ZOE Science & Nutrition - Most replayed moment: Fix Your Gut: Improve Your Mind | Dr Will Bulsiewicz

Episode Date: April 28, 2026

Today, we’re exploring the brain-gut axis We’ve all heard phrases like “gut instinct” or “trust your gut”. These sayings hint towards a sort of mind in your midriff - and it turns out there is some truth to it. Emerging science reveals that the brain and gut are, in fact, closely connected, constantly exchanging signals through this intricate network of nerves. So, the question is: if they’re so tightly intertwined, can treating one help heal the other? I’m joined by gastroenterologist Dr. Will Bulsiewicz to unravel the mystery of this connection - and find out how we can use it to our advantage. 🌱 Try our science-backed and tasty wholefood supplement Daily 30+ Get our brand-new app and Gut Health Test designed by world-leading gut health and nutrition scientists to build healthy eating habits 👉 Join ZOE Follow ZOE on Instagram. 📚Books by our ZOE Scientists The Food For Life Cookbook Every Body Should Know This by Dr Federica Amati Food For Life by Prof. Tim Spector Ferment by Prof. Tim Spector Free resources from ZOE Eating for Better Brain Health: Your brain-gut blueprint How to eat in 2026 - Discover ZOE’s 8 nutrition principles for long-term health Live Healthier: Top 10 Tips From ZOE Science & Nutrition Gut Guide - For a Healthier Microbiome in Weeks  Better Breakfast Guide

Hello and welcome to Zoe Recap, where each week we find the best bits from one of our podcast episodes to help you improve your health. We've all heard phrases like gut instinct or trust your gut. And these sayings hint towards a sort of mind in your midriff. And it turns out there's actually some truth to it. Emerging science reveals that the brain and the gut are closely connected, constantly exchanging signals through an intricate network of nerves. So the question is, if they are so tightly intertwined, can treating one help heal the other? I'm joined by gastroenterologist Dr. Will Bolshewish to unravel the mystery of this connection and find out how we can use it to our advantage.

There is tremendous overlap between digestive health and mood. And we see this represented in a powerful way in this particular condition your oral bowel syndrome. Because, yes, the criteria are not based upon your bowel syndrome. mood, the criteria are based upon your gut, based upon your gut symptoms. But the issue is that a huge percentage of these people simultaneously are dealing with mood issues. So it's roughly 50% of people that have Eural bowel syndrome that actually can be diagnosed with major depression or generalized anxiety disorder or both. 50% of people with IBS can be diagnosed with like serious depression or anxiety or...

Up to 50% at a minimum 50% are suffering in a way where if you were to measure their mood, compare that to normal people. At a minimum, they're in a depressed mood relative to other people. But it is a very large percentage of people that are potentially able to be diagnosed with one of these two conditions or both, which are mood disorders. So there's this overlap between irritable bowel syndrome and these disorders of our mood. And the question that has come up is like sort of a chicken or egg thing, right? what causes what? And in the past, like in the very beginning, Jonathan, when they first started

studying this, they thought that actually started in the brain and that these people had mood issues, they were depressed, and because they were depressed, they were feeling it in their gut. But then we actually lined up studies where we took a group of people that don't have any of these symptoms and we track them. You know, you take 5,000 people, track them and see what comes first. and it's actually quite fascinating, and it teaches us quite a bit, where they discovered that actually most of the time it starts with gut symptoms. By the way, this is the exact same thing that happens in Parkinson's disease, where there's now research that Parkinson's disease starts with constipation and changes in the gut, and then subsequently manifests with the neurologic health condition.

That's amazing. So I just want to make sure, again, I've heard that right, saying Parkinson's, which is this terrible sort of neurological disease, actually starts with gut issues? And gastrologists, any gastrologists who's listening to the show right now is going to raise their hand to be like, yes, I agree. 100% of my patients that have Parkinson's disease are constipated. They're all constipated. And so is it that they develop Parkinson's disease and then they become constipated? No, the answer is the opposite. They become constipated first and then subsequently develop Parkinson's disease.

