How your gut microbiome affects your health

How Your Gut Microbiome Affects Your Gut and Overall Health

In this episode of A Whole New Level, Levels editorial director Mike Haney talks with Dr. Will Bulsiewicz, a board-certified gastroenterologist and gut health expert, about the profound influence of the gut microbiome on overall health. Dr. Bulsiewicz has a master's degree in clinical investigation and is the author of the bestselling book Fiber Fueled.

They discuss the staggering scale of the gut microbiome---38 trillion microorganisms living inside your colon---and how this invisible ecosystem connects to nearly every system in your body. The conversation covers how the microbiome influences immunity, brain health, metabolism, hormone balance, and even cancer treatment outcomes. Dr. Bulsiewicz also explains how to assess your gut health, the power of dietary fiber, and practical strategies for reshaping your microbiome through food choices.

This is part of a series exploring various systems of the body, how to assess their health, and how they relate to metabolic and overall wellness.

How a gastroenterologist discovered the power of gut health

Mike Haney: To give a little context to this conversation, this is part of a series we're doing on the podcast where we're diving into various systems of the body. This is spurred a little bit by our company's expansion into blood testing, but really it's just trying to help people understand all of the different systems that are going on in their body, how they can assess the health of those systems, and how they relate to other systems, whether that's organs or things like metabolic or cardiovascular. Today we're talking about gut health, which I think is so key to this because it connects to every other system. I wanted to start for folks who don't know you, who haven't read Fiber Fueled, with maybe just a little bit of bio and background. How did you get to be a gastroenterologist and start to communicate a lot of the ideas that you communicate?

Dr. Will Bulsiewicz: This was not my plan. My plan was to become a medical doctor and take care of patients. I started working on that when I was in my teenage years. From the time I started college to the time that I finished without taking any breaks, it took me 16 years of full-scale, six days a week, 15 to 18, sometimes 30 hours a day. I ultimately emerged from that as a board-certified internal medicine doctor, board-certified gastroenterologist. I also have a background in research. So I have a master's degree in clinical investigation and I was on a grant from the NIH.

I went through all this, but in this process, there was a moment where my lifestyle choices, which I thought were very normal---at least they were for people my age and among my peers---they caught up to me. I was in my late 20s and early 30s, and I started having health issues. I was overweight, high blood pressure, high cholesterol. I was also depressed. I was anxious. And even though on paper it looked like every single one of my professional dreams were coming true, which they were, I had very low self-esteem and I needed to find a way to fix that. Ultimately it was by healing my gut. It changed everything.

As a medical doctor who cares deeply about the type of work that I do, when I discovered something that really improved my life and my health, then I was compelled to bring this and deliver it to my patients who suffer with a different set of diseases like irritable bowel syndrome or acid reflux or inflammatory bowel disease, Crohn's disease, ulcerative colitis. I witnessed them having incredible transformations, which eventually led me to a place where I said, sitting in this room one-on-one with a person in privacy, it's not enough for me to be this doctor in Charleston, South Carolina, alone. I need to share what's happening here because I think there's a story that people deserve to hear, because there's an opportunity for prevention, there's an opportunity for treatment that goes beyond what we do with pills and procedures. That ultimately led me on this journey where now here we are today. That journey has included two books and it has included me going on other people's podcasts or my social media or my online courses or my supplement company.

Mike Haney: We should mention, the Gut Health MD is your handle on Instagram, and you put out a lot of content on there. There's a lot of the kind of advice and ideas and explanations of things. The book is Fiber Fueled if people haven't read that. I'm curious, before we move on---you talk about this a little bit in the book---it's so interesting the idea that you're deep in the research and the clinical practice of gut health essentially, but also having your own issues. The idea that you would be unhealthy yourself is not that surprising given the insane demands that you go through to get to that point of medical school and all the things you were doing, master's degrees on top of it. But tell me about that moment of connecting the dots of going, "Oh, the things that I am studying, the things that I am understanding, I guess they're going on in me, too." When did you realize your gut health wasn't where it should be or that that was the key to making yourself feel better?

Dr. Will Bulsiewicz: I think that I could see it in the mirror and I could feel it. You didn't have to convince me that there was a problem. I knew that there was a problem. I was under no illusions that my health was okay even though it was failing. The illusion that I was under was that the food that I was raised on and that frankly in our family that we celebrated and that most people eat in America, I didn't realize that this was the source of all of my issues, which is shocking because I went to Georgetown for medical school. I was the chief medical resident at Northwestern and I was on a grant from the NIH at the University of North Carolina. Despite all of that training and education, that's just not part of what I was taught.

In the moment as I'm going through this, I know that I need to find a solution. I actually didn't want my own medicine. I didn't want to treat five different health conditions with five different pills. I was on my own search for a solution. I was a single male in my early 30s, and in that context, it kind of made sense. I'm going to go to the gym. That's where I'm going to start. I think that's an extremely important thing, but that did not fix my issues. It wasn't until I changed my diet that really things started to fall into alignment with my body. From a gut perspective, I didn't know that it was my gut that was responsible for all of these things. I didn't understand that at that point, but I did know that I was suffering with gut issues and that I wanted to feel better when I ate food. Those things definitely got better quite quickly. But over the coming years I would see a signal starting to emerge within my field as a gastroenterologist. We were on the front lines of what now has sort of become open conversation at the dinner table with gut health. People are aware that this exists, but back in the early 2010s, people were not discussing gut health at all. But in gastroenterology, there were papers coming out and I was connecting the dots.

My background is in nutrition. I thought I knew everything, but it wasn't until I was tracking that I had to ground truth my assumptions with what was actually happening in my real life.

— Will Bulsiewicz, MD

Mike Haney: One of the things I really appreciated about your story, and like I said in the book you go through it in more detail, is that it was gradual. You were learning as you went, that your move to being very plant-forward was gradual. You talk about experimenting with what if I eat a little bit more and a little bit more, and it wasn't overnight, "I'm just going to become a vegan and everything got better."

Dr. Will Bulsiewicz: I like to think about the gut as a muscle. In the same way that you would apply an approach in the gym to building strength or to addressing an injury that you have, you would basically adapt your plan based upon what your capabilities are and meet your own body's capabilities and then slowly start to progress over time. We're all capable of building strength and building muscle in the gym using this approach. What people don't realize is that the same approach can be applied to our gut. That's ultimately how we get there.

To be totally honest with you, I don't particularly care or love the term vegan because it's not about that. Veganism is an ethical choice that people make, and I would encourage everyone to think about those things. But that's not actually what I'm here for. What I'm here for is talking about the power that exists within our gut microbiome and the levers that we can pull with our dietary choices that can empower this community of microorganisms to help us to achieve better health. There are many versions of diets that can accomplish that.

The staggering scale of the gut microbiome

Mike Haney: You mentioned the microbiome, and that's where I want to start because I find the physiology of this so fascinating. The idea that there are estimates that vary, but 40%, 50%, 60% of the cells in our body are not us, they're bacteria. I think people understand that, yeah, there's some bacteria in my gut and it's breaking down food and it's doing something, but the sheer scale and diversity is something I think most people don't realize. Maybe talk a little bit about that and about just how enormous the gut microbiome is and how diverse it is.

Dr. Will Bulsiewicz: The numbers are staggering and quite overwhelming to be honest. The estimate for the total number of microbes that we have living as a part of our body is 39 trillion. Of the 39 trillion, the vast majority, 38 trillion, live inside of our gut, mostly concentrated in the colon, which is our large intestine. In that place you will find these microorganisms that we can't see them, but if you had a microscope you could take a look. What you would see is an entire community. This is a microcosm of our life that exists inside of us. They're not human. They're not a part of our body, but they are deeply intertwined with our physiology.

Mostly bacteria. By the way, when we hear bacteria, we've been conditioned to think, "Oh, that's bad," right? No, this is good. They are our friends. They want us to be healthy. But there's also yeasts. Again, many of them are very good for us. There are these things called archaea, which we think, as far as we know, this is the first life on the planet, the archaea. They're not bacteria. They're not yeasts. They're a little bit different. And then there may be protozoa, which are in the family of parasites. Once again, we hear parasite, you think, "Oh, it must be bad." Actually some of them, like for example blastocystis, there's metabolic benefits to blastocystis, which is one of the protozoa.

