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What is the gut-brain axis?

A link between the gut and the brain has been theorised for thousands of years. The science is beginning to catch up.

Heights
Heights
March 09, 2021
7 min read

Believe it or not, braincare isn’t just confined to the brain. In recent years, there’s been more and more research into the gut-brain axis, and the mind-gut connection, as a potential contributing factor in our mental health, performance, and mood. But what exactly is the gut-brain axis, and how might we go about using it to look after our brains.

Article breakdown

What is the gut-brain axis?

At its most basic level, the gut-brain axis is the connection between the gut and the brain. That’s it. The most recognisable example is probably the nervous tummy feelings you might get before a first date or interview. 

The gut is connected to the brain both physically and chemically. Physically, there are millions of nerves between the two, most importantly the vagus nerve, the longest nerve in the body. And chemically, our gut microbiomes—the term for the ecosystem made up of bacteria and other microorganisms in our guts—can also communicate with our brains. They produce various compounds that can cross the blood-brain barrier and influence how we feel. This means our gut health can impact our mental health, and vice versa.

In understanding the relationship between the brain, the gut, and the enteric nervous system (the network of neurons in the digestive system also known as the gut second brain), we can open up all sorts of new areas for research.

Why are gut bacteria important?

Bacteria are a specific group of single-celled organisms. They are everywhere and coat almost every part of us. In fact, there are more bacteria in your body right now than there are cells.

For the last 100 years or so, science and medicine have approached them by killing them with antibiotics. However, not all bacteria are harmful—in fact, they are our first line of defence against pathogens. Without our bacteria, we would never be able to sustain a potent immune system.

Our genes take 10,000 years to change, but bacteria can evolve in a matter of hours. Pathogens are genealogically outrunning us, but a healthy set of bacteria allow us to keep up. And that healthy set of bacteria predominantly live in our gut.

How does stress affect the gut?

The effects of acute stress on our gut could manifest as those stomach-churning feelings you get before a high-pressure situation. That’s the gut-brain axis in action. The muscles in the gut start squeezing and relaxing more, resulting in the need to go to the bathroom. And with levels of stress only going up post-pandemic, the connection between stress in the brain and stress in the gut is more relevant than ever.

For example, elevated stress hormones can potentially reduce or suppress the growth of beneficial bacteria and other microorganisms. Over time, this could disrupt the balance of microorganisms in the gut and affect how we digest food. As a result, nerves in the enteric system become hyper-activated and can be switched on all the time. This can become a vicious cycle. 

It’s theorised that when we are more stressed, we produce more digestive enzymes, more stomach acid, and things move more rapidly through our digestive system. As a result, the bacteria that live in our colons have more substrates and food to ferment, creating more of these symptoms and allowing more space for the less beneficial bacteria in our bodies.

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Can certain foods trigger symptoms?

Specific foods can also trigger the same symptoms that you’d get from a stress response. An understandable reaction is to cut foods out of your diet, but while this may help in the short term, it’s not recommended to limit foods in your diet in the long term without professional medical advice.

Mood and the gut-brain axis

The gut communicates with the brain directly through the vagus nerve. There is also a three-way connection between the brain, the gut bacteria, and the enteric nervous system (that gut second brain again).

Many gastrointestinal disorders can be explained by stress, anxiety, or mood disorders. For example, if you get an inflamed leaky gut, the bacteria in the gut wall will send signals to the brain through the bloodstream and create even more inflammation.

What is the connection between gut bacteria and mental health?

This is a relatively new area of research, but scientists have looked at the gut bacteria and mental health of patients who have anxiety or depression. There’s data to suggest that when someone is diagnosed with anxiety, they often have a different profile of gut bacteria in their colon. 

Scientists are currently trying to establish where the causality is. Do the bacteria in the gut drive the mood disorder, or does a mood disorder affect the gut enough to change the microbiome? Understanding this relationship is key to the gut-brain axis.

Another way that mood and the gut can interact is via the blood. Neurotransmitters produced in the brain, like serotonin and dopamine, cannot cross the blood-brain barrier, but they produce certain compounds that can. While there’s not enough research at the moment, this does indicate another possible means of connection between gut and brain.

At this point, we should stress that it’s essential that you do not replace prescribed treatment with a change in diet. If you’ve been prescribed anything by a medical practitioner, think about your diet and the gut-brain axis as a way to supplement that treatment. It’s also important to avoid ‘diet-shaming’ patients for their mental health problems.

Probiotics, prebiotics, and your gut bacteria

The bacteria and other microorganisms that make up your gut microbiome are pretty important, so it’s important we do what we can to maintain that system. Perhaps unsurprisingly, what you eat is one of the best indicators of the general state of your gut bacteria (and your mental health, incidentally), and there are specific things that we eat that can help.

Probiotics—supplementing the microbiome

Probiotics have been trendy for a while now. The term refers to any food or drink that contains live bacteria or other microorganisms with health benefits. Probiotics are thought to help restore the balance of bacteria in your gut, including when it's been altered by external factors like illness or treatment. Examples of probiotics include:

  • Sauerkraut

  • Kimchi

  • Live yoghurt

  • Kefir

  • Kombucha

  • Fermented pickles

  • Some unpasteurised cheese

Prebiotics—feeding the gut

While probiotics provide the bacteria directly to your gut, prebiotics prepare the environment for bacteria to grow healthily. These are typically non-digestible fibre, which can provide both nutrition and a habitat for microorganisms in the small and large intestine. Examples of prebiotics include:

  • Jerusalem artichoke

  • Garlic

  • Leeks

  • Asparagus

  • Barley

  • Oats

Psychobiotics—the mind-gut connection

A psychobiotic is anything that works with the gut to affect the mind, improve mood, or reduce levels of depression or anxiety. More concretely, this is usually some combination of both pre- and probiotics that are designed to work in unison to nurture a healthy gut microbiome. This can then have a positive impact on mental health and mood. 

It’s possible to get the requisite combination of pre- and probiotics from a balanced diet, but recently, specific psychobiotic supplements aimed at the gut-brain axis have started to come to market.

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What you should eat for a healthy gut

That said, before supplementing, it’s a good idea to work on what you’re eating first. Over the past 50 years, industrialisation in food processing has reduced the amount of fibre in much of what we eat. But by taking out fibre, you’re inhibiting the growth and development of bacteria in your gut. Research has suggested that some species of gut microorganisms have been driven to extinction. 

To combat this, and as an excellent starting point for exploring psychobiotics, aim to incorporate more probiotics and prebiotics into your everyday diet. Rather than a soft drink, have some kombucha; add some kefir to your breakfast; and incorporate more high-fibre vegetables into simple weeknight dinners, like stir-fries. 

There’s still a lot that we’re learning about the gut-brain axis, but the link between gut bacteria and mental health has the potential to be a game-changer in the world of braincare. If you want to learn more about the science behind psychobiotics and the gut-brain axis, take a look at our discussion with Scott Anderson, the author of the ground-breaking Psychobiotic Revolution.

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