The brain-gut mystery: What’s really causing your IBS? With Dr. Will Bulsiewicz

Irritable bowel syndrome (IBS) affects over 300 million people globally, yet remains one of the most misunderstood gut conditions.

In this episode, we explore the latest science uncovering the real drivers behind IBS — and why your gut microbiome may hold the key to understanding it.

We then dive into the science: How the gut-brain axis influences everything from pain to food intolerances, why women are disproportionately affected, and how a disrupted microbiome plays a central role in IBS.

Finally, Dr. Will shares practical strategies backed by the latest research —  from dietary changes to cutting-edge treatments — offering hope for healing and prevention.

Whether you're living with IBS or simply want to understand your gut better, this episode is packed with insights into one of the most common yet elusive health conditions of our time.

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Transcript

Jonathan Wolf: Will, thank you for joining me today. 

Will Bulsiewicz: It's my pleasure. Great to see you. 

Jonathan Wolf: Always such fun to have you with us, and I don't feel that I need to explain our tradition here, so I'm going to jump straight into the quick-fire round of questions from our listeners. Are you ready to go? 

Will Bulsiewicz: I'm ready. Let's go

Jonathan Wolf:. Alright. Is bloating and stomach pain something we should just live with? 

Will Bulsiewicz: No, definitely not.

Jonathan Wolf:  Are you seeing more people with IBS than in the past? 

Will Bulsiewicz: Yes. 

Jonathan Wolf: Do doctors understand IBS differently today than they did 10 or 20 years ago? 

Will Bulsiewicz: Definitely.

Jonathan Wolf: Do certain eating habits cause IBS?

Will Bulsiewicz: They may. 

Jonathan Wolf: Is IBS all in the mind? 

Will Bulsiewicz: No, definitely not.

Jonathan Wolf: Finally, you have a whole sentence on this. What is the biggest misconception when it comes to IBS? 

Will Bulsiewicz: The greatest misconception is that people don't understand what's causing it. So, as the result of that, they just treat the symptoms, and I want to address the root of the issue. 

Jonathan Wolf: Well, I think there were some really strong answers there, Will, which is great.

I love the idea of addressing the root of this today. I actually have a personal story. Almost 30 years ago now, after an infection, I started to suffer various gut issues, and it was quite scary. And at first, I had all sorts of tests for potential cancer, and in the end, the doctors couldn't find anything.

I was diagnosed with IBS, irritable bowel syndrome. My doctor attributed it to stress and said I should try to relax and avoid high fiber foods. And I think it's important to say this was a time before GI doctors knew anything about the gut microbiome and its role. So, you know, a long time ago.

As a result of all of this, I ended up avoiding a lot of foods as a way to deal with it. And I thought I was eating a really healthy diet, actually, at the point that I met you and Tim Specter and others.

You helped me to realize that my diet was actually very heavily processed and sort of hurting my gut, not helping it.

I'm telling this story because today I can eat a huge variety of foods that I could never have touched in my twenties and thirties. And I think, honestly, that is amazing. 

Having said all of that, I’d just like to start at the beginning, Will, for people listening to this, what is IBS, what are the symptoms?

Will Bulsiewicz: IBS is the acronym that we use to describe irritable bowel syndrome. Irritable bowel syndrome is defined based on a pattern of symptoms, so it's really important for people to understand that we don't have a test. There's no blood test or CAT scan or procedure, or even microbiome test yet available to allow us to make this diagnosis.

Ultimately, it's the pattern. If you walk into your doctor's office and you say, I'm having abdominal discomfort, it's been going on for three months, at least once per week on average, and I've had a change in my bowel habits. 

What I mean by that is you could be moving more towards diarrhea, you could be moving more towards constipation. There's a change in the way that your bowel movements look. There's a change in how often you're going, and you may also experience symptoms like your pain may get better or get worse when you have a bowel movement. This is how we define irritable bowel syndrome. So the key to really simplify it is the combination of abdominal discomfort, abdominal pain, with a change in your bowel habits. If you have those things over the last three months, then we have what we need to potentially diagnose irritable bowel syndrome. 

Jonathan Wolf: Will, how many people in the U.S. or the U.K. or something like that are having these regular gut issues? 

Will Bulsiewicz: Yeah, so it's a lot. It's staggering. To contextualize this as a gastroenterologist, I have a biased clinic because people come to me with gut issues, but the research says that about one out of three patients, and I will validate that this is in fact true about one in three patients that go to their GI doctor, this is what they have. 

It's a lot of people, and if you look out across the United States, somewhere in the range of between 6% and some estimates, 10% or even 25% of people in the United States have irritable bowel syndrome. That means that they can actually meet these rigorous criteria, which are specifically abdominal discomfort, change in bowel habits, and three months.

But what do you say to the people who have bloating, who have occasional upset stomach, and don't necessarily meet these criteria? That's a far higher number. 

So the estimates would be that in general, about 40% of people on the planet, not just in the United States on the planet, 40% of people have some sort of chronic digestive issues.

Jonathan Wolf: 40% of people on the planet have some sort of chronic, ongoing digestive issues. 

Will Bulsiewicz: This is the estimate that's been made. Now I think that it's going to be more in industrialized countries such as the United States and the U.K., but the estimate is that this is way more prevalent than we realize.

So the number that we diagnose with IBS is going to be a lot less than this, but the number of people who are dealing with something in terms of their gut symptoms that they don't want, that are affecting their quality of life, it's a very high number. 