This is what we're talking about---these invisible microorganisms, and they have intertwined themselves into our physiology. It's quite fascinating to think about that many of our systems as humans actually predate us. Humans have only been around for a few million years. But I would argue that the development of, for example, our immune system started billions of years ago with these organisms and ultimately it consummated in us where we have this interaction with them.

Just to put this into perspective for people, if you were to look up to the sky on the most clear night possible and actually be able to see every single star that exists in our solar system, you would need 380 solar systems full of stars to equal 38 trillion microorganisms that live just inside your colon.

Mike Haney: It's, as you said, the numbers are sort of staggering, both the numbers are staggering, but also that concept of how much of what's going on in those systems is not human cells, but is this ecosystem. I think that gets at what you talk about in the rest of the physiology of understanding that and then working within that context. But I'm curious, given that scale, how well do we understand in the state of the research the makeup of that 38 trillion creatures? Do we know all of the different types that are in there, or is it still like the bottom of the seafloor? There's still a lot we don't know.

Dr. Will Bulsiewicz: It's like going into the Amazon rainforest and discovering all kinds of new species. We're still discovering. Just to put this into perspective, prior to roughly 2005, you were using culture plates, which is the old-fashioned thing where you would swab something onto this jelly and then watch it grow to grow bacteria. Well, the problem is that these bacteria that I'm talking about, they don't grow on culture plates. We didn't have a way to measure them.

Then in 2005, it was a combination of our computers being advanced enough to handle massive data and simultaneously developing a technique that allows us to see the DNA or the RNA of these microorganisms. Suddenly for the first time scientists are like, "Oh my goodness, look at this. This is crazy." We're still in the process of sorting out what's going on there. I would argue, is there this one version of a healthy gut? To date, we don't have evidence that it's just this one thing. Instead, it seems that there's many versions of a healthy gut and there's many versions of an unhealthy gut. When we think about what's happening there, depending on what's happening with your microbiome and how it interacts with your genetics, ultimately that I think is playing a powerful role in determining whether or not we manifest disease.

Understanding the massive individuality of the microbiome

Mike Haney: How much do we understand that level of individuality, both from a research perspective of we are genetically sequencing what's in there or we're looking at and just trying to map essentially what's there, but also from a clinical perspective, how much variation do you see among patients?

Dr. Will Bulsiewicz: The individuality is massive. If you had an identical twin sitting next to you and you could come from the same home, you obviously have the same mother, you probably ate similar foods as a kid, you would still only share about 30% of the same microbes. There's a massive amount of individuality that exists between us, and we think that it's like a fingerprint where out of 8 billion people on the planet, we don't think that there's actually two people who are identically the same in terms of their microbes.

I think that this raises some of the things that we weren't really sure how to explain. I'll give you an example. As a medical doctor, I could prescribe a medicine and that medicine for nine out of 10 people could be life-changing in a positive way. But there's also that one person who has an adverse effect, and I wish that I could predict that. I think that this individuality that takes into account our microbiome probably explains things like that.

Mike Haney: That's a really interesting idea because it is something you see across things, right? Whether it's how does your body respond to cholesterol, what foods spike your glucose, there's so much individuality across all of these systems. How much of that is rooted in this massively diverse system within your body that we know is incredibly individual? Feels like there's a lot of ground to be mined there.

Dr. Will Bulsiewicz: There's actually a paper that was published in Nature Medicine looking at this question of how does our gut microbiome interact with other potential variables to explain our blood sugar after a meal or our blood fat after a meal or the amount of insulin that our body pumps into our system after a meal. With blood sugar, the number one predictor is the choice of food that you make. Whatever that meal is, the carbohydrate makeup and other factors are going to ultimately determine how that interacts with your body more powerfully than the microbiome itself.

But your insulin response, which I think from a metabolic perspective is what people really are interested in---that's the measure of metabolic health from my perspective---actually, your gut microbiome is one of the most powerful predictors. If you wanted to understand how much insulin a person's going to pump out after a meal, actually looking at the gut microbiome helps to explain that. By the way, the same is also true for our blood fat response after a meal.

Mike Haney: How much do we know about the individuality being due to genetics versus environment or external causes?

Dr. Will Bulsiewicz: I think it comes back to this concept of an identical twin. With an identical twin, you have exactly the same genetics, and you also share many common characteristics from your childhood. And yet, despite all of that, you are 70% different from one another. We do think that our environment is a powerful determinant of what's happening with our microbiome.

I actually see this as a beautiful, optimistic thing because basically what that says to me is you're not cursed with a problem. You are born with an opportunity to make changes at any point in your life. If you start to make the changes that I'm sure we will be discussing in a short moment, you can reshape your microbiome. There's a number of different levers that can be pulled to help you to achieve that.

In a way, your gut microbiome, you can think of it as like a Polaroid photo of your choices---basically the foods that you eat and your sleep patterns and exercise and your stress and the people that you surround yourself with, your connections to those people and whether or not you have pets. We can keep going. These are the things that ultimately are going to shape this ecosystem and make it look a certain way.

In a way, your gut microbiome, you can think of it as like a Polaroid photo of your choices.

— Will Bulsiewicz, MD

Mike Haney: Given that amount of individuality---it's both massive, there's 38 trillion bugs in there, it's highly individual we know, it's also very dynamic---it's not the same the day we're born as it is, both given those environmental changes, but also just changes as we age. Given all of that, how can we be confident in the cause-effect relationship of some of these interventions? How do we know that feeding certain things or feeding certain particular bugs are going to work the way that we want them to?

Dr. Will Bulsiewicz: I think sometimes we're limited in some ways by our own intellectual capabilities as humans. I don't think that it comes down to that one bug. We tend to think of these things that way because it's very easy for us to understand, "Oh, if I have more Faecalibacterium prausnitzii, I will have a healthier gut microbiome." And that is generally considered to be true. But I think that instead what we're talking about is communities, networks of microbes. I think ultimately artificial intelligence is going to be necessary for us to really start to understand the complexity as opposed to thinking about it as associations between one bacteria and some specific outcome.

That being said, your question was how can we be confident that the choices that we make make a difference, and this is where intervention studies come in and they show us this. Not only what we see as we build towards human intervention studies, but mechanistic studies that involve animal models or ex vivo microbiome studies, we see what's happening there and then that leads us to a human intervention study. Many times the human studies don't play out exactly the same, but in the microbiome space, you can generally take the animal model studies, get a feel for what is likely to happen with humans, and then you're going to see an effect.

Examples of this---you mentioned that the microbiome changes rather quickly. One of the first studies that really got me excited about this space was published in Nature in 2014. It was a scientist who's at Duke University. His name is Lawrence David. I don't know if he knows what a fan I am of him, but this was the study that got me started. He took a group of people and said, "We don't know in humans whether nutrition can change the microbiome. So I'm going to feed them two extreme opposite diets and we're going to see what happens." In the process of doing this, they would check the microbiome every single day.

What they discovered is that the choices that they made, within 24 hours, were already starting to institute change. The reason that happens, by the way, is that time for our microbes is very different than it is for us as humans. Their experience is that a generation comes every 20 minutes, whereas for us as humans, a generation comes every 30 years. If you think about that 20-minute thing, in one hour they could be spawning off three or four generations of new microbes, and over 24 hours you've spawned a lot of new microbes.

Mike Haney: That's positive in the sense that you can make changes very quickly, but I imagine also then it can go back, right? If you start to change your habits, you could probably see a fall-off effect pretty quickly if you don't keep up the healthy habits.

Dr. Will Bulsiewicz: Absolutely. But that to me is no different---once again, back to the exercise analogy---you can work yourself up and get in great shape, but if you stop working out, then over time that starts to diminish.

How the gut microbiome connects to your immune system

Mike Haney: Let's talk about some of the other systems that the gut microbiome is connected to because I mentioned that's one of the things that I think is so key, is that it really does connect to so much. Now that we have a sense of the scale and what's going on, you can understand why. Maybe get into a little bit more of the physiology of how it connects to some of the other systems. Let's talk about immunity and autoimmunity. How does the gut microbiome affect your immunity?

