The idea that many of our emotions and feelings are linked to the gut is an ancient one. More than 2,000 years ago, the Romans made reference to what we today describe as a “gut feeling”. Now, modern science indicates that the gut may indeed play a role in mood disorders and our mental health.

The gut is covered in nerve cells known as neurons – cells that transmit signals to other neurons, and which are also fundamental to our brains. Research suggests that this network, known as the enteric nervous system, contains more than 100m neurons. This complex network of cells in our gut is able to function independently of the brain and spinal cord, and is often referred to as the body’s “second brain”.

And although this “second brain” doesn’t actually do any “thinking”, the gut is far more than just a mechanism for processing and digesting our food. The gut communicates with the brain in a number of ways, and plays a vital role in providing it with neurochemicals, such as serotonin which influences mood, among other functions.

In addition to the bloodstream, one of the main pathways between the two is the vagus nerve – a long bundle of nerve fibres that stretch from the brain to the abdomen. “Roughly 80% of the nerve fibres which make up the vagus nerve are signalling in the direction from the gut to the brain, while only 20% are the other way around,” says Dr Katerina Johnson, a psychiatry researcher at the University of Oxford. “It highlights how much the brain is a receiver of information from our gut.”

Developing research shows that much of this information comes from gut microbes, which respond to a variety of external influences ranging from diet to stress. It is through the actions and composition of these microbes that the gut is thought to play a role in impacting mental health.

Along with various other factors such as our genes, our diets may play a role in determining how we cope with stress and deal with life traumas. Ted Dinan, emeritus professor of psychiatry at University College Cork, in Ireland, points out that both human and animal studies have established links between poor diet and poor mental health.

“In a study some years ago, we profiled the gut microbes of patients with depression and of healthy subjects, and we found that there was much less microbial diversity in the intestines of the individuals with depression,” says Dinan.

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“And when we transplanted the microbiota of a depressed patient into a rat, the rat’s behaviour altered and it developed depressive-like behaviours and a more inflamed immunology. Whereas if you do the same thing from a healthy human to a rat, the animal’s behaviour doesn’t change at all.”

One of the reasons that diet is important is because the large intestine – a component of the gut which is relatively heavily populated from a microbial perspective – acts as a giant fermentation organ. The microbes within it break down certain fibres producing various substances – some of which have been linked to positive health outcomes.

Johnson explains how scientists have found that patients with depression have less of a particular type of bacteria called bacteroides, which is known to produce an important chemical called GABA (gamma-aminobutyric acid). This neurochemical helps minimise the impact of stress, and can therefore help us sleep soundly through the night. If the brain does not get enough GABA, this leads to increased anxiety and insomnia.

One particularly important product for the brain is the amino acid tryptophan. Tryptophan in our diet is broken down – by a group of gut microbes called bifidobacteria – into smaller metabolites that can cross the blood-brain barrier. The brain needs a constant supply of tryptophan metabolites, as they are the building blocks for the mood-regulating hormone serotonin. It appears that if we don’t have a sufficient abundance of bifidobacteria in our gut, due to eating a poor diet, this process will be inhibited, with consequences for our mental health.

“Numerous studies over the past decade have illustrated the importance of tryptophan,” says Dinan. “If you take patients who have recovered from depression and you deplete their brains of tryptophan, their depression will reoccur within a really short period of time, within minutes to hours.”

Making sure the right types of bacteria are flourishing in your gut appears to be a crucial part of keeping your brain working effectively, yet because people’s guts are so complex – and so different – scientists are still trying to establish the exact definition of a healthy microbiome. What we do know for now is that a good diet plays an important role in keeping your gut – and therefore your brain – healthy.

“We know that if somebody starts off on a good diet, and then goes on a diet of fast food, their microbiota changes rapidly in a very short period of time, and good microbes in the gut tend to be lost or decreased very dramatically,” Dinan says. “There’s no doubt that an appropriate diet gives us a good microbiota, which helps us deal more appropriately with stress.”