I'm quite sure that the listeners at home can vouch for this, where there's going to be a lot of people who are listening to this episode right now and go, yep, that's me.

Jonathan Wolf: I mean, it is amazing. It's a staggering number of people. 

You were differentiating between the 40% with some sort of gut issues and then a smaller number, you said, between six and 25% of all people with IBS. So what takes you to that sort of higher level that you're describing of sort of meeting these strict criteria of IBS?

Will Bulsiewicz: There is a family of diagnoses that exist within the space that I work as a gastroenterologist, and we call these functional GI disorders, or more recently, we call them disorders of the brain-gut axis. And we can talk about that more in a moment. 

But this family it's an umbrella. And within that umbrella, one of the diagnoses is irritable bowel syndrome. And this may be the most prevalent of all of the ones that are possible, but there's all of these other ones. 

There's disorders of functional dyspepsia and chronic constipation, and functional diarrhea, and you can go down the line, and there's many different diagnoses that are possible. 

There's also people that just have symptoms and they don't necessarily fit perfectly into a box where the criteria match exactly what they're experiencing yet it's really important to acknowledge that there are people out there who are suffering, they have chronic digestive symptoms, and they want to a solution, and it doesn't matter what we call that. 

Ultimately, our job is to try to deliver a solution to these people to improve their wellness, to make them feel better.

Jonathan Wolf: And you were saying that with IBS, one part is sort of abdominal pain, so you're feeling some discomfort. Then you also use this wonderful medical term, change in bowel habits. Yeah. What is that all about? 

Will Bulsiewicz: So Jonathan, there's this thing called the Bristol Stool Scale, and basically it's this series of seven pictures that allow you to look at pictures of poop, seven different types of poop, and compare that to what you see in your toilet bowl and know where you stand in terms of these seven different types. 

The seven types are important because there are types that move more towards constipation. So types one, two, and three are on a spectrum of constipation, where type one is basically like rabbit pellets. It's hard, hard, impacted balls.

Type two is like if you took a whole bag of marbles and you started to piece those marbles together. Not to be too graphic, but this is what people will see if they look at the pictures. Alright, so it's important for our listeners to hear, you know that there's these certain types of how your poop looks that would provide us with insight that you're having constipation.

Then there's the opposite end of the spectrum where types five, but particularly types six and seven, is moving towards diarrhea. And as anyone can imagine, six is basically a plop, and seven is water. 

In between these is type four. And type four is nice and balanced, right in the middle. It's exactly what you want it to be in terms of the way that it appears is a sausage. It's soft, it's formed.

With this, generally, people that experience a type four bowel movement, it's easy to come out. It feels effortless. Generally. This is a complete bowel movement, it's very satisfying. There's a little bit of pleasure involved there. I hope you don't mind me talking about this, is the way that bathroom experience should be, whether you're talking to your friends about it or not. 

Jonathan Wolf: You're making me smile because I don't think this is a conversation I would ever have had on air until very recently, but I feel very proud that most of the time, I am now delivering a sausage as you described.

So that feels like a real achievement, and I like the fact that for the first time in my life, I've been able to humble brag about that on air with you, Will. 

Because what I've learned from you and from others is that's where we should be. If our gut is working well, that is just sort of normal, as it were, for our ancestors.

But I think the point about this chart is the reason why there's a chart is a lot of people are not just having that sausage. And I certainly wasn't in the past either. 

Will Bulsiewicz: Yeah. And the origins of this chart are from the early nineties, out of the city of Bristol in the U.K., but it was a limited study. It was about 20 people.

Yet, clearly, we needed to define what a change in bowel habits actually looks like. So now we have a way to actually do this. And it's important because the information that you can gather from this is actually quite powerful and profound. It's not just whether or not you're constipated or whether or not you have diarrhea.

From my perspective as a gastroenterologist, when there's a change towards constipation or towards diarrhea, there's a lot more going on than just, Hey, your poop changed. It goes deeper than that. 

Jonathan Wolf: Is your poop problematic? If it's anything other than that sort of number four sausage you were describing, does that mean that if you have that and some pain, you have IBS?

Will Bulsiewicz: If you have pain, and then any sort of change, whether it be in the way that it looks in terms of how often you're going or in terms of the experiences around a bowel movement, where a quick example, Jonathan. 

Many people who have IBS, they go to poop and they actually feel so much better and their pain gets better after the bowel movement. These would all be things that are indicative of IBS.

But moving away from a type four, whether you have IBS or not, whether you have pain or not, when we drift away from a type four, then we are moving in a direction where we don't necessarily want to be, and there are potential consequences to that. 

So, ultimately, one of the things that I would love to do is to help people build a diet and lifestyle approach that puts them in a position where this is effortless and they're having good, healthy number four bowel movements.

Jonathan Wolf: So why might we be seeing an increase in these sorts of digestive disorders like IBS over the last 30 or 40 years? 

Will Bulsiewicz: Yeah. So I think that there's a number of different factors, yet at the same time, the whole thing is quite mysterious.

Jonathan, I would like to take a moment to sort of explore the history of IBS, and it helps to frame this in a way so that the listeners can understand the way that I'm thinking about this, where you see this historical element.

People have suffered with these types of symptoms for a very long time, and there were doctors in the early 20th century starting to notice this. 

