Most of us intuitively know that there is a strong link between our gut and brain. Almost everyone has experienced ‘butterflies in the tummy’ when they are feeling nervous. For some people, stress and anxiety can cause or worsen stomach problems, such as with irritable bowel syndrome (IBS). Our gut has been referred to as ‘the second brain’ and it certainly seems to have the ability to mirror our emotions and mood. There is an intricate system of nerves in the gut which are connected to the brain by a large nerve called the vagus nerve. The vagus nerve can transmit signals between the gut and brain very rapidly and efficiently, allowing both organs to respond to each other almost simultaneously. Over the past decade, researchers have discovered that our gut bacteria play a huge role in the communication between our gut and brain and can actually influence how we feel and behave. This is both an exciting and startling finding! We have known for a long time that there were bacteria in our gut but we presumed that these were a harmless bunch of bugs that lived off our food and, in return, didn’t cause us any trouble. We now realise that they influence almost all aspects of our health, not least of all our mental well-being.

The sheer number of bacteria in our gut, termed our ‘gut microbiome’ is staggering. For every one human cell in our body we have at least ten bacterial cells. How exactly do these bacteria influence our brain? Researchers all over the world have been trying to uncover the answers and it appears that our microbes can impact our psychological state in various different ways.

  • Gut bacteria play a role in the development of our body’s stress response system which reacts to stress by producing the hormone cortisol. Researchers have shown that animals who lack gut bacteria are far more easily stressed and release more cortisol than animals with a normal microbiome. In humans, studies have shown that the consumption of certain probiotics seems to reduce a person’s response to stress.
  • Bacteria themselves can produce many of the chemical messengers that we use in our human brains. They have the ability to make serotonin, noradrenaline, dopamine and GABA, all chemicals which have been shown to play a role in depression and anxiety
  • As well as producing serotonin directly, the gut microbiome influences the way serotonin is used in our body. Many people will have heard of serotonin, a chemical messenger which plays a role in mood regulation. It is thought that low levels of serotonin contribute to the development of depression and anxiety disorders and most anti-depressant and anti-anxiety medications act by boosting the brains serotonin levels.
  • Our gut bacteria have a huge influence on our immune system, both in terms of development of immunity as an infant and ongoing functioning into adulthood. Immune changes are thought to be important in the development of many different mental illnesses.
  • Bacteria break down carbohydrates in our gut to produce butyrate, a tiny molecule which has been shown to have a protective effect on the brain. It is currently being researched for potential benefits in brain disorders such as Alzheimer’s disease and autism.

So which bacteria, of the trillions out there, are good for our brains? Well it is early days and a lot more research and clinical trials need to be done before we can definitively answer this question. However, studies to date would suggest that various Lactobacillus and Bifidobacterium strains appear to offer psychological benefits. One of the easiest ways to improve and maintain a healthy gut microbiome is through our diet. Green leafy vegetables, fibre, wholegrains and fermented foods such as yoghurt, kefir, kombucha and sauerkraut all promote the growth of good ‘probiotic’ bacteria in the gut. In contrast, the highly processed, high-sugar foods that are typical of the Western diet appear to negatively impact mental health. This concept about healthy eating is not a new one but research is now putting the science behind the message and demonstrating the effects of food on our gut microbiome and subsequently our mental health.

There has only been one study looking at the link between social anxiety disorder and the gut microbiome. In 2015, a research team in the US asked 710 university students to complete questionnaires on diet, personality factors and social anxiety. They found that people with higher levels of ‘neuroticism’ on personality tests were more likely to demonstrate social anxiety symptoms, a finding which is unsurprising. What was interesting was that in those with high neuroticism scores (i.e. at higher risk of social anxiety), greater consumption of fermented foods was associated with fewer symptoms of social anxiety. Although far from conclusive, this would seem to suggest that consumption of probiotic-containing fermented foods may offer some protection against the development of social anxiety disorder in those who are susceptible.

The gut microbiome is a new area of research and one which has generated a great deal of excitement and interest. The potential for improving mental well-bring and treating mental illness through targeting the gut bacteria with dietary change and probiotics is very appealing. It is not suggested that this will replace more traditional treatments, including psychotherapy and medication, but it is likely to become an important part of holistic care. Social anxiety disorder has been largely ignored in the research world despite the fact that is extremely common and causes a great deal of suffering. The APC Microbiome Institute based at University College Cork is currently running a research study investigating the composition of the gut microbiome in people with social anxiety disorder. If you are interested in this study you can find more details at or contact Dr Mary Butler at

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