The Gut - Brain - Immune Connection

The Gut – Brain – Immune Connection

What is the Gut – Brain – Immune Connection?

This can also be called the gut-brain-immune-axis, or ‘psychoneuroimmunology’.

It’s not just your brain that controls your emotions and your feelings. There are neurotransmitters and neurochemicals in other parts of our body that can govern our emotions. In fact, the gut also has a complex array of neurotransmitters that travel along the vagus nerve to and from our brain. In addition to this, many hormones are actually made in the gut; for example serotonin is produced in the gut and can be transported into dopamine. Not only this, but 70% of our immune system lives in our gut, and this is why it’s so important that we look after our gut health. 

There is in fact a correlation between IBS and ADHD, derived from a possible imbalance in our gut bacteria. Our gut doesn’t only help us to digest and absorb the food that we eat, but it is also the largest endocrine (hormone-secreting) organ, and it also plays a role in the immune response.

The psychoneuroimmunological link doesn’t stop there. In fact, the most crucial link may in fact be the trillions of microbes that live in the gut called our microbiome. These include bacteria, fungi, viruses and yes, even parasites. This is a sophisticated system, like a large rainforest within us, and to stay healthy, we need a diverse and abundant array of beneficial microbes living in symbiosis with the entire ecosystem.

When a food, pathogen, chemical, toxin or any foreign particle enters our system it is first assessed by the gut microbiome which may send signals of high alert to the immune system should it think that there is a threat to our system. And this is the link to the immune, neuro and psychological pathways within us. 

In fact, the microbiome even helps to send important messages to the brain via possible ‘post-biotic’ communication molecules. There are certain beneficial strains that may send more ‘positive’ messages to our brain, for example, akkermansia may increase the satiety hormone leptin and decrease the hunger hormone ghrelin, and this may be why there is a correlation between low beneficial bacteria, sugar cravings, weight gain and mood disorders. 

On the other hand, high amounts of unhealthy bugs can summon a pro-inflammatory response via molecules known as cytokines. These can travel to the brain, causing brain fog, poor concentration and mood disorders. There is in fact a symptom called ‘sickness behaviour’ whereby mammals have evolved to protect the herd, so if the immune system is firing off cytokines because it is fighting an infection, these cytokines will send inflammatory messages to the brain creating a message to ‘retreat to the cave’ – and this is often why, when we are fighting an infection we don’t want to socialise. 

In addition to this, when we’re feeling depressed, these feelings can also produce the same inflammatory markers, and that is why it’s important that we’re aware of our emotions and what might be causing them. 

On the contrary, and as mentioned above, serotonin-releasing hormones made in the gut can be activated by gut cells named ‘enterochromaffin’ cells which detect different compounds created by the different microbes in our gut. They then tell the serotonin neurons how much and where to be released into the nervous system. So when gut microbial diversity is compromised this can adversely impact our mental health. 

We must then think of the vagus nerve as a complex superhighway of information traveling around our body and sending messages from our gut to our brain and our brain to our gut. The speed at which this happens can take milliseconds, and the beneficial bacteria known as Lactobacillus rhamnosus is known to improve anxiety when working along this vagus nerve. 

But when it comes to probiotics it is important to note that everyone’s ecosystem is very different. In the first instance, the most logical step is to modify your diet to increase certain levels of beneficial bacteria. This means increasing complex fibre and prebiotics such as kefir, sauerkraut and kimchi. Not only will feeding the good bugs give them a chance to thrive, but they will in turn make vital vitamins for us (such as B vitamins and K2) as well as SCFA’s which help to improve the health of our colon. 

In addition, eating complex fibre not only helps to feed the good bugs, but it also provides bulk to the stool and helps to eliminate toxins. Examples of fibrous foods include lentils, tempeh, apples, dark green leafy vegetables, nuts and seeds. 

So you can see how what we eat can impact our mood, but there is something we haven’t touched on yet and that is ‘stress’. It’s a word that is used a lot and can refer to any number of things and not all negative. The mechanisms of action begin in the hypothalamus and travel to the pituitary and then adrenal glands (HPA) axis, and the hormones that are fired off may cause a burden to our immune system, which as we’ve mentioned have close contact with our gut microbiome. 

When we sense danger, be that a real life threat, or a full email inbox, adrenaline is released, the immune system goes into high alert, and inflammatory cytokines are released along with cortisol. As you can see, there is a cascade of events. Cortisol then in fact dampens the immune response, because the immune system requires quite a bit of energy, and the body and mind are preparing for a battle of other sorts. We’re in survival mode; blood rushes to parts of our body that need it (brain, muscles, limbs) and away from our digestive system too, so you can see that the rest and digest function is turned off by our emotions, and now you can see how there is a link between what we think and feel and the impact it can have on our immune system and our capacity for optimal health. 


Harper, C., 2020. Immunity. 1st ed. London: CPI Group, pp.129-164.

Ramsey, D., 2021. Eat to beat depression and anxiety. New York: HarperWave, pp.65-180.

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