Two things with this. Number one, if you are listening to this and you suffer with constipation, I don't want. want you to hear that you're going to develop Parkinson's disease. The odds of that are very small. There are some people that this happens, but the vast majority of people who suffer with constipation, they don't move on to have Parkinson's disease. The point is, in this brain-gut connection that exists, there can be the manifestation of disease in the gut and the manifestation of disease in the brain or in our mood, and they can be happening in a fashion that is actually they're connected to one another. And how do we understand this connection today?

There's multiple ways that your gut is talking to your brain right now. As you and I sit here and speak to one another, your gut is talking to your brain. And some of the ways are with the production of chemicals, which are what we call bioactive. So that means they can actually impact the physiology in your body. These chemicals, by the way, are often produced by your gut microbes. 70% of your immune system exists in your gut, and the immune system is able to activate things that can affect the entirety of your body and create inflammation. Also, your hormones and your neurotransmitters that are produced in your gut.

So it's interesting when I finished medical school in 2006, I had learned about serotonin and things like this, and I didn't realize that there's over 30 neurotransmitters. I didn't realize there's that many. I thought they were like sex. there's over 30 neurotransmitters. They are produced in the gut. 90 to 95% of serotonin is produced in the gut and 50% of dopamine is produced in the gut. And so now these things can have influence in the gut but also throughout the body.

And particularly with serotonin, I just want to be totally clear because there's going to be someone who's a scientist who's going to want to make sure that I'm clear on this. The serotonin that's produced in your gut does not actually cross into the brain. but there's a precursor to serotonin called 5 hydroxy-tryptophan. And this 5-HTP actually has the ability to cross into the brain and signal and change how your brain works. And then the last but not least, perhaps the biggest and most important is a pair of nerves, which are probably my favorite nerves in the entire body, Jonathan, called the vagus nerves. All right. Now, the vagus nerves start in our brain, pass through our skull, and descends down to our gut.

And they're collecting information from your gut. There are in your gut 500 million nerves feeling and sensing by the microsecond. And all that information that gets collected by 500 million nerves. By the way, I should add that 500 million is a ridiculous number. So, I mean, I know that sounds like a lot. I mean, it sounds ridiculous. But like just to compare this to your spinal cord. This is five times what you will find in your spinal cord.

Hang on. So I've got five times more nerves in my gut, which as far as I'm concerned, I can't have any control over, versus my spinal cord, which ends up controlling my legs and other things I know I'm moving around all the time. That's right. So, yeah, so you have five times more nerves in your gut. This is why we call it the enteric nervous system. Or some people will call it the second brain. I actually would challenge that. I would call it the first brain.

Serious. Okay, so here's why. From an evolutionary perspective, for you and I to be here, it required us to evolve and mature a brain. But the problem is that a tree can't grow until it puts down roots. The roots must come before the tree grows up. The brain cannot be strong without nutrients. We developed our interic nervous system before we developed our central nervous system, which is our brain. So from an evolutionary perspective, this actually came first.

And these 500 million nerves that we have are constantly feeling and sensing everything that's happening. That includes the immune system. You have 1.3 trillion cells in your gut, all right, that are literally right next door to these nerves. Like, they're right there. And then feeling and sensing the 38 trillion microbes that are across a paper thin wall that may or may not have holes in it, which is what we call the gut barrier.