Dr. Will Bulsiewicz: It starts here, which is that if you ask the question, what is home base for your immune system? It's not your bone marrow. It's not your lymph nodes. And it's not your spleen. It's your gut. 70% of your immune cells live within the lining of your gut. What separates 38 trillion microbes from 1.3 trillion immune cells is a single layer of cells which we call the gut epithelial barrier. It's the gut barrier.

Now that gut barrier becomes a key piece of this conversation because with the microbes and the immune cells being neighbors like this, they're talking to one another across that gut barrier. But also if the gut barrier breaks down, then things can sneak across that are not supposed to be crossing that gut barrier. It's a barrier for a reason. If, for example, an inflammatory bacteria like E. coli sneaks across the gut barrier, your immune cells are going to intercept it. That interception is what we call inflammation.

Inflammation is at the root, at the core of these health conditions---autoimmune and allergic diseases. Autoimmune basically means that the immune system is turning against you. There's something within your body that the immune system has said that's a problem. It needs to be taken out. It's confused. In allergic issues, the immune system is also equally confused, but the difference being that it's a response to something from outside your body that normally it should just be fine with. It could be a food or it could be pet dander or whatever it might be that creates seasonal allergies.

Ultimately what we come back to is that if you had that gut barrier intact, then the gut microbes are on one side, the immune cells are on the other side, and the immune cells are protected and they're able to do their job without being stimulated and triggered. That's ultimately what we want if we want a healthy immune system, and that's ultimately what we should strive towards, for example, for people who have autoimmune and allergic diseases.

Mike Haney: Do we see an impact on those kinds of conditions from changes in gut health?

Dr. Will Bulsiewicz: That is an interesting question because if you look at animal model studies, the easiest way in humans to answer this question would be to do a fecal transplant.

Mike Haney: Talk a little bit about fecal transplants for people who don't know that idea. You go into it in some length in the book, but it's such a fascinating idea.

Dr. Will Bulsiewicz: It is a fascinating idea and I used to be disgusted by this idea back in the day. I heard that they were doing this at the University of Iowa and I was like, "What are they doing in Iowa?" That was roughly 2005. The idea is this: a fecal transplant is where, yes, you are taking the poop from a person and transferring it to another person. By the way, for the record, this can be done in a very reasonable way where a person is asleep, they're receiving a colonoscopy, they don't even know what's happening. It's not nearly as gross as it sounds.

We won't get into the details. There was a period of time where it was kind of gross. When I was a GI fellow in 2010, 2011, this used to be kind of complex, but eventually we developed ways to streamline this and it's not so bad anymore. It turns out that there are scenarios where what you need is to restore a healthy gut microbiome to fix a problem.

An example of this is an infection called C. diff. C. diff is a bacteria that many of us have living inside of us. When you take antibiotics, those antibiotics destroy microbes---not just the bad ones, they often destroy the good ones too---disrupting the balance that exists within this ecosystem. Where you end up, the word that I would use is dysbiosis. Dysbiosis is the term that means a gut microbiome that is damaged and not capable of doing its job.

When we take antibiotics and we create this dysbiosis, it actually creates an opportunity for this nasty bacteria that is not being hurt by the antibiotic to multiply, grow, and take over. That takeover manifests with what we call colitis, which is inflammation of the colon. People get diarrhea around the clock, potentially blood. It's a mess. It's not fun. It can be life-threatening. We went through a period of time where our antibiotics were not working---roughly 2010, 2011, our antibiotics were not working. We needed something to save these people.

Ultimately what we came to is, "Well let's try this. Let's try to restore their microbiome by giving them a poop transplant." When you do this during a procedure like this, the C. diff infection, the vast majority of the time in my experience, 90 to 95% of the time in my experience, the C. diff infection immediately gets suppressed by the healthy microbiome that you just installed and it goes away. The poop transplant works for this infection and doctors are allowed to use this therapy in practice routinely. That's what it's there for.

The question that comes up is could this same technique be applied to ulcerative colitis, which is a form of inflammatory bowel disease. The problem is that if I give a new microbiome, but I don't help that new microbiome to stick around through food choices and other lifestyle choices, if I don't do that, then that new microbiome is going to fade away over the course of a few weeks. The ulcerative colitis is still there. Ultimately where we're at with this is that there is some promising data that you can, for example, treat some autoimmune diseases through fecal transplant.

I think that the more convincing thing from my perspective is actually through nutritional interventions where we have a number of different studies that have suggested that nutrition can be a powerful way for us to potentially impact, for example, ulcerative colitis or other autoimmune diseases by reshaping the microbiome. We do know that if you take a group of people that have autoimmune diseases, whether that be rheumatoid arthritis, psoriasis, ulcerative colitis, and Crohn's disease, if you look under the hood, you will see that their microbiome is dysbiotic---damaged gut microbiome. I've never seen a study say otherwise. When you institute the changes that we know with a number of different studies lead to improvements in the microbiome, we do see that many people have profound benefit in terms of their disease. It may not make the disease go away permanently, but there can be profound benefits.

The gut-brain connection

Mike Haney: How about brain health? I know there's a lot of systems we can talk about, but this is another one I find pretty fascinating. How does the gut, all my bacteria going on down here, interact with my brain which has its own very strict barrier about what gets in there? How do these two systems connect?

Dr. Will Bulsiewicz: That's an interesting question because there's a number of different ways that our gut microbes are communicating to our brain and vice versa. This is a two-way form of communication. I think of them, even though they are anatomically separate---the gut is downstairs, the brain is upstairs---I think of them almost as being right next to each other because that communication is so powerful that they can really manipulate each other.

We think of the gut---we actually have called it the second brain or the enteric nervous system. The reason why is because it's completely carpeted with nerves. There's 500 million nerves in the gut constantly feeling and sensing. So again, 38 trillion microbes, 1.3 trillion immune cells, and then right in that same area, 500 million nerves, feeling, sensing, collecting information that gets consolidated into a pair of nerves called the vagus nerve. The vagus nerve travels all the way from your gut all the way upstairs directly to the brain. The brain is taking that information and actioning based upon that. That's one of the more powerful ways that our gut and brain are connecting.

But there's also the chemicals that are produced by our microbes that we might call bioactive, where our gut microbiome is almost like a chemical factory, changing, transforming our food, releasing things. An example of this---I'm a big fan of dietary fiber---is that when fiber comes into contact with our microbes, it stops being fiber and it's transformed into what are called the short-chain fatty acids. These short-chain fatty acids like butyrate and acetate and propionate, they have powerful anti-inflammatory healing effects that will impact the gut barrier right there, will impact the immune system right there, but also will enter the bloodstream and travel to the brain.

Our brain, you mentioned that the brain has a barrier. We call that the blood-brain barrier. Well, it turns out that the blood-brain barrier is made up of the exact same cellular material as the gut barrier. It's a single layer of cells being held together with the same types of proteins. In the same way that these short-chain fatty acids that we get from fiber can heal our gut barrier, they also can heal our blood-brain barrier.

That's one of the powerful ways that our gut and an anti-inflammatory diet can ultimately manipulate what's happening with our brain. It's interesting because the concept of brain fog---we have millions of people who go to their doctor and say, "I feel like I have brain fog." And the doctor doesn't know what to do about this because they don't understand it. I'm convinced that brain fog is the blood-brain barrier being leaky. That's leaky brain, and if we could do these things in our gut, it's the choices that we make that are impacting our gut that ultimately can help us to heal this particular issue.

These are the different ways---the vagus nerve and the chemicals that we produce---these are powerful ways in which our gut is able to communicate with our brain.

Mike Haney: I understand that in the research, we're starting to see some connections with Alzheimer's, dementia, Parkinson's and microbiome. I know there's a fair amount of research going on with the microbiome and even mood to some degree. Is there anything you've seen in that space that you think is interesting or the most promising?

Dr. Will Bulsiewicz: There's a few things. First of all, what you just mentioned---Alzheimer's disease, Parkinson's disease, mood disorders like major depression---there's a common tie that binds them together, which is inflammation. But the inflammation is not what's happening in your gut. It is related to your gut, but it's the inflammation that's occurring in your brain. We call this neuroinflammation.

