The truth about SIBO, with Will Bulsiewicz

Could rebalancing your gut microbiome be the answer you're looking for?

In today’s episode, Jonathan and Dr. Will Bulsiewicz dive into small intestinal bacterial overgrowth (SIBO), which may underlie common health issues from irritable bowel syndrome to brain fog.

Together, Will and Jonathan tackle myths and share insights into SIBO diagnosis and treatment. They also discuss why understanding SIBO could be key to unlocking better health.

Dr. Will Bulsiewicz is board-certified in internal medicine and gastroenterology. He’s also a New York Times bestselling author. Dr. B has won multiple awards and distinctions for his work as a clinician. 

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Mentioned in today’s episode: 

Fiber supplementation protects from antibiotic-induced gut microbiome dysbiosis by modulating gut redox potential from Nature Communications

Our earlier podcast on the low-FODMAP diet 

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Episode transcripts are available here.


[00:00:00] Jonathan Wolf: Hello, and welcome to ZOE Shorts, the bite-sized podcast where we discuss one topic around science and nutrition. I'm Jonathan Wolf and as always, I'm joined by Dr. Will Bulsiewicz, and today's subject is SIBO. 

[00:00:17] Dr Will Bulsiewicz: Yes SIBO, or small intestine bacterial overgrowth. Interest and research into this condition has been growing year on year, Jonathan. There's still a lot we don't know about SIBO, but there are signs that it could be linked to a host of other conditions, ranging from irritable bowel syndrome to Alzheimer's disease. 

[00:00:33] Jonathan Wolf: So Will, what exactly is SIBO? I always find these four-letter acronyms slightly terrifying. And are you saying that treating it could be a key to tackling these other conditions? 

[00:00:44] Dr Will Bulsiewicz: Well, I think it's important for our listeners to understand that what we know about SIBO is a work in progress at the moment. So, we don't want to make promises that the scientific data can't yet keep. However, there are some encouraging discoveries as well as some SIBO myths that frankly, I think we need to bust. 

[00:01:01] Jonathan Wolf: Alright, well let's not hang about then. So Will, let's start at the very beginning. What on earth is SIBO?  

[00:01:32] Dr Will Bulsiewicz: Okay, so to understand this, let's take a journey into the body, like literally into the body, Jonathan. So imagine that we're both inside of a stomach, and we're just floating around in some gastric acid.  

[00:01:44] Jonathan Wolf: Delightful. Will, you always take me to the best holiday destinations. 

[00:01:49] Dr Will Bulsiewicz: So if we continue our journey down South, we'll move from the stomach into the small intestine. And what you'll see around you is that the walls are tightly wrinkled and this is to increase the surface area. There's a lot of absorption happening here. This is actually where 95% of nutrient absorption takes place is in the small intestine.

[00:02:05] Jonathan Wolf: 95%, that's amazing. I didn't realize.  

[00:02:07] Dr Will Bulsiewicz: Yeah. Now, the small intestine is about 7 meters or 22 feet long. It would be like taking a very long walk through the countryside. And while on this journey, in a healthy person, we would occasionally encounter bacteria, but it would just be a passerby. 

Then most of the bacteria live in the large intestine, which is further down. And so the large intestine is almost like the metropolis for our gut microbes.  

[00:02:34] Jonathan Wolf: And Will, as I've learned through many conversations with you and Tim and so many other scientists, these bacteria we know are really important for our digestion and our immune system and they make chemicals that are really important for our health, right? 

[00:02:52] Dr Will Bulsiewicz: That's absolutely correct. In a healthy body, we have a symbiotic relationship with these bacteria, which means that they're in balance. But in SIBO, this relationship has shifted and things, unfortunately, have fallen out of balance.  

[00:03:05] Jonathan Wolf: What exactly do you mean by shifted, Will?  

[00:03:06] Dr Will Bulsiewicz: Well, like I said earlier, SIBO stands for small intestine bacterial overgrowth. And the key word here is overgrowth. 

Now, let's reimagine our walk through the small bowel countryside. Because in a SIBO sufferer, suddenly, this has become a hostile place. It's overrun with bacteria, they're not supposed to be there, they're spilling out of the city in huge numbers, and unfortunately, many of them are hooligans. 

[00:03:37] Jonathan Wolf: So this is your overgrowth idea, right? Like, I guess overgrowth didn't sound good when you mentioned it. And I guess overgrowth is not good in practice.  

[00:03:45] Dr Will Bulsiewicz: Yeah, this is, so overgrowth in the sense that there's more of them, they're multiplying, they're forming a mob and that mob is angry. And rather than you and I having a cordial walk through the countryside and waving hello to the friendly microbes, we are now defending ourselves. 

So that bacterial balance that is supposed to be there is completely thrown off, Jonathan. And this brings me to myth number one that I want to bust. Which is that there's a rumor out there that SIBO is an infection and that is not true, it's a bacterial imbalance. Which basically means that SIBO is a form of dysbiosis.