And there's the 38 trillion cells. They're right there too. And so these nerves get to feel inflammatory cytokines, things that your immune cells create, that's inflammation, and also come into contact with the chemicals that are being produced by your gut microbes. And they collect this information, Jonathan, consolidate it all into two nerves that run from your gut all the way up through your chest, past your heart, through your lungs, through your neck, and to your brain. That's the vagus nerve. this is one of the 12 cranial nerves and then that information your brain can action based upon this

you think that there might be this essential explanation around the gut microbiome how does that play into all this complexity of the nerves and the two-way gut brain access you were talking about the chemicals that are produced by our gut microbes have an influence throughout our entire body and that includes on your brain and if we think about these individual things whether it be the serotonin levels which impact our gut rhythm, which affects whether or not we have diarrhea or constipation or a Bristol 4. That's influenced by our gut microbes. When we think about the sensitivity of these nerves, actually there's research, Jonathan, that with fiber, fiber comes into contact with our gut microbes, produces short chain fatty acids like butyrate, acetate,

propionate, and these short chain fatty acids actually have a soothing effect on the nerves that are lying in our gut. So if you're asking me the question, what's a natural way to reduce gut sensitivity? That's the answer. And simultaneously, we have discussed how our mood is affected. Research is now showing, Jonathan, that the common tie between major depression

and Parkinson's disease, and by the way, also Alzheimer's disease, is inflammation inside the brain. So our immune system is getting activated in both of these places, both in the gut and in the brain. And if you want to reduce these things, the most anti-inflammatory chemical that exists

are the short chain fatty acids, butyrate acetate and propitiate. And so it's kind of interesting to take that into consideration when we think about people that have earol bowel syndrome, and if you zoom in on their gut, what you would see is the gut microbiome is damaged. There's a loss of diversity. They've lost the anti-inflammatory microvioletion

microbes like bifidobacteria and lactobacilli, they're not as well represented as they used to be. And there's an increase in the pathogenic microbes, the inflammatory ones. All right. So this is what we see in Erobialial syndrome. Well, actually, it kind of is in parallel to the same changes that we would see in a person who has a major depression. Similar changes in terms of loss of diversity and more inflammatory microbes and less of the protective ones. So when we look at this whole picture, we start to see that the changes are occurring within the gut, and the manifestation can affect gut rhythm, gut nerves, sensitivity, and simultaneously can affect our mood.

So it feels like the perfect time to switch to, so what can you do about it? And I think, you know, well, we always like to make sure we cover really actionable advice. So if we understand that the mechanisms that end up leading to IBS, you know, start back with this damage to the gut micro, what can you do about it? Right. So the advice that you received was to cut fiber. Correct. I understand why they said that.

On some level, if the only thing we cared about was improving your symptoms, fiber is hard for the gut microbes to digest. So in theory, if you cut fiber, you may feel better. There's research with a dietary approach called a low FODMAP diet, which is something that we've discussed before on the show. So FODMAP is an acronym. It's super nerdy. I'm going to put it out there, but you don't have to memorize this.

No test afterwards. Yeah. So FodMAPE, FOMAPE, MAPE, fermentable oligosaccharides, di-saccharides, monosaccharides, and polyols. Basically, what this is saying is these are carbohydrate-based foods that are fermented by our gut, by our gut microbes. and in the process of fermenting them, they produce gas. And you know what gas can do? Gas can distend. Gas can activate those nerves that are very sensitive.

So the low-fodmap diet, the concept, is if we reduce these specific fermentable things, then we may actually make it easier on our gut microbes, make it easier on those sensitive nerves, and the result is that you feel better in terms of your oral bowel syndrome. The data are clear. people who followed this dietary pattern if you have ibs will generally experience an improvement in their gut symptoms but there's a key with this which is that you know and this is something that you actually experienced you were told okay go vo fiber and then ultimately when you followed the zoe program you found actually an approach that's focused on abundance and variety is a superior choice

So when we take this sort of restrictive approach of reducing, it should be temporary. And ultimately, we want to work towards from this, like start with this, reduce FODMAP intake, and we want to start to reintroduce these things gently, increasing them over time, because ultimately where we want to be is to include fiber, to include FODMAPs, to include a lot of variety of different plant-based foods in our diet, because that's what makes our gut microbes thrive. So it's just a matter of getting there. It's the process.

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