It's interesting to think about because there have been studies where you lift up the hood and you take a look at the gut microbiome, again separate from the brain, within the context of these health conditions---Alzheimer's, Parkinson's, major depression---and what you see is an inflammatory microbiome. The gut microbiome is inflammatory. When we take steps to address that---an example of this is the SMILES trial where Felice Jacka, who's at the Food and Mood Center in Australia, introduced a high-plant Mediterranean diet (was not vegan) to a group of people with major depression. What they found is over the course of a few weeks, their improvements in mood were conceptually similar to what you would expect to see with medication.

Another thing that's kind of interesting is Parkinson's disease because I can tell you as a gastroenterologist, and any gastroenterologist who's listening to the show right now will nod their head and agree with me, they all have constipation. Every person who has Parkinson's disease has constipation. What's interesting is that actually there have been studies that are now emerging saying that the constipation and what's happening in the gut predates the manifestation of Parkinson's disease in the brain.

Now this is not to say---I don't want people to freak out---constipation is highly prevalent in the United States. It does not cause Parkinson's, and the vast majority of people who have constipation are not going to develop Parkinson's disease. Nor am I saying that constipation doesn't cause the Parkinson's. It's that you have this health condition that's developing, but the manifestation happens in the gut first and then in the brain second, which is interesting.

Mike Haney: It feels like given the complexity, and we could keep going through all these different systems and there's interesting research in all of them whether it's cardiovascular, metabolic, cancer. I know there's a lot of work going on. It feels to me like we're at the tip of understanding how many tentacles into your body and into these systems the microbiome has. I'm curious, without going through all the systems, is there anything else that you're seeing in the research in the last couple of years that you're really excited about that you think we're about to uncover some really interesting connections?

Exciting frontiers: the microbiome and cancer immunotherapy

Dr. Will Bulsiewicz: I think that the way that I would characterize our journey with the microbiome is 2005, we developed a new laboratory technique. We start to discover and our initial approach is, "Oh my goodness, there's thousands of species in there. We just need to look at them and try to understand them." So we spent a big period of time just kind of describing what we were seeing.

Now here we are and we're starting to move into a new phase, which is manipulation. Manipulation for health outcomes. I would characterize the front line of this new approach as occurring in cancer therapy because the new approach to treating cancer is less about poisoning the entire body so that you can also poison the tumor. The new approach is what we call immunotherapy, where you empower your own immune system to fight the cancer for you.

What we're discovering is that the gut microbiome is able to work along with the immunotherapy drugs that are being administered to help us to achieve our goals. It starts with this, particularly in melanoma, where we have mature research that have shown us that if you give antibiotics to a person and then you give them immunotherapy drug, they don't do as well. They showed that by destroying the gut microbiome with antibiotics, you actually are disrupting your ability to treat cancer.

Then they asked the question, "Well, what if we go the opposite direction and rebuild the microbiome?" There was a case series---this is not, we need more research to flush this out---but there was a case series where they took a group of people who had failed immunotherapy for melanoma, and by the way I should mention, melanoma is a skin cancer and it's the number one most deadly skin cancer in the United States. They took a group of people who had failed immunotherapy for their melanoma and they gave them a fecal transplant and then they redid the immunotherapy, and a very large percentage of them actually achieved a lasting response in terms of fighting their cancer. That's life-changing for those people. They were given another chance.

More recently, there's emerging data with the power of fiber where they identified that people that were on a higher fiber diet---it's not what I would classify as a high-fiber diet, it was actually below the minimum recommended amount of fiber, but people who were on a higher fiber diet actually had better survival in terms of their cancer when they were receiving this treatment. There's a story that's developing there.

Mike Haney: Again, it seems to fit with that larger theme of the microbiome is absolutely interacting with other things that are going on. I mean, I guess it's not surprising given the connections with immunity, but something like immunotherapy for melanoma could seem like a very disconnected idea from what's the health of your microbiome, how much fiber are you eating. But nope, they're connected. They do seem to have some interaction there.

Dr. Will Bulsiewicz: I think it comes back to the limitations of us as humans, which is that clinical research has accelerated and there are new---in my field as a gastroenterologist, there's hundreds of papers published on a daily basis. It's hard enough for me to try to keep up with that. The problem is then you become a bit myopic where you're focusing on your field, your specialization. What you lose sight of is the big picture, which is that your body doesn't care that you're a gastroenterologist and someone else is a neurologist. The body doesn't care because it's one body and it's all interconnected, and at the center of it, really the command center for our health turns out to be the microbiome. It's just fascinating to consider that it's not even human and not even a part of our body, yet that is the center of what's happening.

How to evaluate scientific research on the microbiome

Mike Haney: That perfectly tees up---I have one more large meta question and then we'll dive into all the specifics that I know folks want to hear about how they can understand their gut health and all the great tips on how to improve it. But it's around this idea of research because one of the things I loved about your book is it is heavily sourced. It's heavily referenced. You're clearly somebody who takes in a copious amount of research and then wants to translate that through to people and help them understand. You do that through your social channels as well.

One of the things that I find difficult as a journalist---I'm not a doctor, I'm a journalist---so I'm doing the same thing, but without all the training that you have and trying to make sense of when I see a new study come through PubMed or come through some other channel and it's got a really flashy headline on it and we've made some really interesting discovery or connection or association. Is this a big deal? How confident should we be in this? Is this an idea that we should put forth more broadly as, "Hey, look, turns out melanoma and fiber are connected, eat more fiber and you'll have a better chance if you're doing immunotherapy"? Or is it, "Well, we have a connection, maybe we're not quite sure"? Just broadly, how do you take in research? How do you decide and evaluate what is "huh, it's interesting, okay, cool, hopefully we learn more" and what is "oh man, this is, we got to tell the world about this, this is amazing" or "this is going to change what I'm doing as a clinician"? Because it's one thing for me to decide whether or not to write a blog post. You're working with humans who are dealing with real conditions. How do you make sense of research and how to incorporate it?

Dr. Will Bulsiewicz: This is one of the challenges that we have in the year 2025---dissemination of information and finding reliable sources of information. This is a challenge across all fields. It's not just science. Within science, from my perspective, I feel that I was trained to look at studies, understand the details, and then be able to pull out whether or not this study is something that's trustworthy and what I can pull from this to share ultimately more broadly. That's something that I spent years working on. But for the layperson who's at home, it's like, "Okay, that's great, but what do we do?"

The way that I would approach it---it starts with this. There's a hierarchy of evidence and I think that the hierarchy of evidence is something that we all actually should be aware of. It shouldn't just be a scientist-level thing. The highest level of information that we can collect is what we would call a systematic review and meta-analysis. Essentially what we're doing there is we're compiling human data, ideally without bias. Now, this is not perfect because people, if they want to, can manipulate things, but if we're being honest and we're going to pose a question---does fiber protect us from melanoma?---in a perfect world, what we would do is we would pose that question, we would identify all of the studies that are relevant, and then we would bring all of those studies together into one analysis and that would answer the question for us. That's at the peak, that's at the absolute top.

Just below that are human randomized control trials. We call those RCTs. That's where we take a group of people and we randomize them. Why do we randomize them? What is that word about? Well, the problem is that if people are given the opportunity to make the choice themselves, those biases could be part of the problem. When we randomize, we're basically going to take a group of people and we're going to just shake it all up, try to make it as even as possible, and then we're going to see how it plays out between usually two choices. Those to me are the top two.

I want people to be aware that there's some human research that's interesting, but you got to be careful. And those are anecdotes. The problem with the anecdote is they're great stories. And we can all, myself included, get caught up in them. It's a bit like you have an uncle who drank a 12-pack every day and smoked two packs every day and he lived to 95. Does that mean that we should all drink a 12-pack every day and smoke two packs? No. I think that we're all in a position where we can see that and understand that. But in a lot of things in medicine, in a lot of things in science, it gets a little more convoluted and challenging.

We also need to be aware that laboratory studies and animal model studies are what we would describe as mechanistic studies. They help us to explain and understand what we see occurring in humans. But they by themselves should not be the basis of what we do because I'm not a mouse and neither are you, and many times those studies don't actually bear out. That's why we need human studies.

Then epidemiology, which is basically the study of large populations of people, by themselves epidemiology studies are not perfect. Yet at the same time, I see people who dismiss them in totality. And I think that's insane, because for latent diseases like our top killers, heart disease and cancer, we're not going to be able to afford to do randomized control trials for all of these things. There's no RCT that proves that smoking causes lung cancer. It was epidemiology.

