Understanding the gut-brain connection

 

Have you ever felt ‘butterflies’ in your stomach before a big event, lost your appetite when stressed, or had an intuitive ‘hunch’ about someone? These intriguing sensations are all down to a highly sophisticated network called the gut-brain axis.
 
The gut-brain axis is a complex communication system between your gut and brain. Until recently, scientists believed that only the brain could influence the gut. However, research now suggests the inverse can also be true. In other words, your brain can affect your gut just as much as your gut can affect your brain.
 
In this article, we delve deeper into the gut-brain connection, exploring its effects on mental health and outlining the best dietary changes to support this important communication highway.
 

What is the gut-brain connection?

 
The gut-brain axis bridges the central nervous system, including the brain, with the enteric nervous system in the gut – often dubbed the ‘second brain’.
 
The enteric nervous system contains 100 million nerve cells, which are divided into the parasympathetic (‘rest and digest’) or sympathetic (‘fight or flight’) nervous systems.
 
The parasympathetic nervous system is active during times of rest, supporting contractions in the gastrointestinal tract. Conversely, the sympathetic nervous system kicks in during moments of stress, inhibiting digestion to focus on prioritising survival. 
 
For a happy, balanced gut and brain, we want to be in a predominantly parasympathetic state, which paves the way for a harmonious coexistence between the gut and the brain.
 
However, chronic stress can disrupt this delicate relationship, throwing the gut-brain connection off-kilter and compromising both digestion and mental wellbeing.
 

How are the brain and gut connected?

 
The brain and the gut are biochemically and physically tied through the vagus nerve, neurotransmitter production, and the gut microbiome.
 

The vagus nerve

 
The gut and brain connect via the vagus nerve, a large nerve running from the colon to the amygdala – the region of the brain responsible for processing primal emotions like nerves and stress (1).
 
This connection highlights how mind-body practices like diaphragmatic breathing can reduce stress so quickly. Consciously breathing from your diaphragm (stomach) massages the vagus nerve and activates a parasympathetic response (‘the rest-and-digest’ state), helping the body relax and rebalance.
 

Neurotransmitters

 
The gut and brain are also closely intertwined through neurotransmitters, chemical messengers controlling feelings, emotions, and digestion. 
 
Studies suggest that gut bacteria are responsible for producing around 95 per cent of serotonin, an important neurotransmitter influencing mood, sleep, and gastrointestinal activity (2).
 

The gut microbiome

 
The trillions of microbes in the digestive tract – collectively known as the gut microbiome – and the brain have an important relationship, too (3).
 
When your gut microbes digest fibre, they produce short-chain fatty acids (SFCAs), including butyrate, propionate, and acetate, which affect brain function (4).
 
Moreover, the gut microbiome impacts inflammation due to its close relationship with the immune system. For context, approximately 70% of the immune system, particularly the pathogen-fighting white blood cells, resides in the large intestine, where they constantly interact with gut microbes.
 
A well-balanced gut microbiome with ‘good’ strains of bacteria may reduce chronic inflammation, which, for clarification, is the overactivity of the immune system (5).
 
However, an imbalance favouring ‘bad’ communities of bacteria can lead to unwanted inflammation associated with mood changes and neurodegenerative diseases (6).
 

How does the gut-brain connection affect mental health?

 
Digestive issues, such as bloating, constipation, nausea, and abdominal pain, have long been associated with stress and low mood (7).
 
Traditionally, scientists believed mood disturbances contributed to digestive problems. Indeed, mental health conditions like anxiety are often associated with chronic gut disorders (8).
 
However, research now suggests poor gut health may also drive psychological changes.  For instance, a recent observational study found people with low mood often have depleted levels of Coprococcus and Dialister (species of gut bacteria), compared with those who reported better emotional health (9).
 
What is clear is that gastrointestinal stress can be the cause or the result of anxiety, stress, and low mood. And that’s because the gut-brain axis goes both ways.
 

Can diet affect the gut-brain connection?

 
Absolutely. According to the emerging field of nutritional psychiatry, food can significantly impact the gut-brain axis.
 
A healthy digestive system should be able to digest food, absorb nutrients, and effectively excrete waste from the body. By maintaining these functions, you can support a happy gut and, consequently, a happy brain.  
 
Here are some basic dietary principles to nurture the gut-brain axis.
 

Eat more plants

 
The gut microbiome loves nothing more than plants. Plant foods – specifically, fruit, vegetables, whole grains, legumes, nuts, seeds, herbs, and spices – contain dietary fibre and polyphenols, which feed your hungry microbes.
 
Research suggests people who eat a typical Western diet, often loaded with ultra-processed foods, refined sugar, and unhealthy fats, have more ‘bad’ microbes in their gut than those who avoid these foods (10).
 
The Mediterranean diet, on the other hand – which celebrates plant foods – is often linked to positive health outcomes. And some scientists believe this might be down to positive changes in the gut microbiome (11).
 
The Smiles Trial is one of the best demonstrations of the power of plant foods, highlighting how a healthy, balanced gut can set the stage for good emotional wellbeing (12).
 
