Have you given a presentation at work and experienced a nervous stomach beforehand? Or walked into a restaurant for a date with “butterflies” in your tummy? These everyday expressions we use to describe the effect our thoughts, feelings and emotions have on our digestive system are rooted in the two-way communication between our gut - also known as our “second brain” - and our actual brain.
This second brain is the enteric nervous system (or ENS) and is a matrix of around 500 million neurons that line the gastrointestinal tract, from oesophagus to rectum. Although it’s not capable of thought per se, it communicates expertly with our brain.
The communication between the gut (a handily succinct word to describe the vast gastrointestinal tract in its entirety) and the brain is complex and intricate. It’s a two-way street and conversations passing in each direction inform, direct and respond to each other in order to achieve physiological balance in our body. The signals are sent both biochemically and physically in a mix of neurons, hormones, immune cells and microbial molecules.
It has been well established that the signalling between the gut and brain influences a number of things, including the way we behave and the actions we take linked with hunger and the anticipation of food. But in recent years, research is mounting that shows the influence of the gut–brain connection going far beyond feeding and digestion.
Just after you’re born, the colonisation of millions of bacteria (microbes) begins in the gut and continues in parallel with cognitive development. Research has shown brain development, subsequent behaviour and cognitive function are shaped by this, as the microbiota are pivotal to the maturation of the central nervous system (CNS) and ENS in those early weeks of life.
An emerging body of animal-based studies demonstrates that alterations in the gut microbiota can affect brain neurochemistry. These studies are conducted on germ-free animals and tend to focus on introducing antibiotics, probiotics and faecal transplants to observe how altered microbiota influences brain activity. Early research suggests the gut microbiome is able to influence numerous brain functions including (but not limited to) mood, emotion regulation, stress response and social behaviour.
When we experience stress, the gut–brain axis and the HPA axis (hypothalamic-pituitary-adrenal axis) are activated, leading to the release of stress hormones such as cortisol. This is one aspect of our natural fight-or-flight response. This response is vital for survival and is what has kept humans alive for millennia. However, if we are in state of heightened stress (often the case!), the elevated cortisol levels have been shown to affect the composition of bacteria in the gut.
Our brains, and specifically the limbic system that manages the fight-or-flight response, have not had an upgrade since the days of the sabre-toothed tiger. Stress can now be triggered by a full email inbox, a train running late or no toilet paper on supermarket shelves.
You may have heard the fact floating around that 90 per cent of our feel-good hormone serotonin (or 5-hydroxytryptamine) is made in the gut. Whilst structurally the same as the brain, our gut-derived serotonin has localised effects, such as the perception of nausea, pain and peristalsis (the rhythmical movement of the gut muscles to transport food through the tract).
This being said, there is still much to be understood about the effect of peripheral serotonin on mood. What has been determined is the production and release of serotonin from the enterochromaffin (EC) cells in the gut can be controlled by our gut bacteria. It’s clear that getting to grips with this mechanism might be important for understanding, and potentially influencing, levels of serotonin in the brain and revolutionising how we think about mental health.
This is really just the tip of the iceberg when it comes to our gut bacteria and its possible influence on mood. We’d need a few more cups of tea to work our way through this topic.
So, what does this mean for us? How can we make sure we’re getting the most out of our gut microbiome?
We can start by feeding it well…
Diverse range of foods
Scientific consensus is that an ‘ecosystem’ of varied bacteria is a huge plus for our health. Each strain of bacteria performs a different beneficial function and each strain needs different foods to thrive. When you eat a diverse variety of foods, you’re giving your microbiome the best chance at this.
"Eat food, not too much, mostly plants" – Michael Pollan
Fibre found in complex-carbohydrate plant foods is an important part of our diet, yet we don’t have the enzymes to break it down and digest it. That job falls to our gut microbiome and is the reason fibre is known as “prebiotic”, it nourishes our gut microbiome and allows it to flourish. Recent figures show that in the UK, most adults are only reaching 60 per cent of the recommended 30 grams per day. Good sources include broccoli, apple, chia seeds, leeks, onion, asparagus, raspberries, lentils, kidney beans, chickpeas, quinoa and oats.
Aim for whole foods first where possible as additives, artificial sweeteners and preservatives have been shown to upset gut microbiome balance.
Fermented foods and live yoghurts
There is conflicting evidence on whether the beneficial bacteria in fermented foods helps the gut. To be able to go on to colonise the lower gut where it becomes useful it has to survive the acidic environment of the stomach. The same applies for the bacteria in live yoghurt. So, eat these if you enjoy them and any potential protection is an added bonus!
Polyphenols are micronutrient compounds found in certain plant foods. They are well known for their antioxidant and anti-inflammatory properties, but studies show they also promote the growth of beneficial gut bacteria, whilst fending off some bad bacteria that may be loitering. They can be found in foods like green tea, cacao, olive oils, red wine and berries to name but a few.
S.M.A.S.H. is a handy acronym for sardines, mackerel, anchovies, salmon and herring. All rich in Omega 3 fatty acids. Current UK guidelines are two portions of oily fish per week. If you’re not hitting that, a good quality Omega 3 supplement taken daily is a quick win.
The emotional response we feel in our gut at the moment may be unwelcome. Some of us feel the effects more keenly or regularly than others but this is by design. Often your “gut feeling” (or instinct) is stepping in at just the right time to protect you. So, listen to your gut and feed it well!
Charlotte Faure Green is a registered nutritionist who provides expert one-to-one guidance both online and in person at her Brighton clinic (when we’re not in lockdown!). She helps stressed bodies and minds regain balance through real-world sustainable changes. You can find her on Instagram @charlottefauregreennutrition or contact her through her website at charlottefauregreen.com.