But it really started to come together in the 1970s, where doctors were all kind of getting together at their meetings and saying, Hey, we keep seeing the same thing coming through the door. Here's what we're seeing: pain change in poops and their, and their pain gets better after they poop. This is what we're seeing, and yet, an X-ray is not showing anything. Our blood tests are not showing anything, so what's going on here? 

Ultimately, it led them to where my mentor, actually, at the University of North Carolina, created an international organization of gastroenterologists and scientists, and they met in Rome, Italy.

They came together and they said, there's something going on, we need to define this. So they created what are called the Rome criteria. And for the first time, what you and I have been discussing so far, we actually have this in a way where a doctor can now use this. So a doctor who's anywhere in the world can say, okay, I can use the Rome criteria to make the diagnosis of irritable bowel syndrome. 

Jonathan Wolf: And I love that they decided this was a very important problem, which it clearly is. And then they decided that they'd all get together, not in Kansas City, but somehow it required going to Rome. 

Will Bulsiewicz: And they kept revising it because it's a great place to go and get together with your friends.

Jonathan Wolf: So that's not the only visit to Rome on this, is that what you're saying? 

Will Bulsiewicz: Yeah. So we're up to the fourth Rome criteria now. Right? Let's just keep getting together and working on this because we got to work on this. So this is sort of historically where it came from.

The challenge with this is that in there, again, there's no test. So it can easily sort of devolve into this symptom-based thing where all you see are the symptoms. You check the box to make the diagnosis, and then that triggers some sort of knee-jerk thing out of the doctor to here are treatments.

The problem with this entire approach, Jonathan, is that people don't really get better, the treatments aren't super effective. Why? Because you're not actually getting after the root of the issue. And so if we want to understand why IBS is taking off and becoming more prevalent, more common than it used to be, ultimately, that requires us to dig into the root of the issue. I think that's what we have to do. 

Jonathan Wolf: It sounds like what you are describing in a full scale is very similar to my own personal experience 30 years ago, where they get to a diagnosis by saying, Well, you haven't got these other very serious diseases, which is great. So we say that you have IBS, and then we don't really understand what to do, so we can't really tell you how to deal with it.

It was, I have to say, quite unsatisfactory, and you sort of try to figure this out on your own. You're saying that, in a way, my own experience was quite representative of what was true up until fairly recently. 

Will Bulsiewicz: Yeah. Well, I think that your experience on some levels is continuing to be true. As an example, one of the things that I heard you say is that you were told to stop stressing out. 

Jonathan Wolf: Correct.

Will Bulsiewicz: Right. And that to me is something that happens all the time with people that have irritable bowel syndrome, and they're left with the feeling that this is all in my head. I'm here to tell you that it's not and that this is real.

The reason why you're experiencing this does involve your brain, but it also involves your gut, and it's about the connection between the two. 

Jonathan Wolf: So, Will, I've got to ask the question then, what is that missing key in our understanding of IBS that these doctors didn't understand in the past, you know, in their beautiful field trip to Rome, but that we do understand today 

Will Bulsiewicz: In the process of these multiple Rome visits, we went from thinking of this as a symptom-based condition to thinking of this as a gut issue. To now, it has been expanded with clarity and is being defined as a disorder of the brain-gut axis. 

So it's not just the gut, actually, it's a combination of the things, the changes that are occurring in the gut and also in the brain, and the interplay between the gut and the brain that ultimately are resulting in the manifestation of this illness.

Jonathan Wolf: And can you help us to understand a bit more what that means? What is the gut-brain axis? What have we discovered that we didn't know about in the past that that is explaining what you just said? 

Will Bulsiewicz: Yeah, so let's start with this, that there is tremendous overlap. You mentioned the comment, oh, you just need to stop stressing out. Or we hear things like it's all in your head.

There is tremendous overlap between digestive health and mood. We see this represented in a powerful way in this particular condition, irritable bowel syndrome. Because yes, the criteria are not based upon your 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 irritable bowel syndrome that actually can be diagnosed with major depression or generalized anxiety disorder or both. 

Jonathan Wolf: Fifty percent of people with IBS can be diagnosed with serious depression or anxiety.  

Will Bulsiewicz: 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 a chicken or egg thing, right? What causes what?

In the past, in the very beginning, Jonathan, when they first started studying this, they thought that actually that it 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. We track them. You take 5,000 people, track them, and see what comes first. 

It's actually quite fascinating, and it teaches us quite a bit. 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.

Jonathan Wolf: That's amazing. So I just want to make sure again that I've heard that, right? You're saying Parkinson's, which is this terrible sort of neurological disease, actually starts with gut issues.

Will Bulsiewicz: Yes. Any gastroenterologist who listens to the show right now is going to raise their hand to be, Yes, I agree. A hundred percent of my patients who have Parkinson's disease are constipated. They're all constipated.

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 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 go 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.

Jonathan Wolf: How do we understand this connection today? 

Will Bulsiewicz: 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. 

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. I didn't realize that there's over 30 neurotransmitters. I didn't realize there's that many. I thought there were like six. 

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. So now these things can have an influence in the gut, but also throughout the body.

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-hydroxytryptophan. 

This 5-HTP actually has the ability to cross into the brain and signal and change how your brain works. 

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. Alright, now the vagus nerves start in our brain, pass through our skull, and descend down to our gut. 

