Gut health and immune system: what's the link?

'The last undiscovered human organ'

You could be forgiven for thinking that the gut is simply the place where we digest the food we eat. After all, that is its main function; absorbing the nutrients from our diet so they can be used in vital processes throughout our bodies.

It's only recently that the scientific world has begun to understand just how much more complicated things are in our guts.

A sudden new interest in gut health in the wellness world has sprung from new research focusing on the gut microbiome. This is the population of bacteria living in your gut. Some are beneficial, some less so, but they all exist in a delicate balance which is as unique to us as our fingerprint.

The possibilities are so vast that scientists have called the gut 'the last undiscovered human organ'. There's evidence that the processes within this fascinating system can have links to our mental health, overall wellbeing and even our immune system.

So, what do we know about the goings-on in our gut, and how is it related to protecting us from germs?

The gut microbiome and the immune system

Your immune system is a collection of organs and cells involved in protecting you from pathogens (dangerous microbes like bacteria and viruses).

When we talk about the immune system, we often think of bone marrow and white blood cells. But, through these cells and others, your immune system is essentially present everywhere in your body.

That said, there are certain places where your immune defences are concentrated. It appears that the gut is one of the most important sites for immunity.

Indeed, that makes a lot of sense when you consider that the gut is where germs that have entered the digestive system through food have a chance to hop over into your body.

There, they can wreak havoc, causing cell damage and disease. Hence, it's vitally important that your gut can provide a solid line of defence against unwanted visitors.

Part of this defence is undertaken by your gut microbiome.

What is the gut microbiome and where does it come from?

We start to develop a population of different types of bacteria in our gut from birth. The bacteria living in our gut is collectively called the gut microbiome.

Our digestive tract is thought to be sterile while we're in the womb. However, our passage through the birth canal exposes us to the first of the bacteria which will eventually form our microbiome.

It is then topped up through breast milk, which contains more live microbes which contribute to the population.(4)

Later on in life, we can somewhat influence the types of bacteria in our gut through our diet. This can be both positive and negative.

For example, a diet high in fat can reduce the presence of two types of bacteria, A. muciniphila and Lactobacillus. Both are associated with good health.(5)

The Western diet, which is usually rather too high in fat and too low in fibre, is associated with a lower diversity in the microbiome. This includes lower numbers of beneficial Bifidobacterium and Eubacterium bacteria.(5)

On the other hand, people who follow a healthy, Mediterranean-style diet show improved health. This can be seen in terms of both obesity and inflammation, for example.

Researchers think this may be linked to higher levels of Lactobacillus, Bifidobacterium, and Prevotella, gained through diet, and lower levels of Clostridium bacteria. (5)

Telling the difference between helpful and harmful bacteria

One of the most important things our immune system needs to do is tell which bacteria, or other pathogens, can do us harm.

That becomes quite complicated when we consider that, in the gut, you have the helpful bacteria of the microbiome alongside potentially harmful bacteria coming in from your food. Hence, the immune system and the gut need to work together to identify which bacteria should be there and which should not.

Your immune system learns to recognise commensal bacteria (i.e. bacteria from the microbiome) as 'self'. Other bacteria are recognised as 'other', and can provoke an immune response.

It does this through the workings of special proteins called 'toll-like receptors'. These receptors act as little guards in the gut. They learn to recognise the familiar bacteria and allow them to carry on with their business. Meanwhile, they alert the immune system to any foreign bacteria attempting to get past their defences.(2)

What happens when we disrupt the microbiome?

When something is not quite right in the gut, it has consequences throughout the rest of the body.

Scientists have suggested that dysfunction in the gut can make us more vulnerable to infections. It can also produce hypersensitive responses (such as allergies). This occurs when the immune system misinterprets benign stimuli as being harmful to our bodies, and attacks.(2)

There are other, more serious consequences to poor gut health too. These include a higher susceptibility to cancer and increased autoimmune responses. The latter is another type of hypersensitive reaction which causes the body to mistakenly attack itself.

Further, chronic inflammation goes up. A simple way to understand inflammation is to think about how your body normally reacts to a paper cut. The skin around the cut puckers, swells, becomes hot, and goes a bit red.

This is a natural and healing reaction to the body being invaded. But when inflammation is out of proportion, it becomes a serious threat to health. It is cited as a factor in a long list of disorders, from diabetes to asthma and arthritis and even mental illness.(3)

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Gut health: How gut microbiome impacts mental wellbeing

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