Did you know your whole digestive system uses a special set of neurones and neurotransmitters that are found in our brain? That might explain why we sometimes use the phrase “gut feeling” because the gut can communicate with our brain through the Gut-Brain Axis (GBA).

Gut microbiome - the central role in the gut

Have you wondered what “probiotics“, “pre-biotics“ do to us? They all feed into our gut microbiome. As the name suggests, the gut microbiome is a mini ecosystem in our guts, with trillions of bacteria mutually living with us every day. We get most of our gut bacteria when passing the vaginal canal at birth and from our environment. These bacteria reside on the wall of our digestive system, living upon the food we eat. Their by-products and activities are influential to almost all body functions.

What bacteria are there?

Like in any biome, more than one type of bacteria is living in our guts. In fact, thousands of families of bacteria are discovered in our guts. Some of which are more well known as they have been marketed as components in food and supplements, such as Lactobacillus and Bifidobacterium, and other vital genera of bacteria include Bacteroidetes, Firmicutes, Ruminococcus, Streptococcus, Clostridium, Actinobacteria, etc. These are only the generic names (like dogs and cats) of the different bacteria.

So what do they do?

Gut microbiota resides on our intestinal wall, metabolising the (residual) food in the system to grow their families. The balance of different bacteria families can either protect or damage the gut integrity. When the “good bacteria” and “bad bacteria” are well balanced with each other, which we call eubiosis (the peaceful mode), they can form a strong protective layer and maintain the wellbeing of the gut wall. On the other hand, when the balance is broken, one side can overgrow and inhibit the other side, which is called dysbiosis (the war mode), and this can lead to bacteria overgrowth, sensitive guts and damage the gut wall integrity (leaky guts).

The two-way communications

The gut and brain communicate via 3 main channels - the immune, endocrine (hormonal), and nerve connections. This is called the Gut-Brain Axis (GBA).

1.The Immune Connection

The immune connection is strongly linked with the gut wall integrity that is maintained by the gut microbiome. Leaky guts can trigger the body’s immune system, which releases inflammatory signalling molecules, cytokines. These molecules travel through our blood and reach an area in the brain and activate a special mechanism - hypothalamus-pituitary-adrenal axis (HPA axis)*. In eubiosis, the microbiota can help regulate cytokines level and decrease the inflammatory level.

On the other hand, the brain can interfere with the gut microbiome balance through their immune connection via the HPA axis. The HPA axis activation produces cortisol, the stress hormone, which can activate the immune system and release cytokines. Both cytokines and cortisol can travel to the gut via blood. They might trigger the war mode between bacteria, damaging the gut integrity.

*The HPA axis consists of the hypothalamus - the decision-making centre for passive body functions; the pituitary gland - the messenger centre connecting the hypothalamus with other departments of the body; and the adrenal gland - one of the departments, which is responsible for stress and emergency actions.

2.The Endocrine (hormone) Connection

Cytokines and cortisol are signalling and hormone molecules which also participate in the hormone connection. The HPA axis allows the brain to increase stress and inflammatory level in the guts by sending cytokines and cortisol. The stress and inflammatory level can affect gut microbiota metabolism, including tryptophan metabolism, short-chain fatty acids, serotonin and other neurotransmitters production, etc.

Our second brain can also trigger the hormonal signalling which feed back to the brain. In the process of metabolising the food in our guts, some microbiota will produce short-chain fatty acids (SCFAs) as by-products. These short-chain fatty acids can directly interact with our brain which can regulate inflammation, immune response, and mood (1-2). In addition, short-chain fatty acids can trigger the production of lots of metabolic and satiety hormones (e.g. GLP-1, PYY, leptin, insulin), which can help regulate hunger and satiety, blood sugar, and might be protective towards heart diseases, cancer and diabetes (2).

3.The Neural Connection

The neural connection between the gut and the brain lies in 2 main neural networks - the vagus nerve and the enteric nervous system (ENS). The vagus nerve is body’s main control network for background functions, such as breathing, basic heart beat, digestion and maintaining intestinal wall. The enteric nervous system on the other hand is less well-known, but it is the largest nervous system outside the brain which embraces the whole digestive system to signal gut movement, secretion of enzyme and absorption of nutrients. It is a nervous system that shares so many similarities with the our brain in terms of the neurones and transmitters used.

Did you know that about 95% of serotonin (the happy neurotransmitter) is produced by your gut? Besides serotonin, the gut microbiome also helps produce several neurotransmitters - acetylcholine, GABA, dopamine and norepinephrine. Short-chain fatty acids can release these neurotransmitters to the nervous system. Tryptophan (one of the essential amino acids) metabolites are also neuroactive, sending neural signals to the brain. The gut microbiome also plays a role in other nutrient metabolism, such as vitamins B2, B9, B12 and K, which can protect our nervous system (3).

How do we protect our guts?

1. Enjoy physical activities and reduce sedentary time

2. Manage your stress and optimise your mental wellbeing

3. Do a general routine check-up with your doctor every year

4. Balanced nutrition!

Stay tuned for our next gut-featured blog. We will talk about

• What to eat for a healthy gut microbiome

• Probiotic supplements and prebiotic foods

• Some astonishing findings about a healthy gut microbiome.

References

1. Appleton J. The Gut-Brain Axis: Influence of microbiota on mood and mental health. Integrity Med.2018;17(4):28-32

2. Tan J., McKenzie C., Potamitis M., Thorburn A.N., Mackay C.R., Macia L. The role of short-chain fatty acids in health and disease. Adv, in Immunol. 2014;121:91-119

3. Parker, A., Fonseca, S., Carding, S. R. Gut microbes and metabolites as modulators of blood-brain barrier integrity and brain health. Gut Microbes.2020;11(2):135–157

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