What is the gut-brain axis exactly? In its simplest form, the gut-brain axis is the direct, back and forth communication between your enteric nervous system (ENS), which is the nervous system that lives in your gut, and your central nervous system (CNS), the nervous system located in your brain and spinal cord.
The more we learn about the gut and gut microbiota, the more it’s clear that it really is our ‘second brain.’ So, your nervous stomach may not just be a coincidence after all.
The complex communication between your gut and brain is vital for your everyday functioning. For example, it helps you recognise when you're hungry, and monitors how your gut functions in response to certain situations, like stress.
The gut-brain axis: How do the gut and brain connect?
In physical terms, your brain is connected to your gut by the vagus nerve. The vagus nerve sends information to and from your brain, coordinating a variety of key but unconscious bodily functions from heart rate to digestion.
The second brain
You might have viewed your gut as relatively simple, but it actually has its own highly complex nervous system, the enteric nervous system. This contains more nerve cells than your spinal cord (about 100 million) and these cells help your gut digest food.
Because the gut holds such a large number of nerve cells, it has been named the second brain. While the second brain doesn’t get involved in day to day thought processes like what you want to make for dinner, or what exercise routine to do, it is thought that it can control behaviour on its own. This is believed to have come about to make digestion more efficient.
Because this second brain is so complex, scientists aren’t convinced that it was purely designed for digestion. While it isn’t capable of thoughts as such, it does ‘talk’ to the brain in major ways.
Gut bacteria and the brain
The trillions of microbes that live in your gut (otherwise known as your gut microbiota) play a significant role in the function of your body, especially digestion, nutrient absorption, immunity and mood.
Studies suggest these gut microbes communicate to the brain via the vagus nerve. They also make chemicals that affect brain function, for example, short chain fatty acids that reduce appetite.
Gut microbes also make neurotransmitters, which control emotions. A good example is serotonin, a large proportion of which is produced in the gut. Serotonin contributes to feelings of happiness and also helps control your body clock.
Your gut bacteria also produce gamma-aminobutyric acid (GABA), which helps control feelings of fear and anxiety.
Can gut health affect mental health?
Initially, it was thought that anxiety, stress and depression contributed to gut conditions like IBS, but there has now been evidence to show that it may also be the other way round.
Gut health and mood
If your gut microbiota becomes imbalanced - which means certain good strains of bacteria are lacking and bad bacteria are thriving - it can affect your brain and mood.
Gut health and anxiety
Research shows that stress is very closely linked to the gut.
We respond to stress with a ‘fight or flight system,’ which is a physical response to the stress. During fight or flight, is the digestive system is switched off as it's not viewed as "essential" in the moment. This is only temporary and when the stressful situation is fleeting, your body returns to normal.
However, if you’re constantly stressed, your body gets stuck in that fight or flight phase over an extended period of time. And this can lead to chronically bad digestion, such as constipation and IBS-type symptoms.
Natural ways to improve your gut-brain axis
While there’s still a lot we don’t know about the gut and all it affects, there are a few things that we know can help improve the gut-brain axis.
1. Eat a whole foods-based diet
Diet is one of the most important factors when it comes to the health of the gut-brain axis. The food you eat directly influences good gut bacteria and a whole foods-based diet leads to a much healthier gut than one consisting of mainly refined and processed foods. Eat a variety of colourful fresh fruits and vegetables, oily fish, fermented foods, high fibre foods and foods rich in tryptophan such as turkey, eggs and cheese. You’ll also want to include polyphenols such as cocoa, green tea, olive oil and coffee in your diet.
2. Eat pre and probiotic foods
Eating probiotic-rich foods, like kefir and sauerkraut, taking probiotic supplements to achieve a high dose, and prebiotic foods like artichokes, leeks, onion and oats, will improve the health of your gut-brain axis. Probiotics are good bacteria that line your gut and are responsible for nutrient absorption and supporting your immune system. Prebiotics, fibres that are fermented by your gut bacteria, may also affect brain health and help reduce the amount of stress hormone, cortisol, in your body.
3. Spend time in nature
To get a diverse range of bacteria populating your gut, you need to come into contact with a lot of different environments. There’s a huge difference between the bacteria that live in cities compared to those that live in farms with livestock. Taking a trip to the countryside can actually lessen your risk of allergies because exposure to this kind of bacteria trains your immune system to tolerate non-harmful bacteria and substances. It has also been shown to help to reduce stress which can further support a healthy gut and strong gut-brain axis!
4. Move your body
Physical activity can improve conditions in your gut so that beneficial bacteria can thrive. If you can combine exercise with getting outdoors, like running in the woods or doing your workout in a park, it will further benefit your gut health. Exercise also can prevent constipation by keeping the muscles in your gut moving food through.
Summary
There’s still a lot that we're learning about the gut-brain axis. Although the connection between the two is quite complicated, it is clear that if we can maintain a diverse, balanced and healthy gut microbiota by implementing some healthy dietary and lifestyle changes, we will be positively supporting our mental health.
References
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Control of appetite and energy intake by SCFA: what are the potential underlying mechanisms?
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Serotonin, tryptophan metabolism and the brain-gut-microbiome axis
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Bifidobacterium adolescentis as a key member of the human gut microbiota in the production of GABA
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The role of the hypothalamic-pituitary-adrenal axis in neuroendocrine responses to stress
