different signaling pathways are used: the nervous, hormonal and immune systems, most of which are located in the intestine.
Relaxed vagus nerve, relaxed gut
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The vagus nerve plays a central role in communication via the nervous system. As the main component of the parasympathetic nervous system, it is crucial for rest, relaxation and social interaction. “In order to recognize emotions on the other person’s face, the vagus nerve has to function well,” says psychiatrist and nutritional doctor Sabrina Leal Garcia from the Medical University of Graz. She heads the special outpatient clinic for Nutritional Psychosomatics in Graz.
Studies suggest that certain probiotic bacteria such as lactobacilli and bifidobacteria can positively stimulate the vagus nerve via the gut-brain axis. Most studies that examine effects on mood or autonomic nerve function use periods of use ranging from several weeks to around three months.
How the gut influences the brain – and vice versa
At the center of this axis communication is the microbiome, i.e. the entirety of microorganisms in the intestine. They produce metabolic products such as short-chain fatty acids. “These either migrate in their original form or are converted to the brain and send signals there,” explains Birgit Terjung. She is chief physician in the department of internal medicine and gastroenterology at the St. Josef Hospital in Bonn-Beuel.
The short-chain fatty acids have a local anti-inflammatory effect, but they can also have a positive effect on neurological diseases such as Parkinson’s or dementia.
Messenger substances such as the hormone serotonin also play a role. Around 90 percent of the body’s serotonin is produced with the help of bacteria in the intestine. “It is responsible for intestinal movement,” says Sabrina Leal Garcia. Too much serotonin can therefore lead to diarrhea, while too little serotonin can lead to depressive moods. Although this serotonin does not reach the brain directly from the intestine, it can indirectly influence psychological processes via the vagus nerve.
Some bacteria promote inflammation, others inhibit it
Conversely, a stressed brain can send signals to the adrenal gland, where stress hormones such as adrenaline are produced. These can in turn have a negative effect on the entire body and also on the intestines.
The immune system also mediates between the intestine and the brain – for example via cytokines. These are proteins that control inflammatory processes and can influence neuronal activity. Certain gut bacteria promote anti-inflammatory immune responses, while others promote inflammation that is linked to depression or anxiety disorders.
Mediterranean diet promotes “good” intestinal bacteria
Diet is crucial for a diverse microbiome in the intestine. According to the experts, it should be Mediterranean, i.e. lots of fiber, a large amount of fruit and vegetables, nuts, seeds, oils such as olive oil with a high proportion of omega-3 fatty acids, little meat, lots of fish. Fermented foods such as sauerkraut, kombucha or kimchi are also so-called synbiotics. On the one hand, they contain living intestinal bacteria (probiotics), and on the other hand, they contain food for the intestinal bacteria (prebiotics).
But: “It has not been scientifically proven that a certain number of vegetables and fruits are necessary,” says Birgit Terjung. Diversity is more important than individual supposed “superfoods”. Choosing different colored plant foods can also be useful as they provide different ingredients. According to Sabrina Leal Garcia, the first changes in the microbiome often become apparent after just a few days of a change in diet, but it takes months for the entire system to change measurably.
Stool should not be too soft and not too hard
The diet must suit the person, says Leal Garcia. Trends like the low-carb, high-fat keto diet were initially thought to be dangerous. Studies have now shown that this form of nutrition provides relief from mental illnesses. “But you definitely need medical support, especially if you already have mental illnesses,” says the nutritionist.
If you are not sure whether your own digestion is in order, you can first focus on bowel movements. This should take place once a day, be well formed, i.e. not too soft and not too hard. The so-called Bristol Stool Scale is used for assessment. “Type 4 would be optimal,” says Leal Garcia.
How are you after eating? A regular bloated stomach, pain or heartburn can be signs of impaired digestion.
Analysis of the microbiome is not useful
However, analyzing the microbiome is difficult, says Birgit Terjung. A stool sample only provides limited information because the bacterial composition in the intestine varies greatly depending on the section. “Many bacteria also adhere to the intestinal wall and are not detected by a sample,” says the gastroenterologist. A stool sample can therefore show a trend, but – contrary to all promises – a meaningful therapy cannot be based on it.
