Have you got the gut-brain connection?

At some point in your life, you might have experienced a gut-brain connection. Have you felt butterflies when going for an interview, important meeting or date? Even the thought of eating can release the stomach’s juices before foods make their way in. This is your brain interacting with your gut, and the connection goes both ways. For example, an uncomfortable situation might have pushed you to make a decision based on your ‘gut feeling’. This is a clear example of your mind reacting to the bodily system.

An upset brain can send signals to the gut, just as an upset gut can send the signals to the brain. It means a person’s gut distress may also be the cause of some mental stressors – even anxiety and depression. Conversely, these stressors can upset the gut, too. Speaking to your healthcare professional for more personalised support is the best way forward if you’ve experienced this sensation.

How is it all connected?

The gut and brain are connected physically and biochemically in several different ways, including through gut microbiota, the nervous system, neurotransmitters, hormones and the immune system.

Gut microbiota

Trillions of bacteria living inside our gut, helping in the digestion process, are collectively referred to as gut microbiota. This bacteria works by digesting dietary fibre to produce certain chemicals such as short chain fatty acids – butyrate, propionate and acetate – which have a positive influence on our health.

These chemicals also affect brain activity. Propionate induces the feeling of fullness while butyrate is an energy source for intestinal cells. Both have anti-inflammatory properties with the ability to influence brain functions and behaviour.

The nervous system

‍Your gut contains more than 500 million neurons (nerve cells or nerve fibres responsible for transmitting information throughout the body). Together, these are collectively called the enteric nervous system (ENS).

Put simply, the ENS is responsible for digestion, controlled release of enzymes to blood flow, nutrient absorption and the elimination of waste.

ENS works together with the vagus nerve – a nerve that connects the brain to the body. The vagus nerve, along with the ENS, communicates with the brain. Animal research has observed that feeding with some probiotics reduces the level of blood stress hormones. However, when the vagus nerve was removed, probiotics have no effect, highlighting the importance of the vagus nerve in the gut-brain connection.

Neurotransmitters

Neurotransmitters are the biochemicals necessary for communication between neurons. They also control your feeling and emotions. For example, serotonin contributes to feelings of happiness and wellbeing. Many of these neurotransmitters and their building blocks are produced in the gut – up to 90 per cent of serotonin is produced in the gut. Gut bacteria produces gamma-aminobutyric acid (GABA) which helps control feelings of fear and anxiety. Certain probiotics can increase the production of GABA and reduce depression and anxiety-like behaviours.

Immune system

‍The gut microbiota and your immune system work in coordination towards maintaining the body’s homeostasis (balance) and controlling what can and can’t be passed into your system, protecting the body from pathogens and toxins.

If the immune system is over stimulated for long periods of time, it can lead to chronic inflammation. Research has demonstrated that chronic inflammation is known to increase the risk of many health concerns including brain disorders such as dementia and Alzheimer’s disease.

Hormones

The stress hormone cortisol is regulated by the hypothalamus, pituitary and adrenal gland, in response to environmental stress. Cortisol affects the immune cells both in the body and locally in the gut. It can alter the gut permeability and microbial composition, leading to conditions like irritable bowel disease (IBD). Conversely, certain probiotics can help reduce the level of cortisol and improve depression-related behaviour effects.

Your gut and brain are connected physically via the nervous system and biochemically by immune and hormonal systems. Gut microbiota and various metabolites produced by them can regulate the immune system and affect brain health.

This article was written by Dr Shakuntla Gondalia

Dr Shakuntla Gondalia is a postdoctoral scientist at CSIRO Health and Biosecurity Business Unit in the Food and Nutrition Program. Dr Gondalia has been in the CSIRO team since 2019, leading projects focused on modulating the gut microbiome composition through diet, prebiotics and probiotics for healthy aging. Prior to joining CSIRO Dr Gondalia was a postdoctoral fellow at Swinburne University where she has led the project evaluating the microbial signature of cognitive health in elderly individuals. Her work has established a system level approach to detect changes in gut microbiota associated with cognition. She completed a PhD in 2014 from Deakin University investigating the gut microbiome associated with severity of Autism. Her research continues to investigate the underpinning mechanisms of gut microbiome in influencing brain functions through Microbiome-Gut-Brain axis and developing microbiome targeted strategies to improve individual health and wellbeing. Dr Gondalia is the current secretary of the Australian Nutrition Society and holds an adjunct position at Australian University providing supervisions to higher degree researchers.