On some level we have to be willing to take those pieces. At the end of the day, I think that being aware that mechanistic studies and anecdotes are not as powerful as human randomized control trials and systematic reviews---if you literally just know that, you're already off to a good start.

Mike Haney: I think that's a really great takeaway to recognize that different studies are trying to do different things, that because a study is only showing a mechanism or only showing an association is not a failure of that study. That's what the study was setting out to do. And it is the case I find that the researchers are not doing the study to create the headline that will then tell everybody that blueberries cure cancer. That's not what they're trying to do. It's in the translation of it out to the mass population that that message kind of gets twisted. So I think your point of if you can at least go a level deeper to understand was this a review study, is this a mechanistic study or is this an epidemiology, is this giving us associations, is it giving us mechanisms, is it giving us the review of the reviews, the study of studies, at least helps you gauge a little bit what to do with that information and what to do with that headline.

Dr. Will Bulsiewicz: For me, when I'm assessing the science, it's usually not this one study that determines everything. Most of the time what it is, I'm looking at---I have my hand on the pulse of the science, in general what do we see. Because most of the time it's not perfect and there are definitely examples. There were examples that existed where smoking did not actually increase the risk of lung cancer. There were examples where smoking was associated with less risk of lung cancer. So we have to be able to see this bigger picture.

Mike Haney: I think looking for those themes that emerge---and this is why I think microbiome, and this is something I've certainly seen over the last five years in microbiome---if you ask me is the microbiome connected to a lot of things, I can look at just the volume of research that has come out over the last five years that is continually connecting the microbiome to other systems and we are finding out more and more about it. That's a theme that's emerging and I think paying attention to those---I asked Joel Fuhrman, I interviewed him a few weeks ago, and I asked him the same question because nutrition studies obviously are hard to do. He went, "Yeah, but if you take a big enough view, you will see truths that emerge. There's a ton of things we know, we really do know now that we didn't know 30 years ago because of the research that's done." So any one study, sure, but if you just pay attention to the totality of it, things will emerge from there that we can count on.

Dr. Will Bulsiewicz: I think the other thing too is that sometimes among medical doctors I hear people pose the question, is it literally the microbiome? Basically what they're getting at is, are the microbes really responsible or are the microbes just kind of along for the ride and there by association? From my perspective, yes, I do want to prove whether the microbes are responsible or not, and I think the fecal transplant studies help us to do that. But at the end of the day, in some ways, who cares? Because truly, if the point is that I can use the microbiome to get people to change their diet, and that's a diet that reduces their risk of disease, results in them being healthier on the other side, that's what matters. It's the outcome that matters.

Three free tools to assess your gut health

Mike Haney: Well, so let's get into some of those outcomes and some of the practical bits. Where I want to start with this is this concept of measurement or understanding. We've talked about some of these other systems where I've got blood markers and I can understand the health of my kidneys through a series of blood markers. How do I understand the health of my gut microbiome? How do I know whether or not I've got a good one?

Dr. Will Bulsiewicz: There are the free tools and that's where I'm going to focus our time. But there are also the emerging tests and the way that I would describe those emerging tests are very interesting, but I wouldn't really describe them as totally ready for prime time. There's still a lot that we need to learn to be able to confidently use what we see in a microbiome test and apply it in a clinical way. There's still a lot for us to learn there.

That being said, there's some general things that any person could listen to our podcast and take away if they do microbiome testing. The diversity of species within your gut microbiome is a marker of health. We see this across ecosystems that biodiversity means that the ecosystem is healthy. When the ecosystem is eroding and failing, we lose biodiversity. That could be true in the Great Barrier Reef and the Amazon rainforest, and it also can be true inside of our intestines in the gut microbiome.

Biodiversity, the diversity of species within the gut microbiome, starts to contract when the microbiome is not doing well. At the same time, this is what we see in the manifestation of disease, whether that be metabolic disease, obesity, diabetes, whether that be immune disease, which could be autoimmune health conditions or other chronic inflammatory health conditions, or neurologic disease. We see that there's a loss of diversity within the gut microbiome. That's a general rule.

That's a decent place for us to start if you were to do microbiome testing. Now there are the free tools and what I would say is that as a gastroenterologist, this is where I would start. This is where I would start with a patient. I want to know how do you feel? If you said to me, "I'm having a lot of bloating. I have abdominal discomfort. I can't eat food. I'm breaking out in rashes. I have brain fog. I have tremendously low energy, lots of fatigue, and yet I'm not sleeping well," you just told me a microbiome story. I already know the microbiome is struggling right now because it's manifesting with these symptoms throughout your body. That's number one. How do you feel and what are your symptoms?

Mike Haney: Sorry, just to interrupt there. Are there symptoms in that list that you just gave that jump out at you more than others?

Dr. Will Bulsiewicz: I would say the consummate microbiome symptom is bloating. Now this is not to say that any bloating indicates that your microbiome is unwell. We all get bloated once in a while, and also our food choices---if you are not someone who really eats beans and you go and have chili that has five different beans in it, I expect you to get bloated because what you ate was a microbiome food. But if you have chronic bloating, if you have a chronic issue that you wake up in the morning and you say, "Gosh, I hope today's a good day. I hope I don't have any bloating today," that's to me---I already am hearing that there's an issue there. That to me is the classic.

Going head to toe in these different bodily systems, there's different symptoms that could manifest. To me, inflammation is often associated with fatigue. There's an example. Symptom checking, how you feel and how you do and what's wrong---I think on some level, let's not lose sight of this. I appreciate that we have biometric data and that complements all of these things, yet at the same time, how do you feel remains just as important as ever.

Number two, if I were a cardiologist, I would listen to your heart. As a gastroenterologist, I want to know how are you pooping? Ultimately, our poop is a reflection of what's happening in our gut microbiome. It turns out, first of all, that our poop is not predominantly the waste of our food or leftover food that we didn't digest. It turns out that actually the majority of the weight of our stool is microbial, which explains why I could put a fiber supplement into a cup of coffee and you'll have a big bowel movement. It's not because the fiber supplement was grit---I'm talking about fiber that has dissolved in your coffee. The reason why that happened is because we created new microbes with that fiber supplement.

The other thing is that what we want, the ideal, is a soft, formed, sausage-shaped bowel movement. I apologize for using my hands as I do this. I'm a creature of habit and my wife often tells me I get weird with this kind of stuff. We want a soft but formed bowel movement that's like a sausage. That's something that we actually have a classification system for called the Bristol stool scale. There's seven types. People can look this up. Seven different types of bowel movements and you can collect information about what's happening with your bowel movements based upon just looking at them.

What I just described is right in the middle. It's a Bristol 4. If you move towards Bristol one, whether it's three, two, or one, you're moving towards constipation. If you move towards the higher numbers, five, six, and seven, you're moving towards diarrhea. What we know is that every single one of those is associated with certain changes in the gut microbiome. The gut microbiome is associated with changes in your bowel habits.

I want to know also how often are you going and how do you feel when you go? It's sort of overlooked. We think about how often and we think about the shape, but the part that gets often overlooked is are you straining? Are you struggling? Do you feel like you didn't really empty? Do you have to go again 45 minutes later? These are signs that this is not going well. Whereas feeling an urge, following that urge to the restroom, having a relaxed, complete evacuation that I'm just going to say it is satisfying and feels a little bit good---that's where we want to be. I think it's important to try to get people there. Number two is your bowel movements. What's going on with them? How often are you going? What do they look like and how do you feel?

Then the last thing is to take and look at a person's health history. We've discussed how the gut microbiome is connected to our digestion, our immune system, our metabolism, to our brain and our mood. The one thing we haven't really touched on yet is our hormones. For a person who comes in to see their medical doctor and they have gut health issues, meaning they have IBS, irritable bowel syndrome or something of that variety, chronic constipation, but they also have migraine headaches and chronic anxiety and eczema, and they also have high blood pressure and high cholesterol, and you see this list that's whole body---I know that every single one of those conditions is associated with a damaged gut microbiome. I don't need any more evidence. I already know it's there. The writing's on the wall.

What to know about microbiome testing

Mike Haney: You mentioned the microbiome testing earlier and this pops up in my Instagram feed. Talk a little bit about the microbiome tests that are out there, at-home tests you can get. There's lots of companies that do this. Are they worth doing? How reliable are they? What should people think about when they see something about microbiome testing, test your gut?