They're collecting information from your gut. There are in your gut 5 million nerves, feeling and sensing by the microsecond, and all that information that gets collected by 5 million nerves, by the way, I should add that 5 million is a ridiculous number. 

So I mean, I know that sounds like a lot, but just to compare this to your spinal cord, this is five times what you will find in your spinal cord. 

Jonathan Wolf: 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, you know, my legs and other things that I know I'm moving around all the time.

Will Bulsiewicz: 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 call it the second brain. I actually would challenge that. I would call it the first brain.

So here's why. From an evolutionary perspective, for you and I to be here, it required us to evolve and mature a brain, right? 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 enteric nervous system before we developed our central nervous system, which is our brain. So, from an evolutionary perspective, this actually came first.

These 5 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 that are literally right next door to these nerves,  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.

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. So this is how the gut is communicating to the brain. 

Now, flip side, your brain is talking to your gut, and the way that your brain is talking to your gut is through number one, the vagus nerve. So about 20% of the fibers in the vagus nerve are for taking information from your brain back down. 80% is sucking up information from 5 million nerves, 20%, taking information from your brain back down. 

Vagus nerve is one, but also your brain produces hormones that can be released by the hypothalamus or the pituitary glands, and that can have physiologic effects throughout your entire body.

So, an example of this, if I jump out at you from behind a door and I try to scare you, I would never do this to you, Jonathan. I just want you to know that. Okay? But if I jump out at you, that shock gets your heart racing, your blood pressure goes up, adrenaline surges, where does that come from? That's your sympathetic nervous system.

Sympathetic nervous system is your action, your get up and go, and your brain actually controls the gas. Your brain is what's pushing the gas on the sympathetic nervous system. 

This is one of the ways in which your brain actually can communicate down to the entirety of your gut because the hormone that your brain is producing, it can impact your gut barrier, it can impact your immune system, and then it can impact the muscle that lines your intestines, which basically means motility.

Jonathan Wolf: There's a very complex sort of two-way thing going here where you're saying an enormous amount of my nerves going into my gut, getting information from it, but also my brain has this ability. So I create all of these chemicals and other things that then shape the way that my gut is working. So it's sort of going in both directions.

Will Bulsiewicz: It's going in both directions. And so now thinking about this, the hormone, by the way, for the nerds who are listening, is called corticotropin-releasing hormone, or CRH. Okay? 

So we have CRH receptors, again, in the muscle of our gut. That means motility. We have CRH receptors in our immune cells, that's inflammation. And we have CRH receptors in our gut barrier. 

When this CRH, in the moments of stress, comes down, many people can relate to this. You're feeling stressed out, you manifest the symptoms within your gut. It could be nausea, it could be some bloating, feeling gassy, or it could be discomfort that grows, gets more intense, and turns into waves of cramping, abdominal pain that will fold you over. Right? There's varying levels of intensity. 

But I'm sure that you've experienced this at some point, for sure, and I remember in medical school on test days, there was a line at the bathroom door because it was such a pressure cooker that we were working in. So that's the result of this entire process, right?

That's a true manifestation of the brain-gut axis. But now let's apply this, if we could to irritable bowel syndrome. Alright, so just from a symptoms perspective, making the diagnosis, we talked about abdominal pain, and we talked about having diarrhea or constipation or both. 

What I'm saying is that what's happening in your brain can affect gut motility because it's affecting the muscle cells that line your intestines. Gut motility means the way that your intestines are constantly moving. 

So each one of us has 20 to 25 feet of intestines, so seven to eight and a half meters of actual intestine. And it's constantly moving, right? But it can move in ways that are in rhythm that will propel food forward and allow it to digest, or it can be in a pattern that is not in rhythm and spastic and out of control.

Then your body feels that. And many of the discomforts that we get are the result of abnormal gut motility. 

Jonathan Wolf: The question as you say this is you're describing it's very complex interplay. You're also telling me  IBS isn't something we've always had at the same level forever. You're saying that I think that it's something that doctors have become more aware of in the last 40 or 50 years. So I guess what's causing this system to not work well? 

Will Bulsiewicz: Right. I think the tie that binds it all together is the gut microbiome. And the issue is that we didn't have gut testing in the 1970s, 1980s, 1990s, or most of the 2000s. So, gut testing is something that's much more recent. 

But if we think about this, the abdominal pain, where that comes from, is a change in gut motility and simultaneously the sensitivity of the nerves.

Those 5 million nerves becoming overreactive. You feel things in a different way, and that actually can be motivated by what's happening with our gut microbes. 

So number one, I said that 90- 95% of serotonin is produced in the gut. Well, why, why is serotonin in the gut? The answer to that question is that serotonin is the drum that sets the beat, sets the rhythm for your gut.

And if you were to have excessive amounts of serotonin, that drum beat goes too fast, and you get diarrhea. And if you have inadequate amounts of that serotonin drum beat and it's too slow, you get constipation. Production of serotonin within the gut is, in part, motivated by our gut microbes.

At the same time sensitivity of these 5 million nerves, we call this visceral sensitivity visceral hypersensitivity. And these nerves, they've done these studies, Jonathan, where they'll take a person who has irritable bowel syndrome, and they'll inflate, imagine a tennis-sized ball. Alright.

I'm just going to tell you, having done thousands of colonoscopies, this is not very large in terms of our colon's ability to handle that. Alright. A tennis-sized ball our colon can easily handle; it can stretch. 

