80% Of Your Immune System Lies in Your Gut. Now Is  A Very Good Time to Keep Those Bacteria Happy (Part 1)

80% Of Your Immune System Lies in Your Gut. Now Is A Very Good Time to Keep Those Bacteria Happy (Part 1)

In a nutshell:

  • Why your microbiome is so crucial to your health (and we’re only 1% human)

  • How to keep your gut stocked with good bacteria to keep you healthy

  • Whether probiotics are worth bothering with

  • The Perfect Poop

  • How else to keep your bacterial overlords happy

Next week, in part 2, we delve into prebiotics.


Now would seem an excellent time to do everything we can to max out our immune systems. And here we can look down to our bellies for helping hand.

The gut has been ever-hyped since a breakthrough in 2006 - even being described as "the saviour of modern medicine”. But when it comes to your immune system those 38 trillion microbes - collectively known as the gut microbiota - could well be worth the adulation. Why?

Dr Will Bulsiewicz describes in fabulously understandable ways what the microbiome is, why it’s so important - and how were are only 1% human DNA

Bacteria vs Foreign Invaders

It turns out approximately 80 per cent of your immune system is found in the GI tract - the main link between your gut bacteria and your health. 

 The gut bacteria work with your immune system so that your body is able to recognise the difference between a foreign invader and your body’s own tissues. 

 This is crucial in order for your immune system to recognise when to fight. If there’s a mistake in this process, this can lead to an excess of bad bacteria. The result is that your immune system begins to attack its own cells. 

 So it seems we should be doing anything we can to keep our bacteria in prime fighting condition - but what would those microbes want?

 Are Probiotics Worth It?

 Probiotics - the “good” bacteria heralded as the key to a healthy gut - have often divided opinion. 

While there are claims that probiotics can help with a range of digestive problems, such as IBS, diarrhoea, gas, bloating and constipation, it can be hard to separate the science from clever marketing; particularly when there is a dearth of definitive studies. 

So we spoke with gut expert Dr Will Bulsiewicz - best known as Dr B - about the probiotic debate, calling on his 14 years of experience as a gastroenterologist to help us wade through the sea of information on probiotics and our gut. 

Dr B talks to Tim Samuels about probiotics - whether they’re worth it, and tips for taking them

While Dr B recognises that the gut is “reorganising our idea of human health” he also urges us to be careful “about what we accept as truth versus what we accept as myth”. 

“Probiotics will help some people”, explained Dr B. ‘I would not take the entire industry of probiotics, discard all of it, and say that it has no value.”

However, he is keen to emphasise how unique each individual gut is. 

“Genetically we are 99.9 per cent the same but if you look at our microbiome… we could be virtually 100 per cent different and that’s what counts for a lot of the differences between us including your health.”

This becomes problematic, he explains, when you want to use probiotics since they are mass-produced as standard capsules rather than developed specifically for your individual needs. 

This is a particular problem in the United States, where most probiotics are sold as dietary supplements so do not undergo the same testing and approval that drugs do. This means that while manufacturers are responsible for making sure a product is safe and that the claims are true, there’s no guarantee that the bacteria in the probiotic are effective for your needs. 

Another concern for Dr B is the cost of probiotics - “I think the biggest potential harm is the harm on your wallet.” 

With all this in mind, is there a sensible way to try probiotics? We asked Dr B to pull together his top tips, starting with his recommendation that you begin with one month’s supply.  

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Dr B’s Top Probiotic Tips

  1. Always identify a specific issue you want to fix. Dr B advises against using probiotics unless you have a pre-existing digestive problem or concern. Once you’ve done this, look for a study that shows probiotics are beneficial when dealing with this problem and make sure you get the right type. 

  2. Higher colony counts are always better. Look at the number of CFUs - that’s the colony-forming units. If there are less than 10 billion it’s “virtually worthless”. Aim for higher - 50 billion is a good number to start with. 

  3. Probiotics that have multiple strains tend to do better than single-strain probiotics.

  4. Dr B always refrigerates his probiotics even if the label says it doesn’t require it. If it doesn’t need to be stored in the fridge, this can often be a good sign, though, as it means it’s been developed to survive longer at room temperature. 

  5. Where possible, try to buy capsules that are designed for a delayed-release. This means that the probiotic will travel through your stomach all the way down to your colon - the main destination - before it releases the microbes. If you don’t have this type of capsule, the probiotic’s microbes could potentially be killed in the stomach. 

In terms of the immune system and probiotics, there doesn’t seem to be enough quality evidence out there - and those with immune deficiency shouldn’t use probiotics without seeing a doctor.

The proof is in the pudding, so to speak. Do you feel better after taking a probiotic - do you have better stools? As to what makes the ‘perfect poop’, that’s probably best left for Dr B to describe here…

Gastroenterologist and gut health expert Dr Dr Will Bulsiewicz describes what the perfect stool looks like - and how it's a sign of your gut's health, and wh...

Eat Your Gut Happy

Perhaps the most reliable way to arm your bacteria to boost your immune system is through what you eat. So what’s in a naturally probiotic-laden diet?

Many fermented foods, such as kefir and sauerkraut, are rich in probiotics. Check out our list of the top five probiotic foods for a healthy gut.

You can also help build your gut and strengthen your immune system by increasing your fibre intake which can be achieved through more of a plant-based diet. Another way to keep your gut healthy is to avoid processed foods with lots of chemicals, as well as artificial sweeteners and emulsifiers. 

Get a Dog

Other lifestyle factors also have an impact on your gut - bringing you into contact with a whole range of bacteria. So getting a pet - or even getting muddy - will mean more exposure to diverse microbes. 

As a doctor once said: the two best things you can do for your health are gardening and getting a dog.

And if self-isolation looms, at least it’d be with canine company. 

Technically, popcorn is fibre-rich and falls more into the prebiotic camp which we’re covering in part 2. Stay!

Technically, popcorn is fibre-rich and falls more into the prebiotic camp which we’re covering in part 2. Stay!

Dr Will Bulsiewicz has a splendid new book coming out soon called Fiber Fueled - pre-order on his site.


The gut-mind link is becoming ever clearer - not surprising seeing as 90% of mood-impacting serotonin seems to lurk in the gut. We did a show diving into this - it’s absolutely fascinating. Not least how a mice’s mood was totally transformed by a poop swap. Listen to Sad Mouse

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