The gut-brain connection

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The communication system connecting the intestine and the brain is called the gut-brain axis. The gut and brain are connected in many different ways, both physically and biochemically. The gut-brain axis is the term used to describe the communication network connecting the gut and brain. This axis involves two-way communication between the central and enteric nervous systems and links the brain’s emotional and cognitive centres to the peripheral functions of the intestine. Recent research shows how important the gut microbiota is in influencing these interactions. The interaction between the microbiota and the gut-brain axis appears to be bidirectional, involving signals from the gut microbiota to the brain and vice versa, through neural, endocrine, immunological, and humoral connections.

This two-way communication network comprises the central nervous system, the brain and spinal cord, the autonomic nervous system, the enteric nervous system, and the hypothalamus-pituitary-adrenal axis.

Clinical and experimental evidence suggests that the gut microbiota has a significant influence on the gut-brain axis, interacting not only locally with intestinal cells and the enteric nervous system, but also directly with the central nervous system via neuroendocrine and metabolic pathways.

The most compelling evidence of interactions between the microbiota and the brain dates back more than 20 years, when patients with hepatic encephalopathy often showed dramatic improvements after treatment with oral antibiotics. Meanwhile, new data confirms the impact of microbiota on anxiety and depression-like behaviours, and more recently on dysbiosis in autism. This is because patients with autism show specific changes in their microbiota, which are consistent with the severity of the disease.

The vagus nerve and the nervous system

Neurons are cells in the brain and central nervous system that communicate to the body how to act. The human brain contains around 86 billion neurons.

Remarkably, there are about 168 million neurons in the intestine, which are connected to the brain via the nervous system. The vagus nerve is one of the largest nerves connecting the gut and brain, transmitting signals in both directions.

Neurotransmitters

The gut and brain are also connected through chemicals called neurotransmitters. These are produced in the brain and intestine to control the function of the nervous and digestive systems. The enterochromaffin cells of the small intestine (part of the intestine) produce the highest concentrations of serotonin. Serotonin is associated with intestinal function, nausea, appetite, mood, learning, memory, and sleep. Almost 90% of serotonin is produced in the gut.

The microbiota also produces gamma-aminobutyric acid (GABA), a neurotransmitter that helps manage feelings of fear and anxiety.

Microbiota

The trillions of microbes in the gut also produce other chemicals that affect brain function. Among them are a number of short-chain fatty acids (SCFAs) such as butyrate, propionate and acetate. These can affect brain function in a number of ways, such as by reducing appetite.

Immune system

The gut-brain axis is also linked to the immune system. The gut and microbiota play an important role in the functioning of the immune system and inflammation by controlling what enters and what is eliminated from the body. When the immune system is engaged for too long, this can lead to inflammation associated with a range of conditions such as depression and Alzheimer’s disease. Lipopolysaccharide (LPS) is an inflammatory toxin produced by some bacteria. When too much of it passes from the gut into the blood, it can cause inflammation. This happens when the intestinal barrier becomes permeable and allows bacteria and LPS to enter the blood. Inflammation and high levels of LPS in the blood have been linked to a number of disorders, including major depression, dementia, and schizophrenia.

Probiotics, prebiotics and the gut-brain axis

As the microbiota influences brain health, changing its composition can have a positive effect. Probiotics that have an effect on the brain are often referred to as psychobiotics. Some probiotics have been proven to alleviate symptoms of stress, anxiety and depression.

Prebiotics – typically fibre fermented by gut bacteria – can also have beneficial effects on brain health.

Foods that have a positive effect on the gut-brain axis:

• Omega-3 polyunsaturated fatty acids: found in oily fish and, in large quantities, in the human brain. Studies on humans and animals show that omega-3 fatty acids can have a positive effect on microbiota balance and reduce the risk of brain disorders.

• Fermented foods

• Foods rich in fibre: wholegrain cereals, nuts, seeds, fruit and vegetables contain prebiotic fibre, which is food for probiotics. In humans, prebiotics can reduce the levels of stress hormones.

• Foods rich in polyphenols: cocoa, green tea, olive oil and coffee contain polyphenols, which are plant chemicals that are digested by the microbiota. Polyphenols positively affect the balance of the microbiota.

• Foods rich in tryptophan: tryptophan is an amino acid that is converted into the neurotransmitter serotonin. Foods containing high levels of tryptophan include turkey, eggs and cheese.

The gut-brain axis refers to the physical and chemical connections between the intestine and brain. They are connected by millions of nerves and neurons. Neurotransmitters and other chemical substances produced in the gut also affect the brain. There is strong evidence that the gut microbiota plays an important role in the two-way interactions between the gut and nervous system. By interacting with the central nervous system, it regulates brain chemistry and influences the neuroendocrine system, which is linked to stress response, anxiety, and the functioning of memory.

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