Jonathan Wolf: I'm smiling because it sounds quite big inside me. But you're saying not a problem.

Will Bulsiewicz: Yeah, I get that. But it's not a problem. Okay. That our colon has the capacity, it's kind of like an accordion, and it can stretch and open up and create space to accommodate this. 

So Jonathan, they'll do these studies where they'll take a person who is completely healthy, they do not have IBS, they'll inflate this ball, and the person will sit there, and they'll say, you know, I feel some pressure, but that's about it. 

Then they'll take a person who has irritable bowel syndrome. They will inflate this ball, and that person is ready to jump off the table. 

Jonathan Wolf: So they're in a lot of pain. 

Will Bulsiewicz: With a lot of pain, and they're experiencing the same amount of pressure. It's not that the pressure is different.

What's different is the way that the body is interpreting the pressure. And this is what we call visceral hypersensitivity, where that person with irritable bowel syndrome, those nerves are overreactive, and the brain has finite resources. There's only so many neurons, so it's going to devote a certain number of neurons to certain things.

So the amount of neurons, the amount of brain space that is devoted to your hands, and your ability to feel with your hands is a lot, right? And the amount that's devoted to your physical movement is a lot. The amount that's devoted to your ability to feel the individual nerves in your gut is none. It's very little, right? 

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So you don't have that sort of same level of tactile capacity, but yet the information that's being collected is ultimately consolidated through the vagus nerve and being actioned on in a way that is not conscious, but it's affecting your physiology, the way that you feel, the way that your body works throughout the entire system.

The information goes up to the brain, the brain actions on it, and then it basically disseminates back down throughout the entire body, affecting, yes, the gut and affecting other things too. 

Jonathan Wolf: So I think you're saying when you have this thing like I don't feel very good, those things aren't very well closely linked to I haven't got a sore finger, it's more general actually. 

That could be really these nerves feeding back, for example, from my gut saying that things aren't quite right, it's just that I can't locate it clearly in a particular part of my abdomen in the way that I would do if it was my knee that was hurting. 

Will Bulsiewicz: If I were to touch your hand, you would know exactly where I touch your hand. Right? The way that your gut is wired is in a different way. 

As a gastroenterologist, when people describe, Hey, I feel pain in this location, I'm thinking about general parts of the body because I know that they can't have that exact precision, but it does help me to narrow it down. 

So, a quick example of this, when people feel pain around their belly button, that's their small intestine, right?

And when they feel pain in their right upper quadrant radiating to their back, yes, that could be your gallbladder. That could also absolutely be your colon.

Jonathan Wolf: That's really interesting. So you just mentioned you think that there might be this central explanation around the gut microbiome. How does that play into all this complexity of the nerves and the two-way gut-brain axis you were talking about?

Will Bulsiewicz: The chemicals that are produced by our gut microbes have an influence throughout our entire body, and that includes on your brain. 

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 four, 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, and propionate. And these short-chain fatty acids actually have a soothing effect on the nerves that are lining our gut.

So if you're asking me the question, what's a natural way to reduce gut sensitivity, that's the answer. 

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. If you want to reduce these things, the most anti-inflammatory chemical that exists are the short-chain fatty acids: butyrate, acetate, and propionate. So it's kind of interesting to take that into consideration when we think about people who have irritable bowel syndrome.

If you zoom in on their gut, what you would see is that the gut microbiome is damaged. There's a loss of diversity. They've lost the anti-inflammatory 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.

Alright, so this is what we see in irritable bowel syndrome. Well, actually, it kind of is in parallel to the same changes that we would see in a person who has 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 nerve sensitivity, and simultaneously can affect our mood.

Let's go back to your original story. There was something that I have been waiting to point out about this, which is that you mentioned that it started when you got sick. So you had some sort of stomach bug. 

This is actually very clearly defined, and it is very common, actually. A person gets some sort of stomach illness, and then the stomach illness is gone, but the symptoms carry forward, and it's the manifestation of irritable bowel syndrome.

The reason why this occurs is because in the setting of having some sort of stomach illness, you have caused dysbiosis damage to the gut and activated this new condition. Irritable bowel syndrome is the result of this IBS overlaps with having stomach bugs or a stomach illness. 

IBS also overlaps very powerfully with inflammatory bowel disease. So, people that have ulcerative colitis and Crohn's disease, a huge percentage of them ultimately have IBS, where you could literally take away the inflammatory bowel. They have no inflammation, and yet they're still manifesting symptoms because they also have IBS. 

The same is true for celiac disease. So there are people who develop celiac disease within this setting. So the common tie that binds all these things together, whether it's irritable bowel syndrome or inflammatory bowel disease or celiac disease, is the gut microbiome in all cases. 

Jonathan Wolf: So Will for you, the key thing that GI doctors understand now that they didn't 30 years ago, is that this gut microbiome is really important. From your perspective, it's central to this story. 

Because if I understand right, you're saying damage to the microbiome is firing the starting gun on this set of very complicated interactions you're describing. That can lead me to IBS.

Will Bulsiewicz:  Yes. So we were talking about the history of this condition, and I said that it started with a symptom-based thing and then it moved into a sort of like, hey, it's a gut thing.

Then it became a disorder of the brain-gut axis. And now what I'm saying is the tie that binds all of it together is the microbiome. And we just didn't have the testing to see this, but now we can see that it's the gut microbes that are central to both the gut and the brain. 