Dr. Will Bulsiewicz: I think sometimes people make profoundly viral-friendly statements on this topic, where "this is a total waste of your money." I think this is a moving target and we're moving towards understanding what's happening here. What I would prefer to do is to propose a framework, which is to say that what I'm looking for as a gastroenterologist is microbiome testing that has been validated by a human clinical study. If you can take the microbiome information, apply it to a human and then achieve some sort of improvement in their health, whatever that may be---there's a lot of options on the table---if you can improve their health in that process, you got me. That is worth it. That's the way the testing should be used.

But on the flip side, if we're fulfilling the desire of a person, but we're not grounded in science, an example of this would be, "Okay, so I just tested your microbiome, and using this information, I can tell you which foods you should eat, which foods you shouldn't eat. I can tell you which supplements you should take." I haven't seen a study to back that up.

Mike Haney: Related on that and just on this point of understanding your gut health and what to do. When should somebody come see you? I think there's some systems where if I have cancer, I'm going to see an oncologist. I feel like gut health is one of those things that people probably---I'm curious if you think that this rings true with you---people probably struggle with a long time on their own. Like you said, their bowel movements aren't normal or they might have a ton of bloating. They think, "Well, it's just age or it's just who I am." When should somebody come see you?

Dr. Will Bulsiewicz: I guess what I would say is first of all, there are these red flag symptoms that you shouldn't wait, that there should be no delay. If you're losing weight and you can't explain why, if you see blood in your stool, if you are anemic without a good clear-cut explanation, if you're low on iron, again, without a clear-cut explanation, if you have intense symptoms or chronic symptoms or there's been a change in your symptoms, those are the red flags that we look at.

Where do we draw the line though for people that don't have those red flag symptoms and do have some sort of issue there and they're trying to figure out, "Should I go see a doctor for this?" To me, there's no reason for this to be affecting your quality of life. If it is affecting your quality of life, you've crossed the line. It's time for us to get you better.

The one thing I will say---many people have challenged me and said, "Well, when I go to my gastroenterologist, I'm not seeing you. They're not going to give me nutrition advice." We have to start. Step one is always what are we treating. The value of going to see your medical doctor starts with the diagnostic process. This is where testing in general really comes in. This is where, for example, at-home testing, I think there's value there because identifying problems that exist, there's value to that. That's step one. What is going on, what are we treating?

Then step two is how can we treat this best, and it may be that your gastroenterologist has expertise and a great handle on what are the medication options, but maybe they're not comfortable with giving you nutrition advice. And that's where you got to build a team. In 2025, we're not in a place where there's this one doctor who's going to solve all of your health issues all at once. You have to be prepared for the possibility. Your gastroenterologist, they serve their purpose. They're a part of your team, but you may also have someone else who's doing your nutrition. You may have someone else who's working with you on your physical fitness, etc.

Mike Haney: If anything, I feel like we're in the opposite of that where GPs have less and less time to deal with people and to solve all your problems.

Dr. Will Bulsiewicz: I think so. I think unfortunately they have a horribly challenging job and the system is beating doctors across the board, but particularly primary care doctors who are on the front line. They're getting beat down by our system. I feel bad for them because these are people who are well-intentioned. They don't go to medical school---they could have become a banker. They don't go to medical school because their focus is to go as quickly as possible. The system is forcing them to do this. But I think that at the end of the day the message to people at home is don't be afraid to build your team and think about it. Who are the team members that you need to be successful?

Mike Haney: I think that's really good advice and particularly thinking of folks like a registered dietitian nutritionist can be so helpful.

Dr. Will Bulsiewicz: 100%.

The power of fiber for metabolic health

Mike Haney: All right, so let's get into some nutrition advice. I'm going to start with an easy one. Make the case for fiber.

If I could address this one issue of fiber deficiency, I am convinced that a lot of the issues that we have from a health perspective in America would improve.

— Will Bulsiewicz, MD

Dr. Will Bulsiewicz: I think that there's a highly powerful case here, which is that first and foremost, 95% of America is deficient in fiber. This creates a ripe opportunity for me because if we're going to give nutrition advice, we should be addressing the things that are missing, not getting more of the stuff that we already have plenty of. Here's this thing that we're missing. It turns out that this is the primary food for our gut microbiome. If you believe what I'm saying in this podcast, if you have bought into the concept that our gut microbiome is relevant and important to our health, then you should be buying into this concept of fiber. This is food for our gut microbes. We call that prebiotic.

It plays out and proves itself because the fiber goes in our mouth, it comes into contact with our microbes, the microbes produce short-chain fatty acids. And then what do we get on the back end? In my favorite fiber study of all time, they found that increased dietary fiber was associated with less risk of having a heart attack, less risk of dying from heart disease. That's our number one killer. Less likelihood of being diagnosed with three different types of cancer---colorectal cancer, esophageal cancer, and breast cancer. Less likelihood of dying from cancer. Less likelihood of having a stroke. Less likelihood of being diagnosed with diabetes.

I've now named four of the top 10 causes of death in America that we can reduce our risk by consuming this one nutrient that 95% of America is missing. I could go further. It would reduce the risk---we have data suggesting that it reduces our risk of Alzheimer's. We have data suggesting it reduces our risk of chronic kidney disease. As we think about the connections between our gut and these microbes and our metabolism---diabetes, heart disease---and our immune system and cancer, for example, as we think about these different connections that exist between the gut and these different systems within our body, you see it play out here. The fiber is able to profoundly reduce our risk of these diseases.

From my perspective, if I could address this one issue, and this by the way is not a dietary pattern issue. This is a you're-not-getting-fiber-in-your-diet issue. If I could just address that, then I am convinced that a lot of the issues that we have from a health perspective in America would improve. That was the argument that I made in Fiber Fueled.

Mike Haney: So maybe draw out that connection a little bit more with---fiber is feeding the microbes, producing SCFA, short-chain fatty acids. There's a bunch of different kinds of short-chain fatty acids. Butyrate is one maybe people might have heard of. Make that physiological connection. What are the SCFAs doing in our body that are affecting then all of these other systems?

Dr. Will Bulsiewicz: This could literally be a podcast on its own, but right there in your microbiome, that's where they get to work immediately. You are encouraging the growth of the fiber-producing microbes. You get more of those. Those are, by the way, the anti-inflammatory microbes. You are simultaneously suppressing the inflammatory bacteria. E. coli, salmonella, shigella---you get less of those as a result of short-chain fatty acids. You're repairing and restoring the gut barrier that we talked about earlier in this episode, which is important for protecting our body. People talk about leaky gut. You want to fix leaky gut. This is the answer. Short-chain fatty acids.

They interact with our immune system directly, not just fixing the gut barrier, but by directly influencing and having an anti-inflammatory effect on our immune cells. Which by the way may make you think, "Oh, wow. That sounds scary actually because if it's anti-inflammatory, well, what if I have an infection and I got to clear that infection?" Actually though, it's interesting. The way in which short-chain fatty acids shape our response to infection is by getting the right immune cells in the right position to clear the infection as efficiently as possible. This is the reason why there's been compelling data in the last five years related to COVID-19 and reducing our risk, whether it be through plant-based eating or whether it be through dietary fiber and its effects.

Spreading out into the body, these short-chain fatty acids enter our bloodstream and they influence our metabolism. They affect our blood sugar, our insulin sensitivity. They affect the release of GLP-1, which of course is the popularized gut hormone that has been concentrated in the forms of Ozempic or Wegovy. But GLP-1 in the body when you're working physiologically, it comes from short-chain fatty acids interacting with cells. It helps us to release peptide YY, which is another hormone that helps us to feel full. It affects our fat metabolism. In terms of preventing the accumulation of fat in the body and helping us to activate the burning of fat, actually short-chain fatty acids help us to do that.

Whether it's our lipids or our blood sugar or feeling full, which are all measures of metabolic health, the short-chain fatty acids are right there in the center of this. And then we mentioned earlier that they travel up to the blood-brain barrier, repair the blood-brain barrier, cross into the brain and have an anti-inflammatory effect in the brain where I mentioned that Alzheimer's, Parkinson's disease and major depression are manifestations of neuroinflammation, and short-chain fatty acids actually help us to cool off the inflammation of our brain.

