More research over the years has helped us understand the function of ‘the second brain’ and the interdependence of the gut microbiota and the brain, the so-called gut-brain axis.

The enteric nervous system is the focus of University of Melbourne professor Joel Bornstein’s research.

Bornstein explains that this system of neurons and cells runs the length of the gastrointestinal tract, from the oesophagus to the anus. That’s why, he suggests, the gut-brain axis should really be called the gut-microbiota-ENS-brain axis.

The enteric nervous system is the largest nervous system in the body apart from the brain, with 200 million to 600 million nerve cells which form 20 or more different interconnected functional subgroups.

The gastrointestinal tract is the only organ in the body with a nervous system that can operate without input from the central nervous system.

She explains the difference between microbiota – all live microbes – and the microbiome, which includes microbes, their genetic material and the gut environment.

As a simple analogy, consider the microbiome as a house, and microbiota as the inhabitants.

Gut microbiota produce large amounts of metabolites when the body breaks down food. Depending on what we eat, these might be inflammatory or they may stimulate and regulate the action of nerve cells, influencing mood.

It’s why diet is a key influence on gut microbiota, and has an impact on mental health and other conditions.

For example, gut microbiota are the main source of short chain fatty acids (SCFAs); low levels of these are associated with low mood. “If the gut microbiota is unhealthy, there is a lack of SCFAs and this can lead to depression,” Ng says.

About 95 per cent of the “happy hormone” serotonin is made in the gut, says Holmes. Gut bacteria create other neurotransmitters, she says, and “we need an abundance of helpful ones to be in optimal mental health”.

While good gut bacteria promote good mental health, stress can upset an otherwise happy balance.

The problem, says Holmes, is that many of us are in a persistent state of stress, so our systems become desensitised to high levels of the stress hormone cortisol.

“This is one of the ways our gut can suffer from our brain constantly triggering a stress response,” Holmes says.

Current evidence strongly indicates that gut microbiota can affect mood, notably anxiety, and other elements of brain function.

The reasons for this are many.

Not only do the metabolites act on the enteric nervous system, they can also affect the immune system and gut-hormone release. The intestinal wall contains the largest collection of hormone-releasing cells in the body, which regulate metabolism and appetite.

The microbiota can modify mood and cognitive function through the nervous system, and indirectly through the immune and endocrine systems.

Holmes can attest to this. In 2018, she found that making small, powerful changes in her diet transformed her from the inside out – and helped her recover from an 11-year battle with an eating disorder and pre-diabetes.

Ng notes that an unhealthy gut microbiome is closely related to intestinal and non-intestinal disorders, including irritable bowel syndrome, allergies, asthma, cardiovascular disease and obesity, as well as mental health disorders, so nurturing a healthy environment for our microbiota is crucial.

Everyone’s microbiota differ slightly in the types and quantities of microbes, Bornstein says. Generally, the more diverse the microbiota, the better.

The microbiota get all their nutrients from what we eat, so to change the relative numbers of different microbes we need only change the composition of the diet we eat, he says.

What’s exciting about this is that, while we may not be able to change our genetic make-up, we can modulate our gut microbiome to relieve diseases or promote better health. This is an entry point for the development of precision medicine.

Advances in biotechnology, Ng says, have successfully used the gut microbiome in a number of important clinical conditions.

For example, the US Food and Drug Administration has approved the use of an oral gut microbiome formulation to treat a recurrent and potentially lethal condition, the Clostridium difficile infection.

Kara Holmes’ tips to support a healthy gut-brain balance

1. Ensure your nutrition supports a healthy gut

Ensure adequate fibre and variety in plant foods. Prebiotic fibres, such as those in flax seeds, oats and leeks, aren’t digestible but can help good bacteria grow in your gut.

Consuming a minimum of 25g of fibre per day and eating 25 to 30 different plant foods a week will provide gut bacteria with enough prebiotic fibre to thrive.

2. Reduce inflammation by eating whole foods

Avoid packaged and frozen foods that are laden with hydrogenated oils, artificial sweeteners, stabilisers and emulsifiers which disturb the gut and promote inflammation.

3. Include probiotics in your diet, through food or supplements

Probiotics contain live microorganisms that maintain or improve the “good” gut bacteria. Probiotic foods include tempeh, miso, natto, sauerkraut, kombucha and kimchi.

These foods or probiotic supplements are especially important if you are taking antibiotics – medicines that fight infections caused by bacteria by either killing the bacteria or making it difficult for it to grow and multiply.

Probiotics may reduce the negative effects on your gut flora and potentially boost your immune response.