Jonathan Wolf: So it feels like the perfect time to switch to, so what can you do about it?

I think, you know, Will, 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 microbiome, what can you do about it? 

Will Bulsiewicz: So the advice that you received was to cut fiber. 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 the 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. 

Jonathan Wolf: No test afterwards. 

Will Bulsiewicz: Yeah. So fodmap, F-O-D-M-A-P, fermentable, oligosaccharides, disaccharides, monosaccharides, and polyols. 

Basically what this is saying is these are carbohydrate-based foods that are fermented 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 irritable bowel syndrome.

The data are clear. People who follow this dietary pattern, if you have IBS, will generally experience an improvement in their gut symptoms. But there's a key with this, and this is something that you actually experienced, you were told, okay, go low fiber, and then ultimately, when you followed the ZOE program, you found that 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. Ultimately, we want to work from this, 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. 

Jonathan Wolf: And so is it possible to actually heal your gut and improve your gut microbiome just by changing what you're eating? 

Will Bulsiewicz: A hundred percent. You've proven that to us in a way. Truly. So, a few things, number one, yes, we can do gut testing using the ZOE kit. With retesting, you have the ability to actually see where your gut microbiome is today and where your gut microbiome is after you make the changes that we're discussing. What I'm proposing. Right. So yes, we can do testing.

Do you want to share how your microbiome test changed by doing the ZOE program? Because I know that you retested a few times. 

Jonathan Wolf: I'm happy to do so. So my microbiome scores are significantly better than the first time that I did the test, which was all part of our first ZOE Predict study. 

So it was part of that first clinical trial, and we've actually had to reanalyze it subsequently because the science of understanding the scores has got much better.

The answer is that the number of good bugs I have has gone up immensely, and interestingly, the number of bad bugs has come down a lot. So when I started this,  I had a lot of bad bugs, which I now tie back to eating a lot of processed food and a lot of red meat and things like this.

So my diet today is very, very different. I had a little hiccup in the middle where I had a massive amount of antibiotics that wiped it all out. And I'm sort of heading back on track again. But yes, I've definitely seen a really big change for me. 

Will Bulsiewicz: And your symptoms improved in the process of this.

Jonathan Wolf: Correct. I think that I was already in much better shape at the point that I started ZOE. So my symptoms have got better sort of slowly through my twenties and thirties. But when I think about what I can eat now, it's really amazing.  I can eat anything. I'm very serious about my diet.

So  I do eat very healthily now, but the amount of different plants and fiber and things that I eat, I think I would've thought was impossible. 

If my 25-year-old self could have seen that, because I'd really gone to this…  exclude all the stuff that makes me uncomfortable, and I think I'd understood that was the path to being healthy, right, is getting rid of all of these things. 

Will Bulsiewicz: Yeah. It's a bit counterintuitive, but I think that the thing that's interesting with your story is that I've known you for five years now, which is an amazing thing. And I remember in 2020, we would have more conversations about this topic involving your symptoms, and now this is a non-issue. 

I don't think that we've talked about this in many years. And that by itself is actually proof of progress. It's kind of fascinating to me because a moment ago we were talking about this concept of the tactile sensation, right? Like your gut, these 5 million nerves.

And I found it very interesting because the way that you received that information was to say, oh, well,  I can't feel this at all. I can't feel this at all. But Jonathan, if I asked you this exact same thing six years ago, seven years ago, 10 years ago, back when you had way worse symptoms than you currently do, you wouldn't have asked me that question because the issue is that you would feel it, you would know it.

Jonathan Wolf: And that's back to your sort of tennis ball example, that if, in a way, things are going wrong, you're much more sensitive than if they're going right a hundred percent. 

Will Bulsiewicz: And the mindset of a person who's dealing with chronic pain,  there's actually a pattern to this. We call it the vicious cycle, which is that they wake up…

This is what I'm going to describe, and I'm quite sure that there's going to be some listeners who can relate to this. They wake up in the morning, and they say to themselves, I hope that today is going to be a good day. And the reason why they say that is because they're fearful that they may actually feel discomfort, because they do every day.

When that discomfort does hit, the minute it happens, suddenly their mind goes and focuses on their gut, and they can't think of anything else. So they become obsessive about that. 

The problem with this is that it actually activates their sympathetic nervous system. So the stress of focusing on this chronic pain actually activates the sympathetic nervous system, which releases this CRH hormone, which actually makes your symptoms worse and intensifies them, which makes you focus on them even more. And you're in a vicious cycle of actually spiraling. 

The reason why I wanted to point that out is because the fact that you could go from a place of being there previously and having to restrict your diet and to avoid and having a fear of food. 

To go from that place to a place where you are today, where you have no gut symptoms, you feel well, you enjoy your food, you don't think twice, and you don't have to wake up in the morning and ask the question, I hope today is a good day because you're confident that it is going to be a good day. You don't even think about it. Right? That's proof of healing by itself. This is proof of healing. 

So yes, the gut testing is verifying this, and I love it. Right? And I love that following the program helped you to achieve these results. And at the same time, you can tell when a person's gut is in a better place because they have less symptoms and they're able to tolerate more food. And that by itself is proof. 

Jonathan Wolf: And one last question on this one. I think a lot of people might listen to this and saying, well, that's just an example of one person. So maybe that worked for Jonathan, but that doesn't mean that it works in general.  