We're talking about whole-body healing that we get from this nutrient that is missing in 95% of our diets, which can essentially only be made or is most efficiently made by us taking in fiber as basically the fuel.

Mike Haney: Maybe this is a good moment to explain---you mentioned prebiotics, but you do a nice job of explaining prebiotic, probiotic, postbiotic.

Dr. Will Bulsiewicz: These are terms that you're probably seeing in your supermarket now. It used to be just probiotics. Of course, probiotics are living microorganisms, usually bacteria or yeast, that have been shown to be beneficial for human health. That's what probiotics are.

Now emerging is the concept of a prebiotic, which is actually the food for beneficial bacteria, where if you consume a prebiotic and you feed these microbes, you get a benefit as a result of that. There's a formula that I like that I've presented in my books where if you take prebiotics and you combine them with probiotics, because they need each other---they're not effective by themselves. If you had a great gut but no fiber, you can't really get the benefits. If you had fiber but you were sterile and you had no microbes in your gut, you wouldn't get the benefits. But when you combine these two ingredients together, this is the formula for success because they produce the postbiotics, which are what actually matter. That's the part that's bioactive that actually affects your physiology.

Short-chain fatty acids are the classic postbiotic. But another example of this is that you could take polyphenols. People hear about resveratrol, and this is the most powerful argument for drinking red wine. Resveratrol is this anti-aging polyphenol. Most people don't realize 90 to 95% of polyphenols are not absorbed by the body. When something is not absorbed by the body, that means that it's going to come into contact with your gut microbiome.

Polyphenols come into contact with our gut microbes and they transform them and then they get absorbed. Part of the reason why resveratrol and red wine is probably so good for us is that actually the fermentation process is microbes outside of our body preparing the resveratrol in a way for us in the same way that our gut microbes would.

Mike Haney: Oh, interesting. Okay. So I'm convinced on fiber. Thank you. Talk about the different kinds of fiber. People have probably heard soluble and insoluble, but one of the things you talk about in the book is there's lots of different kinds of fiber.

Understanding different types of fiber

Dr. Will Bulsiewicz: Fiber is perhaps the most biochemically complex nutrient out there because basically different plants have different forms of fiber. The concept of soluble versus insoluble, what we're referring to there is does it dissolve in water? Soluble fiber will dissolve in water. When we were talking about putting a fiber supplement into your coffee, that's soluble fiber that I'm referring to there. Insoluble fiber is grit. It's the roughage. I could put it in---I could heat up the water. It doesn't matter. It's not going to dissolve.

This is really just a characteristic of the fiber. It's a way for us to take complex stuff and simplify it. When we characterize them as soluble and insoluble, the value that we get from that characterization is that insoluble fiber generally speaking runs through us. That's the classic model of fiber that we heard about when we were kids---it just goes through you, you poop out the other end. That's insoluble fiber.

But soluble fiber is where the science of the last 20 years is sharing a story that's incredibly exciting because it's the soluble fiber that tends to be the type that comes into contact with these microbes and gets transformed into the postbiotic short-chain fatty acids.

Now many people hear this and go, "Oh well then it sounds like I need soluble fiber and I don't need insoluble fiber." It doesn't need to be that complicated. It's actually quite simple. Fiber is found in plants. All plants, fruits, vegetables, whole grains, seeds, nuts, legumes, every single one of them has fiber. Every single one of them, it's going to have a mix of many different types of fibers that include both soluble and insoluble.

We want the different types of fiber that come from different types of plants. Because much like protein where the protein in a fish is not the same as a protein in a cow is not the same as the protein in a bean, we want a mix of different types of fiber because that's how we feed many families of microbes and that leads to a diverse microbiome. The way that we approach this is by focusing on variety in our diet.

We've spent this time talking about grams of fiber, but I'm actually here in this moment to say to me, it's not actually about grams of fiber. It's about varieties of plants in your diet. And that we should be striving to eat 30 or more different plants per week.

I know because every time I talk about this, there's some people in the comment section that freak out. "Oh my gosh, that's so many." Time out. Hear me out for a moment. Herbs and spices, they count. You make a smoothie on Monday morning. This is what we do in our house. You make a smoothie. It has easily six, potentially up to 10 or 12 different plants. You're almost halfway there already. And every single time you sit down to have a meal, you have an opportunity to just have this concept cross your mind and it forces you to get outside of the routine and look at what can I add. It's about addition. And that is what I think to be empowering. You don't have to go from five or 10 up to 30 tomorrow. What you need to do is start at five or 10 and shoot to go to 15 and then eventually 20 and just work your way up. Build that muscle.

Practical advice for picky eaters

Mike Haney: You talked about this in the book and I really appreciate it. The idea of low and slow, I think was the phrase that you used about that idea of working up. I think that's probably worth touching on. What if I'm somebody who's getting the typical American diet and when I start tracking my macros I'll easily have a day of under 10 grams of fiber because I don't eat nearly enough plants? If I'm in that camp, how should I think about ramping up to getting to---I mean, you tell me what I should be aiming to get to. I've heard 50 is a number that folks should be targeting, but how should I think about the target and how should I think about ramping it up?

Dr. Will Bulsiewicz: That's interesting. I think that it's very personal to be totally honest with you. The approach is this. Don't feel the pressure of the endpoint. I want to meet you where you are and start a process. This is about the long-term view because if we pull this off, this is going to provide rewards to you that compound over time, because this will be a way in which you are improving the quality of your diet. Every single day that your diet is better is going to have benefits in terms of health throughout your entire body.

Let's pretend that you're doing 10 grams of fiber per day. Mike, I was less than that 15 years ago when I was eating the standard American diet the way that I did. We just want to start with number one. What do you like? What kind of food do you like? Are there plant-based foods that you enjoy? Yes, I imagine avocado has to be one of them.

Mike Haney: I don't mind an avocado, but I am---this is embarrassing to admit, but I'm a picky eater. This is actually one of my questions for you, and I realized you addressed this recently on your Instagram in the context of kids. I'd love to frame this with my kid, but let's be honest, it's about me. I'm a picky eater. Grew up not eating a lot of vegetables. Trained myself as I got older and similar to you, hit a certain age where I just couldn't live off Ho-Hos anymore and had to start eating real food, found a handful of foods that I like. I can eat spinach. I can eat a salad. I can eat broccoli. But there's probably five vegetables that I enjoy eating. When I heard the 30, I had the same reaction. But what I appreciated about it and where I'm curious where you'll go with this is thinking about that whole universe of foods. It includes seeds, includes flax seeds I can throw in there, includes grains, it includes herbs, those kinds of things. So yeah, that's where I'm coming from as somebody who my universe of vegetables is pretty small.

Dr. Will Bulsiewicz: Start off with the food that you love. People hear this and they think of raw broccoli. That is not what I have in my mind. I think that starting off with things that first of all are a bit predigested and more gentle. That to me includes using your blender to create a smoothie. Make it taste good. I don't recommend doing a pure fruit smoothie, but it's not hard to start off with whatever berry you enjoy. For us, we do blueberries in our house. Adding a banana and some greens and then seeds and nuts, and next thing you know, you have a delicious smoothie and you already have done six to 12 different plants.

Soups are a great place to start. A delicious minestrone soup could easily have 10 different plants in there. You're just opening up, whether it's you go to the farmers market or it's you literally opening up your fridge and saying, "What can I throw into that pot?" That's where you start.

When I make sauce, first of all, it starts with the raos. Someone corrected me. I used to call it rouse. It's rao's. I never thought that I would be comfortable spending eight bucks a bottle for sauce and then I started consuming it and now I can't go back. Rao's is pretty great. It's great and so now Costco helps to make it more palatable. But you start with that. You don't have to add anything to it, but you could and it tastes better and you can pull your kids into the kitchen and they want to be a part of that. What can we throw into that sauce and allow it to simmer for 30 or 45 minutes, bringing out the flavors, softening the food, tasting delicious, and complementing whatever you're going to put that sauce on.

To me, these are some of the opportunities---soups, sauces, and smoothies. I guess it's s-words right now.

