The gut-brain axis refers to the connection between the intestine and the brain and works like this:

Anyone who has ever felt “butterflies in their stomach” at the sight of a loved one or has lost their appetite due to stress can guess that the brain and stomach are connected. Constant, two-way communication takes place via the gut-brain axis, which, when disturbed, can trigger intestinal and other health problems.

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Broadly speaking, the gut-brain axis is a communication system between the brain and the trillions of bacteria, fungi and viruses that live in the intestines. Several studies have shown that the composition of gut bacteria can have a profound effect on mental health and nervous system function. A healthy diet plays an important role in shaping this microbiome by promoting the growth of beneficial bacteria and preventing the accumulation of harmful bacteria. Nutrition can also influence communication along the gut-brain axis, which in turn affects the connections between the gastrointestinal tract and the nervous system.

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But how do the various parts of the gut-brain axis work and what is the role of diet in maintaining good gut health?

The gut-brain axis refers to the constant flow of information between intestinal microbes and the central nervous system. Numerous different signaling pathways and microbial metabolites are involved in this two-way communication. It is a highly developed network that can easily be disrupted by many different factors, including environmental irritants, stress, antibiotics, and diet. Even the type of delivery may play a role.

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The gut microbiome is a crucial part of this connection between the intestine and the brain. It develops simultaneously with the central nervous system and has a strong influence on many different mental processes. There is evidence that dysbiosis — a term that describes a disturbed gut microbiome — may play an important role in many mental and neurological disorders.

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In dysbiosis, the pathways of the gut-brain axis are disrupted, which can make the physical barrier between the central nervous system and the cardiovascular system more permeable. If this blood-brain barrier is leaky, this can lead to inflammation of the brain matter. Intestinal neuroinflammation is associated with the development of diseases such as multiple sclerosis, stroke, Alzheimer's and Parkinson's disease.

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New findings also suggest that a disrupted gut-brain axis may promote weight gain through changes in metabolism, satiety control, and eating patterns. In addition, a recent study from 2020 has shown how impaired signaling in the gut-brain axis can cause a strong preference for the taste of sugar but not of artificial sweeteners.

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To better understand the function of the gut-brain axis, we should break it down into the various components.

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The vagus nerve

The human intestine contains almost 500 million neurons, which are connected to the brain via nerves. The vagus nerve is one of the largest nerves that connect the gastrointestinal tract to the nervous system, and it plays many important roles in our body. It has a far-reaching effect on inflammation and the composition of the microbiota in the intestine, and many factors can impair its ability to function. Psychological stress, for example, has a particularly damaging effect on the vagus nerve and has been shown to be involved in the development of gastrointestinal disorders such as irritable bowel syndrome and inflammatory bowel diseases.

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Neurotransmitters

Chemical substances, known as neurotransmitters, are involved in communication between the intestine and the brain. Neurotransmitters, which are synthesized by the brain, are involved in regulating emotions and the “fight or flight” response. Recent studies have shown that these compounds can also play an important role in the intestines. The neurotransmitters norepinephrine, dopamine, and serotonin are able not only to regulate and control blood flow, but also to influence bowel movement, nutrient uptake, the gastrointestinal immune system and the microbiome. Many neurotransmitters that are responsible for maintaining our mental health are actually produced either by intestinal cells or by intestinal microbes.

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The intestine produces 90% of our happiness hormone serotonin, 50% of our pleasure hormone dopamine, melatonin — the sleep hormone — and oxytocin, the cuddling hormone. If the balance of organisms in our intestines is not right, this can have an effect on our mood and behavior.

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Intestinal microbes, neurotransmitters and mental disorders influence each other in a reciprocal relationship that forms a triangle. Dysregulated neurotransmitters may contribute to the onset and progression of inflammatory bowel diseases and neurodegenerative diseases such as Alzheimer's and Parkinson's.

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Chemicals produced by gut microbes

Gut microbes produce a range of chemicals that affect how our brains work. Bacterial fermentation of fiber is the main source of short-chain fatty acids (SCFA), such as butyrate, propionate, and acetate. These compounds have been shown to prevent digestive problems and reduce the risk of obesity and type 2 diabetes. Short-chain fatty acids are able to cross the blood-brain barrier and therefore have an effect on the structure and function of the brain.

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The short-chain fatty acid butyrate plays a key role in communication between intestines and brain and protects the brain from mild inflammation. Butyrate is also the main source of fuel for cells in the intestines and has many health-promoting effects on the nervous system of the gastrointestinal tract.

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The gut-brain axis and mental health: What is the connection?

Dysbiosis and inflammation in the gastrointestinal tract are associated with a wide range of mental illnesses. New findings suggest that intestinal microbes play a crucial role in brain development and the flow of information in the nervous system. Poor gut health can contribute to the onset and progression of depression, anxiety, schizophrenia, autism spectrum disorders, migraines, and epilepsy.

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There is also a significant relationship between the gut-brain axis and our vulnerability to stress. Chronic stress can trigger episodes of depression and anxiety. It is speculated that people with good gut health can withstand pressure better than those who struggle with it. Several studies have also shown how changes in gut microbiota in early childhood due to antibiotic exposure, lack of breastfeeding, cesarean delivery, infections, stress exposure, and other environmental influences can lead to long-term changes in stress-related physiology and behavior.

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Which foods support the gut-brain axis?

Improving our dietary habits is one of the most important things we can do to support our gut microbes. An easy way to improve gut health naturally is to eat various plant foods that are rich in fiber and beneficial plant compounds. The rule here is that the more diversity, the better.

However, certain nutrients are more beneficial to our intestines than others, and even with a healthy, balanced diet, deficiencies can occasionally occur. That's why it's important to know which foods can help our gut-brain axis the most.

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Probiotics

Probiotics are live bacterial strains that, when consumed in sufficient quantities, have various health benefits. There are many probiotic foods that support the intestines, but anyone who is not a fan of fermented foods can also take probiotic supplements.

Recent studies have shown that probiotics can alleviate neurological and psychiatric disorders via the gut-brain axis. Probiotic strains, which have the greatest effect on central nervous system function, are often referred to as “psychobiotics.” New findings suggest that these so-called psychobiotics can improve cognitive functions and symptoms of stress, anxiety and depression.

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Prebiotics

Prebiotics are certain amounts of fiber and indigestible carbohydrates that help our good gut bacteria grow and thrive. Prebiotic foods include certain vegetables, fruits, whole grains, and seeds.

Prebiotics have an extremely positive effect on our gastrointestinal tract, but they have also been proven to improve brain health. Research even suggests that these connections may help people recover from traumatic brain injury and PTSD (post-traumatic stress disorder).

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Omega-3 fatty acids

The polyunsaturated omega-3 fatty acids are a type of essential fat that our bodies cannot produce themselves and which we must therefore obtain through food. Several studies have shown that omega-3 fatty acids have far-reaching effects on our cardiovascular and metabolic health due to their ability to alter the structure and function of cell membranes. In addition, they can promote the health of our intestines.

Foods with lots of omega-3 fatty acids include linseed oil and linseeds, walnuts, herring, salmon, olive oil and olives.

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Polyphenols

Polyphenols are a diverse group of phytochemicals — organic compounds that occur naturally in plants. Numerous studies have shown that they have a positive effect on health due to their antioxidant and anti-inflammatory properties.

Certain dietary polyphenols have the ability to influence intestinal health and therefore protect against Alzheimer's and Parkinson's disease. Polyphenols with neuroprotective properties are found in many different foods, including cocoa, green tea, and olive oil