What's the evidence that, in general, shifting diet towards all these things that are supporting your microbiome can actually reduce symptoms of IBS?

Will Bulsiewicz: Well, we know that at a minimum in the short term, following these dietary approaches of a low FODMAP diet proves to be beneficial. 

But in the very beginning of the show, we started talking about, you know, I said a change in bell habits. And he said, It's a little stiff. Why don't you loosen up, Will? And so I started talking about poop and talking about deviation where you could have a constipation poop or you could have a diarrhea poop.

Okay, let's come back to this. So the key here is that when a person's bowel movement changes, their microbiome has changed, right? So when you move from a type four to a type one, there is a microbiome change that's associated with that. And your microbiome is not happy when you move from a four to a type seven, and you're having diarrhea, your microbiome has changed, and it's not happy.

If we can bring you back to a type four, we are healing your microbiome. So in a way, the proof is in the poop because you can literally see in the toilet bowl an update of where your microbiome is at right now. 

Jonathan Wolf: And can you today feel that you can heal almost everybody with IBS by slowly introducing them towards this sort of plant-based, high fiber, diverse diet?

Will Bulsiewicz: So my general approach has always been that I think that diet should create the backbone of what we do in this type of situation. 

The reason why is because if it's a microbiome-based issue, which is what my argument is, if that's the root of the issue, then the solution is to pull the levers that most powerfully manipulate and change, and reshape the microbiome; diet is the right place to start.

So to me, diet, lifestyle, these are things that absolutely we should be doing. We can unpack lifestyle, if you want me to, but it's shocking what a good night's rest can do for a person, right? It's amazing what exercise can do for a person. 

So yes, we want to do these things. I also see that there's a place where supplements can play a role. When I say supplements, I don't mean generic supplements and micronutrients, I'm talking about specifically supplementing fiber prebiotics. And I'm also talking specifically about supplementing probiotics. 

We can talk more about the whole probiotic question because I think fermented food is like the intersection between these concepts of probiotics and nutrition, and making it work for our gut microbes.

But all that being said, Jonathan, I'm not a hardheaded medical doctor who says the only way to fix issues is with our plate and with our food, I'm of the belief also that medications can play a role. 

My issue with the approach that we have been taking in the past is that we're using only medication, and we're not really addressing the root of the issue. We're covering up the issue by addressing the symptoms. 

Yes, there is benefit to addressing symptoms in many cases with medication, but simultaneously, we should absolutely take the steps that we need to make sure that we're healing the microbiome. 

Jonathan Wolf: So, if you're listening to this and you've got bloating issues or some pain or any of this, how many of the people listening to this, if they were to make a big change to their diet, would be able to reduce their symptoms? 

Will Bulsiewicz: If I were working with a person in a personal way? You have to meet people where they are. You have to tailor it. So to me, there's a personalized element of what is necessary to achieve this. Right? 

But if I'm allowed the opportunity to do that, I'm extremely confident. Extremely confident. Now I must acknowledge that I've had some people who defy my best advice.

Jonathan Wolf: Because knowing you Will, I can imagine that annoys you with your desire to be perfect.

Will Bulsiewicz: Well, right, so I get them on the full plan, right? And I have them eating the right food, sleeping, exercising, meditating, or whatever it might be, doing something to reduce stress. 

You get them doing all these things, and you discover that they're not better. What could that possibly be? The answer to that question is in many cases, it's not a physical issue. It's actually an issue in the mind. 

It's actually quite common, Jonathan, this is actually quite firmly established going back to the 1990s, where it's been shown that people who are the victim of trauma, particularly people that are the victim of childhood abuse, manifest irritable bowel syndrome years later as adults. This is very firmly established. 

So there's this connection that exists not just between the gut and the brain, but also between the gut, the brain, and the non-conscious mind. 

Acknowledging that trauma, if you've been exposed to something that was traumatic, you most likely are not interested in thinking about that. It hurts. So you push it into the corner.

The issue from my perspective is I want the listeners to hear that for some of the people who are tuning into our show right now, this is the solution. The solution is to acknowledge that thing that happened in the past that overwhelmed your ability to cope, that is hard for you to think about, and that you've been trying to avoid. It actually makes sense to turn towards that with the help of someone who's a trained professional to actually guide you towards healing that wound. 

Because the reason why that's problematic is we've actually explored the mechanism already. Trauma activates CRH in the brain. It's a nonstop level of activation of the CRH, which is disturbing and disrupting our gut.

So in order to remove that thing, that's basically constantly having the foot on the gas. In order to take your foot off the gas, we have to heal the underlying issue. 

Jonathan Wolf: Thank you, Will, for sharing that. I'm sort of on time, but I'd like to quickly ask two final questions. The first is, is there anything that we can do to prevent IBS from starting in the first place?

Will Bulsiewicz: I'm of the belief that you should not wait until you are sick to start caring about your gut health. This is so important, yes, for digestion, which can manifest with irritable bowel syndrome, and yes, for our brain health and our mood, but also for our metabolism and our immune system inflammation, and for our hormones and hormonal balance.

So the solution from my perspective is to listen to the other episodes where we have discussed in great detail, here are the strategic approaches that you can take to improve your gut health. Here are the things that you can do: eat 30 different plants per week, add fermented food to your plate, and all these different things.

That to me is where a person should start, and I think that's the roadmap to protecting ourselves from these gut microbiome-based health conditions. 

