The Gut-Brain Connection: Exploring How Your Microbiome Impacts Mental Health

Have you ever considered the possibility that your gut could be influencing your mental health? Research suggests a surprising link between the digestive system and the brain, known as the ‘gut-brain connection’. The human body is home to thousands of bacteria, working in harmony with the rest of your cells to create a healthy internal environment. 

But what happens when things become unbalanced?

Here, we delve into this question to address how the microbes in your gut can impact your mental health and how you can improve this relationship.

What is the human microbiome?

The microbiome is a community of microorganisms, including bacteria, fungi and viruses, that live both in and on the human body. Microbiota is a more specific term that highlights microorganisms in a defined area, like the gut or the skin. Gut microbiota are a vital part of the human microbiome, particularly regarding the gut-brain connection. 

Gut microbiota contributes to maintaining healthy body functions in a variety of ways:

• Aiding digestion
• Providing nutrients we can’t get from food 
• Strengthening the intestinal wall
• Regulating the immune system 
• Preventing disease and infection.¹

What is the microbiome made up of? 

Whilst viruses and fungi are also part of the microbiome, bacteria get the most attention. It is currently believed that each individual has equal numbers of bacteria and human cells.²

Though there are lots of different types of bacteria within the human microbiome, there are 6 main groups under the names of:

• Firmicutes 
• Bacteroidetes 
• Actinobacteria
• Proteobacteria 
• Fusobacteria 
• Verrucomicrobia.³ 

Are there good and bad bacteria in the gut? 

The simple answer is no. With different numbers of bacteria present in each individual due to many factors, it is difficult to define a ‘normal’ human microbiome. That being said, certain ratios of gut bacteria indicate a healthy or unhealthy microbiome. For example, individuals who have more Bacteroidetes than Firmicutes tend to have a healthier gut.⁴ If something offsets this balance, it could lead to a whole host of side effects and implications in conditions including anxiety, depression and even neurodegenerative diseases. 

What is the gut-brain connection?

The gut-brain connection acts like a motorway for continuous two-way communication between the digestive system and the central nervous system. 

Different types of communication occur along a variety of pathways that make up this motorway, including:

• Nerve signals
• Hormones
• Metabolites 
• Inflammatory signals.⁵

Nerve signals

One of the main nerves involved in gut-brain communication is the vagus nerve. It sends information about the gut environment up to the brain so it can act accordingly. The vagus nerve communicates through the use of neurotransmitters. These small signalling molecules send specific information and orders between nerve cells. Messages sent and received depend on the type of neurotransmitter used, for example, a molecule called GABA can inhibit the signal being passed on further.

Hormones 

The gut is home to enteroendocrine cells (hormone-producing cells in the gastrointestinal tract), which release hormones, such as serotonin, in response to a variety of signals. Whilst mainly associated with its role in the brain, 95% of this ‘feel-good hormone’ is produced and stored in the gut!⁵ The release of cortisol, otherwise known as the stress hormone, depends on the activation of the hypothalamic-pituitary-adrenal axis (HPA). Gut microbiota can provide us with nutrients that trigger the release of molecules from enteroendocrine cells and influence the activation of the HPA axis (‘stress’ axis).⁵ 

Metabolites

The bacteria in our gut help to break down some foods that we struggle to digest ourselves. For example, fibre-rich foods, including fruits and vegetables, are broken down into short-chain fatty acids (SCFAs), which are vital in maintaining our gut health. SCFAs are also important in the production of serotonin and other signalling molecules. Reductions in the types of bacterial species that help to break down foods into SCFAs can interfere with gut-brain communication.⁵

Inflammatory signals 

Inflammation of the gut places additional stress on the microbiome, which triggers the release of inflammatory signals into the bloodstream. The barrier that protects the brain (blood-brain barrier) can weaken under this stress, allowing these molecules to pass from the bloodstream into the brain and negatively influence its function.

The link between the gut and mental health

Since the gut and brain are linked so closely, it has become clear that the gut may have more of an influence on mental health than what was originally believed. Several studies have shown that an imbalance of gut microbiota can influence the development of anxiety and depression. 

Mood disorders 

Depression 

Depression is a common mental health condition that is characterised by persistent low mood and a loss of interest in activities.⁶ As with most mood disorders, there are a variety of factors that come together to trigger the onset of depression.

Interestingly, individuals with depression tend to have a lower total number of bacteria in their gut, as well as reduced diversity compared to healthy individuals. Since this environment isn’t as strong, the wall of the intestines can become ‘leaky’ and allow bacteria into the bloodstream.⁷ This series of events can send inflammatory signals along the gut-brain axis, causing a vicious cycle that can worsen the depression.

Anxiety 

Anxiety is characterised by excessive worry, which can impact an individual's quality of life. Similar to depression, people with anxiety also have an imbalance in their gut microbiome. In particular, they seem to have significant reductions in the type of bacteria that produce SCFAs.8 This is thought to interfere with the release of hormones and neurotransmitters, which could lead to anxiety. Additionally, a prolonged stress response is often seen in individuals with anxiety. The increase in cortisol is likely to weaken the intestinal wall, allowing bacteria to enter the bloodstream.⁹

Neurodegenerative diseases

Whilst neurodegenerative diseases, like Alzheimer’s and Parkinson’s disease, aren’t considered under the bracket of mental health, there is increasing evidence that suggests the imbalance of gut bacteria could contribute to their progression or even their onset. 

Parkinson’s disease is a progressive condition, with symptoms including tremors and difficulty initiating movement. These symptoms occur following a build-up of a toxic protein called α-synuclein in the brain.¹⁰

Individuals with Parkinson’s disease have also been reported to have an imbalance in their gut microbiota. α-Synuclein is also located in the gut, and studies have suggested that the altered microbiome may trigger the build-up of this molecule and encourage its transport along the gut-brain connection.¹¹

What is influencing the health of the microbiome?

Many factors influence the composition of the microbiome, with some causing subtle changes and others completely altering it. 

Diet and lifestyle 

Diet is the most influential factor in maintaining a healthy gut microbiome. Eating a varied and balanced diet not only promotes an environment for better physical health but is linked to improved mood and well-being. 

Different food groups have different properties that promote gut health, which is why it is important to consider adding some more variety to your diet!

Here are some examples of food groups that benefit your gut and brain:¹²

Other lifestyle patterns such as smoking, drugs and lack of sleep can reduce microbiota diversity, which in turn may have a negative impact on mental health.¹³

Antibiotics 

Whilst sometimes it is necessary to take antibiotics, their destructive nature means that they strip your gut of all bacteria. This often causes a large imbalance in the microbiome that can increase the risk of depression and anxiety.¹⁴ Taking probiotics or eating fermented foods after a course of antibiotics could help to replenish your microbiome.

Summary 

There is scientific backing behind the phrases ‘gut feeling’ and ‘butterflies in your stomach’. The gut-brain connection allows for constant communication, which is easily influenced by lifestyle choices that affect the health of your gut microbiota. Whilst there are often a variety of factors that can lead to mental health disorders and neurodegenerative diseases, the role of the microbiome is gaining more attention in research and the media. A balanced diet is vital in helping to maintain a healthy gut, which will also promote brain health and improve mood and well-being.