It is now apparent that the bacteria living within our gut (the gut microbiome) are essential for our physical wellbeing. Numerous studies implicate the gut microbiome’s role in maintaining our health through important processes such as developing and modulating our immune system. Allergies, autoimmune diseases, chronic conditions and even obesity are all thought to result from possible alterations in this environment.
More recently, the role of the gut microbiome in maintaining or contributing to our mental health or illness is being the focus of many studies.
The Gut-Brain Axis
There are several ways in which the gut microbiome is thought to interact with the brain – a communication system now called the Gut-Brain Axis (GBA).
This system is comprised of the central nervous system (the brain and the spinal cord) and the gut’s own nervous system (the enteric system). This enteric system is able to function relatively independently from the brain.
The brain contains around 100 billion brain cells called neurons. Interestingly, the gut is also known to contain a significant amount of the same types of cells – thought up to 500 million. One way by which the gut neurons are thought to signal to the brain neurons is by one of the biggest nerves in the body – the vagus nerve – which in turn signals to a brain system known as the hypothalamic and limbic system. This system is known to be responsible for the regulation of emotions. This communication system can work in both directions – hence why feelings of stress can sometimes influence our gut function.
It is now shown that bacteria produce a variety of products that can interact with our GBA. For example, some bacterial species can produce a substance called GABA (gamma-amino butyric acid), a known inhibitory neurotransmitter in the brain, whilst other bacteria produce dopamine – another neurotransmitter involved in numerous functions such as movement, but also the pleasure and reward centre. Serotonin – scientifically known as 5 hydroxy-tryptamine (5HT) – is also known as the happy hormone, as it contributes to happiness and wellbeing.
Interestingly, over 90% of the serotonin within our body is actually produced by a particular type of cell in the intestine. Studies using germ-free mice showed that their production levels of serotonin were reduced by over 60% when compared with mice that had gut bacteria present. When researchers introduced bacteria into the germ-free mice, the levels of serotonin production increased to that of the mice with a normal gut microbiome.
The gut microbiota are also involved in the production of short chain fatty acids – essential fuel for the colonic cells that help the colonic cells to release mucosal serotonin.
How can our mood affect our gut?
Studies have shown that when we suffer with chronic stress, this can lead to a change in how our intestines let substances through its barrier – a term known as increased intestinal permeability. This condition is called leaky gut syndrome. There is still a lot of debate as to whether this condition exists, however the theory is that bacteria, bacterial toxins and food substances can pass through this leaky barrier into the bloodstream and trigger widespread chronic inflammation that can lead to a variety of physical and mental health conditions. Studies show that conditions such as depression and potentially even autism may be linked with this – though evidence is limited at present.
Another focus of research is that the gut microbiota can produce chemicals that may themselves directly influence the development of disorders such as depression and anxiety. A large European study recently showed that there were several species of gut microbiota missing in a group of people suffering with depression – but that these bacteria were present in the group of people who were free from depression.