How the gut-brain axis might be causing your stress, anxiety or depression

The gut-brain axis is a bi-directional communication system between the gastrointestinal tract and the brain. It connects the central nervous system (CNS) to the enteric nervous system (ENS) in the gut. Recent research has shown that it is more important to our health and well-being than previously realised. Let's take a closer look at what it is and how it affects us.

The structure of the gut-brain axis

The gut and the brain are physically connected through multiple channels, including the vagus nerve, the immune system and various chemical signalling pathways such as neural, hormonal, immune and metabolic pathways. The most well-known pathways in the gut-brain communication system are:

1. Neural pathways

The enteric nervous system (ENS), often referred to as the "second brain," is a complex network of neurons embedded in the lining of the gut. It controls digestion and local reflexes, and communicates with the CNS, particularly through the vagus nerve.

This is the longest cranial nerve and plays a key role in sending signals between the gut and brain. The vagus nerve sends signals in both directions, carrying information from the brain to the gut and vice versa.

2. Hormonal and chemical signalling

The gut secretes hormones and neurotransmitters that can influence brain function. For example, around 90 percent of serotonin, which is involved in mood regulation, is produced in the gut, and around 50 percent of your dopamine.

The gut also releases ghrelin (hunger hormone), leptin (satiety hormone) and other signalling molecules that can affect appetite, mood and overall brain function.

3. Immune system interaction

The gut is home to the largest portion of the body’s immune system, with over 70 percent of immune cells residing in the gut-associated lymphoid tissue (GALT). The microbes of the gut can influence the immune system’s communication with the brain, contributing to inflammation, which is increasingly recognised as a key factor in both gastrointestinal and neurological disorders.

The role of the gut microbiota

The gut microbiota is a diverse community of trillions of microorganisms, including bacteria, viruses, fungi and other microbes, that live in the digestive system. The gut microbiota is essential for digestion as it is involved in nutrient absorption and protection against pathogens. However, it also significantly impacts brain health and function.

The microbiome is the entire habitat, including microorganism and their genes, as well as the environmental conditions. The microbiota influences brain activity through various mechanisms:

• Production of neurotransmitters: Certain gut bacteria can produce neurotransmitters such as serotonin, dopamine and GABA (gamma-aminobutyric acid). These chemicals are key players in regulating mood, stress responses, and cognitive functions. Disruptions in the balance of these neurotransmitters due to imbalances in the microbiota have been linked to mood disorders like depression and anxiety.
• Influence on inflammation: The gut microbiota helps regulate the body’s immune response. Dysbiosis, which refers to an imbalance in the microbial community, can lead to chronic low-grade inflammation. This inflammation may affect the brain, potentially contributing to neuro-inflammatory diseases like Alzheimer’s and Parkinson’s disease or mental health conditions such as depression.

When there is an imbalance of the components of the microbiota, some symptoms can start to appear. These symptoms can be physical or mental.

The impact of the gut-brain axis on mental health

The gut-brain axis is being recognised as an important factor in the development and treatment of mental health disorders. The state of the gut microbiota is linked to a variety of conditions, including:

• Depression and anxiety: When the diversity of the gut microbiota is reduced or when harmful bacteria dominate, it can lead to changes in mood and behaviour. The gut's production of serotonin may be compromised, leading to symptoms of depression and anxiety. Furthermore, inflammatory processes can affect brain regions involved in emotional regulation, such as the hippocampus (important for memory) and prefrontal cortex (used for attention, making decisions, solving problems and more).
• Stress: The stress response is activated by the hypothalamic-pituitary-adrenal (HPA) axis, which is influenced by the gut microbiota. Chronic stress or PTSD (Post-traumatic Stress Disorder) can disrupt the gut microbiome due to high amounts of cortisol and adrenaline.

The gut-brain axis and gastrointestinal disorders

The gut-brain axis also plays a critical role in the development of various gastrointestinal disorders. Irritable bowel syndrome (IBS), inflammatory bowel disease (IBD) and other gut-related conditions may have a strong neurobiological component influenced by the gut-brain axis.

Stress and psychological factors can exacerbate gut symptoms. But the inflammation caused by these diseases can lead to symptoms like fatigue, depression, and anxiety. Here, we can see the bi-directional communication of the axis.

Potential therapeutic applications

With greater recognition of the gut-brain axis, we now have new options for the treatment of both gastrointestinal and neurological disorders. These can include:

• Probiotics, prebiotics and psychobiotics: Supplementing the gut with beneficial bacteria (probiotics) or promoting the growth of good bacteria (prebiotics) may help restore balance in the microbiota and improve gut-brain communication, and therefore gastrointestinal and mental problems. Psychobiotics are probiotics that improve psychological states (mood, anxiety and so on).
• Dietary interventions: The Mediterranean diet, rich in fibre, fruits, vegetables and healthy fats, has been shown to support a healthy gut microbiome and reduce inflammation, potentially benefiting both gut and brain health.
• Psychological therapy: Having a space to talk, where we can express ourselves with freedom and without being judged, is important to be able to liberate emotional stress and reduce the levels of cortisol. It is essential to get to the root cause of the emotional issue with an expert. We also need strategies and tools to learn how to manage the stressful situations in our lives and take control and action over them.
• Exercise and sport: Exercising our body or practising any sport regularly helps the diversity of the microbiome and can have an impact on physical and emotional symptoms.

Consider your own gut-brain axis

The gut-brain axis influences both mental and physical health. Take a moment to consider your own physical health and mental well-being - perhaps some of these new discoveries can help improve your condition and solve long-term issues you may have been struggling with.