Mike Haney: I've read out there that smoothies pulverize the fiber and it's not---then you're not getting the benefits if you're blending it up in a smoothie as opposed to just eating the whole food. Any truth to that?

Dr. Will Bulsiewicz: There is some truth to that, but I don't actually view that as being something that should scare you away, particularly when you're crafting a balanced smoothie. If it was a pure fruit smoothie---sometimes when we go to a smoothie shop, many of the popular smoothie shops, it's just fruit. That to me is not the type of smoothie that I'm referring to. I'm referring to a vehicle to get different plant-based foods into your body in a way that is highly enjoyable.

Yes, I would prefer for you to eat a salad instead of throwing it into the blender and making it a smoothie. But at the same time, with my three-year-old, I can hand her the exact same smoothie that I'm gonna enjoy and she's gonna drink it and she enjoys it. When it's properly balanced, I'm not particularly concerned about the fact that it's blended and therefore the fiber is not as intact as it would be if it were in whole food form.

Balancing fiber intake with blood sugar management

Mike Haney: One of the other things you mentioned, a fruit smoothie, reminds me of one of the other challenges that we'll sometimes see. So our company, Levels, is traditionally focused on metabolic health, putting CGMs on people, thinking about blood sugar spikes. And that can lead a lot of people to run away from some fiber foods---fruit for instance, grains for instance. It is true that a lot of folks will see, you know, it varies a lot by person, it varies a lot by food, but some folks will see pretty significant blood sugar spikes from certain kinds of grains, from certain kinds of fruit. How do you think about that balance between trying to maintain relatively stable blood sugar but still eating some of these foods that are good and high in fiber?

Dr. Will Bulsiewicz: This is a great question. Thank you for asking me. I think that there's very interesting and relevant information that exists in monitoring our blood sugar using a CGM. I don't want to sound like I'm dismissing that in any way. Yet at the same time, we have to be smart about how we accept that information because our health throughout our entire body is not defined by one measure. We have to have the ability to take that measurement and put it within the context of everything else.

When we consume carbohydrate-rich foods like fruit, the sugars that naturally exist in that fruit, they may cause your blood sugar to go higher. Probably not as high as it would be if it were a sugar drink. Probably not as high as it would be if it was a refined carbohydrate breakfast food like a donut. I think we have to just accept that that doesn't make it bad for the fruit when it's a whole fruit form. For the vast majority of people, unless you have insulin resistance, that spike shouldn't be so profound that it's what we consider to be out of balance.

Flip side, I think it's important to acknowledge that many of the foods that we're discussing actually prevent diabetes. Many different forms of fruit actually reduce your risk, actually improve your insulin sensitivity. This is an important part of striking that balance.

When it comes to grains, I get a little bit sensitive around the bashing of carbs. I think that the reason why may just be semantics, but I want to be clear on the words that refined carbohydrates are different from complex carbohydrates. Fiber is a complex carbohydrate. Resistant starches which feed and fuel our gut microbes conceptually in the same way that fiber does are complex carbohydrates. We want those. We don't want to deprive our body of those. Refined carbohydrates where you are milling a grain is a very different thing. From my perspective, we don't have a need for those.

Now, I'm open to fermented sourdough bread being consumed on occasion. I don't have bread more than probably one to three times a week total and sandwiches aren't a big part of my life. In general I think it's important for people to understand---separating the refined carbohydrates from complex carbohydrates is important. Whole grains are different from refined grains.

Mike Haney: I think that's a great message and something we find ourselves having to also then give this message of don't freak out about every spike. Blood sugar curve is not the only thing that matters. And ultimately, and this is really I think the message in Fiber Fueled, is worry less about the gram target, worry less about some of these kind of specific measures and step back and say does your plate have a lot of whole foods on it? Does your plate have a lot of diversity? Does your plate have a lot of plants? If it does, you're probably going to be okay if we can get folks to think about more fiber is good. More whole foods are good, then worry a little less unless you're in a situation where you've got type 2 diabetes or you've got pre-diabetes or you know you have insulin resistance and you have to be very very careful about those kinds of excursions.

The role of fiber supplements

Mike Haney: The last thing I want to touch on before I let you go, because I just think everybody is going to hear, "Okay, I need more fiber" and they're going to go to supplements. It's a vast world from whatever you can buy at CVS to the most expensive stuff you'll see hawked on Instagram. How do you think about how supplements fit into this fiber picture?

Dr. Will Bulsiewicz: Full transparency, and I mentioned this in the very beginning, I am the founder of a supplement company. I'll unpack that, but let me be the first person to say first and foremost, diet should be where we start. I am not of the belief, and I actually said this in Fiber Fueled, that you can't outsupplement a bad diet. You can't take a C-minus gut microbiome and lift it all the way to an A+ simply by taking a whole mix of different supplements, including a fiber supplement.

To me, we need to start by instituting the changes that you and I have discussed here today. That being said, we're going to take that as far as we can take that and then there's an opportunity for optimization. That to me is where fiber supplements come in. There are absolutely people who are trying to do everything right with their diet and still don't get to where they want to be, and fiber supplements can help them to get there. I do think that there's value to prebiotic fiber supplements.

There's many that are available out there and classically, as we discussed, the soluble fiber is the type that tends to be prebiotic and have this effect on the gut microbiome. So examples of soluble fiber include acacia fiber, include partially hydrolyzed guar gum, glucomannan, wheat dextrin, and then psyllium husk is the classic---that's Metamucil---and psyllium husk is somewhere in the middle where it has elements that are soluble and it has elements that are insoluble.

That's the lay of the land from my perspective. The reason why I created a supplement company, my supplement company is called 38 Terra which means 38 trillion. The reason why I created 38 Terra is because I was not satisfied with the lay of the land and I felt that we could do better. Do we know that our wheat product or wheat dextrin is not sprayed with glyphosate? No, we don't know that. There's no transparency.

From my perspective, I felt that we could do better harnessing the new science which tells us that variety is important, which tells us that we should get different types of prebiotics, which tells us that we have an opportunity to do third-party testing to really bring transparency into the equation. Those are the things that I live by, but also fully acknowledging that it's not for everyone. If you're somebody who's coming from a deficit of fiber, focusing first on the diet, make the changes where you can and think of the supplement as a kind of addition.

Mike Haney: In other words, don't---I think it's so tempting because it's so much easier for me not to have to find new vegetables that I want to eat and just go, "Well, I'll just have three psyllium scoops a day." But that's not going to give me the diversity. It's not going to give me all the benefits we've talked about in this.

Dr. Will Bulsiewicz: I think it's important to conceptualize this as think about all the different beautiful colors that you see when you go into your supermarket. There's a reason why they put fresh produce out in front of you. It's beautiful, smells nice, highly attractive. It's there for you to be eating. I want people to think about eating the rainbow. I don't want is people to conceptualize this as you're forcing me to eat Brussels sprouts that I don't like. If you don't like Brussels sprouts, you don't need to eat them. There are tons of other options that exist.

But what I do want is I want you to think about expanding those boundaries by addition of adding more variety into your diet. That to me is the step and it includes the seeds and the nuts and includes the legumes and the whole grains in addition to fruits and vegetables. So we have a lot of choices there. Frozen is good too by the way. Frozen is good. You can get organic a lot less expensive and in some cases it's even better for you because they flash freeze it. They basically harvest it and then they freeze it as opposed to it sitting on your supermarket shelf for two weeks.

To me this is the place to start. But I just think that from my perspective as a medical doctor, I'm about results. This is what I ultimately care about. I would not shoehorn a person into a diet that they hate and they don't want to do. That's never going to work out. I want to start with what you love. Allow that to lead the charge as you add more of these types of foods into your diet to help you to achieve your health goals.

In that process, we have other things that we can do that have nothing to do with lifting a fork, which are the lifestyle factors that include exercise and getting a good night's rest and being connected to other humans, putting down our devices and looking a person in the eye and having conversation, feeling bonded to them. There's those things and there's supplements that can be used in addition to these factors. It's not the front line, but it's a part of the package that's on the table. And the last thing is we have medications if we need them. I'm not against medication. I think that sometimes what we do is we say, "Oh, it can only be this." Why? At the end of the day, what we care about is results. What is the best package that's going to allow us to get there?

To me, that package is like, here's the buffet. You make the choices of what works for you, and let's make it work. Let's get you results.