Researchers recruited participants with low moods and randomly assigned them to either social support or nutritional support for 12 weeks.
 
Those in the nutritional support group were given a predominantly plant-based diet – foods that feed the gut – while the other group received therapeutic support.
 
Quite amazingly, more than 30 per cent of those in the nutritional support group saw positive changes to their wellbeing, compared to 8 per cent of those in the social support group.
 
This landmark study demonstrated that a plant-focused, whole-food diet can be an effective and accessible way to manage low mood. And the gut-brain axis is at least in part to thank for this.
 
Its is important to highlight that a plant-based diet alone may not be the answer to low mood, but just part of a treatment plan, which may also include therapeutic support. If you experience low moods, it is important to speak to your healthcare practitioner who can provide support and advice.
 
In essence, the gut-brain connection take-home message is choosing plant foods over processed foods.
 

Diversify your diet

 
And don’t just think plants, think diversity, too! ‘Eating the rainbow’ means getting a range of fibre and polyphenols, appealing to the varying tastebuds of your gut microbes, so mixing and matching plant foods is equally important.
 
According to the American Gut Project, individuals who consumed 30 different types of plant food every week had more diverse gut bacteria (an indication of good gut health) compared to those who ate ten or fewer plants weekly (13).
 
While aiming for 30 plant foods may sound challenging, it becomes easier with practice. You can supercharge salads with mixed seeds, spice up dishes with plenty of herbs, and bring yoghurt to life with berries and nuts.
 

Eat more fermented foods

 
Moreover, health experts now believe incorporating a small number of fermented foods every week may further support the gut microbiome and, therefore, the gut-brain connection.
 
Historically used as a means of preservation, the fermentation process also enhances the natural, live bacteria found in food, which increases the number and diversity of bacteria in the gut.  
 
A study conducted by researchers at Stanford School of Medicine found that participants who followed a ten-week diet rich in fermented foods experienced improvements in microbiome diversity – a key indicator of overall health (14).
 
Fermented foods include:
 

• Kimchi (fermented cabbage Korean-style)

• Kefir (fermented milk)

• Sauerkraut (fermented cabbage)

• Miso (fermented soybean paste)

• Tempeh (fermented soybeans)

• Kombucha (fermented tea)

• Yoghurt (fermented milk)

• Lassi (soured milk)

• Sourdough bread (fermented dough)

• Some cheeses (aged cheddar, parmesan, and Swiss cheeses)

 

Feed your gut microbes

 
Although it’s important to populate your gut with live bacteria, it’s equally important to consider what you feed them. Like us, these bacteria need plenty of nourishment to thrive. And this is where special plant fibres come in. 
 
Your hungry gut microbes particularly enjoy feasting on the following foods, so try to include more of them in your diet.
 

• Fructooligosaccharides (FOS) – asparagus, onions, leeks, garlic, bananas, Jerusalem artichokes, chicory root.

• Galactooligosaccharides (GOS) – legumes like chickpeas and lentils.

• Inulin – asparagus, onions, leeks, garlic, bananas, Jerusalem artichokes, chicory root.

• Beta-glucan – some mushrooms, barley, oats, rye, and other whole grains.

 

Consider fasting

 
Your dietary choices aren’t the only factors that can shape and improve gut health, when you eat is also crucial.  
 
Similar to humans, the gut microbiome operates on a circadian rhythm and requires time to rest, especially since a portion of your stomach lining is replaced and repaired every night.
 
The problem is about half of adults in Western countries consume food over 15 hours or longer each day, depriving the gut of essential recuperation time.
 
Now, new research suggests short periods of fasting, also known as time-restricted feeding, could significantly alter the composition of your gut microbiome, improving overall health and, crucially, supporting the gut-brain connection (15).
 
One of the easiest ways to get started is by implementing an 8-10 hour eating window. For example, this could involve eating between 9 am and 5 pm or between 12 pm and 8 pm.
 
However, there are some caveats to keep in mind. Firstly, menstruating women should only fast from days 1-19 of their cycle to support progesterone production. Pregnant women, those with a history of disordered eating, or people engaged in intensive training should also avoid fasting.
 
If you have any questions about fasting, always consult your GP. Our expert Nutrition Advisors may also be able to help.
 

Add specific nutrients

 
Incorporating the nutrients below can also provide targeted support for gut health, aiding digestion and nurturing the gut-brain connection. 
 

Live cultures

 
Live cultures are made up of good bacteria that play a vital role in gut health. Aside from increasing fermented foods, taking a live bacteria supplement can support the ecosystem in the gut.
 

Fish oil

 
Research suggests the omega 3s found in oily fish may support the production of short-chain fatty acids, supporting gut health (16). You can find omega 3 fatty acids in oily fish like salmon, mackerel, and anchovies. Alternatively, you can take a high-strength supplement or plant-based microalgae.
 

Peppermint oil

 
Peppermint has long been used to support digestion, with research continuing to highlight its soothing and calmative action on the digestive system (17).
 

Bromelain

 
A protein found in the stem of fresh pineapples, bromelain is a digestive enzyme that can support digestion and overall gut health.
 