Jonathan Wolf: Wonderful. And I want to wrap up with one final question. What is something that someone can do tomorrow to start to rebuild their gut microbiome? 

Will Bulsiewicz: Well, the golden rule is to eat a greater diversity of plants.

We throw around the magical number 30. But what I would say is, let me meet you where you are, because 10 years ago I was eating about six, and that includes French fries and that includes iceberg lettuce with a single tomato cut four ways, right? That was my salad. 

So let me meet you where you are, and any progress in this domain is ultimately good for your gut microbiome. And I wouldn't want you to go from five to 30 in one day. I want to work you there. I want you to build it up. 

Think of the gut like a muscle. It can be trained, it can be made stronger. So to me that that is the number one thing that we can do. But if you already are doing this, there's other opportunities that exist as well.

So I would think about things like making sure that you get a good night's rest, getting exposure to morning sunlight because that actually is great for gut serotonin, great for our mood, and exercise. 

Jonathan Wolf: And Will, just to clarify, that's 30 plants per week. 

Will Bulsiewicz: 30 different varieties of plants per week. 

Jonathan Wolf: Amazing. Thank you, Will.

I'd like to try and do a quick summary of what we've covered and correct me where I've got it wrong. So the thing I'm most struck by is that you said 40% of people around the world have a digestive problem, which is an immense fraction of people. 

And that, then, people actually diagnosed with IBS, which has this sort of stricter criteria, is still six to 25% or something. So it's a huge fraction of people.

The first thing is that lots of people may not even really be thinking about this. You describe the fact that you should be looking at your poop and lots of people, you know, are brought up to believe that it's sort of taboo, right? And you shouldn't talk about it, but if you're not looking at it, you can't even know where you are.

And then the other key thing I'm taking away is, you know, when I think back to when I was being diagnosed with issues, doctors didn't even know that the microbiome mattered. And now you are saying, you know, the microbiome is central. 

So all of these bacteria are the way to understand what's going on. And you're saying not just sort of in my gut, but there's this amazing gut-brain axis, which explained a lot, and if you ask me to explain it now, I don't think I've managed to do a very good job, is linked to those bacteria and linked to the way that there's this amazing amount of nerves, between our gut and, and our brain.

Therefore, it's not just about sort of digestion and digestion pain, but also has this really big impact on brain mood, and I think you're suggesting even brain health. 

Will Bulsiewicz: Yeah, absolutely. So neuroinflammation, which is the root cause of many of these neurologic and mood-based disorders.

Jonathan Wolf: And then I think you came onto this incredibly positive message, and I feel actually you've maybe got even more positive than when I first met you five or six years ago, about the idea that for the vast majority of people, if they were able to engage with you one-on-one as you're describing,  you could get to a place where you really heavily reduce your symptoms.

You are now pointing to me and saying, well, actually, Jonathan, think you've almost forgotten that five or six years ago this was still something that was bothering you some of the time.

And now you're saying, oh, it's just not a problem. Right? And that is an example of one person who's been on this journey. Which is, I've been on a sort of eight-year journey on nutrition now. 

That you can feel radically better. And that the center of that is by improving your microbiome, by changing, above all, what you eat, then you can really reshape, you know, you can heal your gut, is what you're describing.

And if I summarize what you said, the golden rule for you is to eat more diversity of plants. Ultimately, you're trying to get to 30 plants a week, but actually start with where you are. So if you're eating five, then even getting to 10 is going to make it much better. 

If you're listening to this and you've already nailed the 30, then you've mentioned fermented foods a number of times, but you've also talked about the fact that exercise and sleep can really have an impact. 

And then I think the final thing I'm left with, which I was really struck with, I'd never heard before, is that half of people with IBS have a depressed mood compared to average.

And that's not starting with the mood leading to the gut symptoms as people used to think, it's the other way around. And this is the point that your mood is directly affected by the state of your gut health. 

So if you can improve your gut health, you can actually feel much better. And I think we've seen in some of our other studies that that's not even just for people with IBS, right?

There's many people who don't even realize that they could just have a much better mood than they're currently living with, if they can sort of wean themselves off the ultra-processed food that we're surrounded by.

Will Bulsiewicz: Totally. In the beginning of the show, you asked during the quick fire whether or not it's okay for people to suffer with bloating and other digestive symptoms.

I said, the answer is no. And the reason why is because the reason you're suffering with these issues and the vast majority of cases is that your gut microbiome is struggling. 

If we could heal your gut microbiome, we can improve those specific digestive symptoms. But we also would be raising the tide on your health. And the other benefits that you could experience may include improvements of mood, but it may also include improvements in energy, improvements in your ability to focus and to think, and do complex tasks with your brain, but also improvements in your metabolism, improvements in your inflammation, and improvements in your hormones. People with menopause or perimenopause. 

So the beauty of this is that because the gut microbiome is command central for human health, you can have this one particular goal to improve your digestion, and then you can experience all of these other benefits that you didn't really anticipate. 

Jonathan Wolf: Will, thank you so much for being here and taking the time to explain this to us. 

Will Bulsiewicz: Thank you, Jonathan. It's been a pleasure.

Jonathan Wolf:  If you enjoyed my conversation with Dr. B, I know you'll enjoy this episode with Dr. Karan Rajan, where we dig deeper into the hidden forces driving up gastrointestinal diseases and how to prevent them.

Thanks for watching, and see you next time.