Digestive enzymes

 
Stress can impact your ability to produce digestive enzymes and absorb nutrients. Many people choose to take a natural blend of enzymes to support their gut and digestive health.
 

Betaine HCl

 
Stress can also affect hydrochloric acid (HCl) production in the stomach, which may lead to digestive issues and gut dysregulation. HCl helps create an acidic environment in the stomach for pepsin, an enzyme that breaks down protein, to work. It also supports the bacteria in the gut microbiome.
 

Want to find out more?

 
We’re only beginning to understand the stunning complexity of the gut-brain connection. What is abundantly clear, however, is that eating the right foods can improve your gut health and support the gut-brain axis.

For more information on gut health, please contact our team of expert Nutrition Advisors, who provide free, confidential advice, or head over to Nutrition Buzz.

References:

1. Breit S, Kupferberg A, Rogler G, Hasler G, Vagus Nerve as Modulator of the Brain-Gut Axis in Psychiatric and Inflammatory Disorders. Front Psychiatry. 2018;9:44. Available online: https://www.ncbi.nlm.nih.gov/pubmed/29483986

2. Yano JM, Yu K, Donaldson GP, Shastri GG, Ann P, Ma L, Nagler CR, Ismagilov RF, Mazmanian SK, Hsiao EY, Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell. 2015;161(2):264-76. Available online: https://www.ncbi.nlm.nih.gov/pubmed/25860609

3. Clarke G, Stilling RM, Kennedy PJ, Stanton C, Cryan JF, Dinan TG, Gut microbiota: the neglected endocrine organ. Mol Endocrinol. 2014;28(8):1221-38. Available online: https://www.ncbi.nlm.nih.gov/pubmed/24911505

4. Ríos-Covián D, Ruas-Madiedo P, Margolles A, Gueimonde M, de Los Reyes-Gavilán CG, Salazar N, Intestinal Short Chain Fatty Acids and their Link with Diet and Human Health. Front Microbiol. 2016;7:185. Available online: https://www.ncbi.nlm.nih.gov/pubmed/26973644

5. Lucas SM, Rothwell NJ, Gibson RM, The role of inflammation in CNS injury and disease. Br J Pharmacol. 2006;147 Suppl 1(Suppl 1):S232-40. Available online: https://www.ncbi.nlm.nih.gov/pubmed/16402113

6. Song Z, Song R, Liu Y, Wu Z, Zhang X, Effects of ultra-processed foods on the microbiota-gut-brain axis: The bread-and-butter issue. Food Research International, 167, 112730. 2023. Available online: https://doi.org/10.1016/j.foodres.2021.112730

7. Haug TT, Mykletun A, Dahl AA, Are anxiety and depression related to gastrointestinal symptoms in the general population? Scand J Gastroenterol. 2002;37(3):294-8. Available online: https://www.ncbi.nlm.nih.gov/pubmed/11989811

8. Popa SL, Dumitrascu DL, Anxiety and IBS revisited: ten years later. Clujul Med. 2015;88(3):253-7. Available online: https://www.ncbi.nlm.nih.gov/pubmed/26417104

9. Valles-Colomer M, Falony G, Darzi Y, et al., The neuroactive potential of the human gut microbiota in quality of life and depression. Nat Microbiol 4, 623–632. 2019. Available online: https://doi.org/10.1038/s41564-018-0337-x

10. Bolte L, Vich Vila A, Imhann F, Collij V, Peters V, Fu J, Kurilshikov A, Campmans-Kuijpers M, Dijkstra G, Wijmenga C, Zhernakova A, Weersma R, Long-term dietary patterns are associated with pro-inflammatory and anti-inflammatory features of the gut microbiome. Journal of Crohn’s and Colitis, 15(Supplement_1), S028-S029. 2021. Available online: https://doi.org/10.1093/ecco-jcc/jjab003

11. Wang DD, Nguyen LH, Li Y, et al., The gut microbiome modulates the protective association between a Mediterranean diet and cardiometabolic disease risk. Nat Med 27, 333–343. 2021. Available online: https://doi.org/10.1038/s41591-020-01183-8

12. Jacka FN, O’Neil A, Opie R, et al., A randomised controlled trial of dietary improvement for adults with major depression (the ‘SMILES’ trial). BMC Med 15, 23. 2017. Available online: https://doi.org/10.1186/s12916-017-0791-y

13. American Gut Project - The Microsetta Initiative. (2021). Available online: https://microsetta.ucsd.edu/about/american-gut-project

14. A fermented-food diet increases microbiome diversity and lowers inflammation, Stanford study finds. (2021). Available online: https://med.stanford.edu/news/all-news/2021/07/fermented-food-diet-increases-microbiome-diversity-lowers-inflammation

15. Zeb F, Wu X, Chen L, et al., Effect of time-restricted feeding on metabolic risk and circadian rhythm associated with gut microbiome in healthy males. British Journal of Nutrition. 2020;123(11):1216-1226. Available online: https://www.ncbi.nlm.nih.gov/pubmed/31964434

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