[00:04:24] Jonathan Wolf: Well, I love it when you take down those myths, Will. Particularly when they're myths I didn't even realize about until you explained they were myths, but that's good. Thank you.  

[00:04:32] Dr Will Bulsiewicz: Yeah, well, this is something that, of course, we find on the internet, and sometimes what we find on the internet, we have to call into question, Jonathan.

Now, it's important also for our listeners to understand SIBO, and the type is determined by the makeup of the flora that are involved. So today, we're going to focus on the classic form of SIBO, which is called hydrogen dominant SIBO, and this is driven by bacteria.  

[00:04:58] Jonathan Wolf: Okay, so Will, that’s suddenly got quite complicated. Can you help us to understand what does it mean to be a classic form of SIBO? What’s this about hydrogen, which I feel somehow shouldn't be inside my body. What are the symptoms?  

[00:05:08] Dr Will Bulsiewicz: With this type of SIBO, which is the, again, the bacterial type, because there's an extra amount of bacteria in the gut, they're producing additional gas. And naturally, this causes bloating and flatulence, and with it, sort of gaseous abdominal discomfort. 

But it also can cause other things, like malabsorption, and difficulty digesting and absorbing our food. And in severe cases, you can even have neurologic effects, such as brain fog, or altered mental status. Now, what's interesting is this link between SIBO and other diseases.  

[00:05:40] Jonathan Wolf: Well, our team did some research and apparently they found a review published last year that reported SIBO to be significantly associated with at least 30 conditions, including things like IBS, diabetes, heart disease,
Parkinson's disease, even pancreatic cancer.  

[00:05:55] Dr Will Bulsiewicz: Yes, that is true. These are associations and I think that it's important though to also take this with a little pinch of salt. To properly frame that, we must understand that those are all conditions associated with gut dysbiosis. And SIBO is a form of gut dysbiosis. 

So that doesn't necessarily mean that SIBO is the cause. Or vice versa. They could both be true and be unrelated. They could also both be true and come from the same root cause, yet be different manifestations of that root cause. So, like I said, SIBO research is a work in progress. We're here to report where we are today and our understanding of it. 

One of the major challenges that we face in sorting this all out is that there's, there's unfortunately no real consensus on how to properly perform the test for the presence of SIBO. And this really impacts our ability to compare the results between institutions, between research studies. And so it kind of becomes a little bit of a mess, unfortunately. 

[00:06:54] Jonathan Wolf: Got it. So you're saying we have to be very careful. Just because you might have SIBO that might just be a symptom of something else. And it's not that the SIBO itself is necessarily causing any of those diseases. Indeed, maybe those diseases are causing SIBO as a byproduct, just like we might feel fatigued or have inflammation or, or whatever else. 

Will, will you tell us a bit about the testing methods because, you know, in fact, I can't go for a walk into your intestine, I'm pleased to say, I think. So how do we understand whether or not we have this?  

[00:07:25] Dr Will Bulsiewicz: Well, the good news is my intestine is a friendly place, Jonathan, but nonetheless, I want to actually kick off with a really important myth. This is myth number two, and this is one that actually I'm very excited to talk about because I think it's important. 

You cannot diagnose SIBO with a poop test. And I've seen this so many times, including in the care of my patients, where someone is diagnosed with SIBO because the stool says they have overgrowth. That is 100% wrong. Poop tests measure what's happening in the colon, not what's happening in the small intestine. So, the bottom line from my perspective is that if you are being told, based upon a poop test, that you have SIBO, don't buy what you're being told.  

[00:08:06] Jonathan Wolf: Got it. And that's back to for example, ZOE, part of this is that we do this poop test, we measure the bacteria in your gut, but this is your large intestine where it's meant to be. And I think, well, if I understand rightly, what you're saying is, SIBO is all about bacteria being in this, you call it the small intestine, which actually sounded quite long and quite large, in fact, but before, and that's the place where you're not expecting to have lots of them. Am I understanding this?  

[00:08:31] Dr Will Bulsiewicz: That's right. If we wanted to properly understand whether or not you have an overgrowth of bacteria in the small intestine, we would want to sample from the small intestine. Or we would want some sort of surrogate marker for what's happening in the small intestine. We wouldn't want a test that really is reflecting what's happening inside your colon, which is what a poop test is. 

[00:08:52] Jonathan Wolf: So what do we do?

[00:08:54] Dr Will Bulsiewicz: Well, so the gold standard is a test called a sterile jejunal aspirate. The issue with this test, by the way, I should mention, is that it's invasive, it's expensive, and frankly, very few clinicians actually do this. So I've worked with dozens of gastroenterologists, none of them were actually doing this test. 

So what we actually do is a breath test, Jonathan. And the advantage of the breath test is that it's non-invasive. Basically what happens is the patient will drink a sugary liquid. And it goes down into their gut and wherever it comes into contact with bacteria, those bacteria will metabolize that sugar and produce hydrogen. 

And so the breath test is measuring the production of that hydrogen and based upon how much hydrogen is produced and how like soon after consuming this beverage, the hydrogen is produced, you can determine effectively whether or not there's SIBO.  

[00:09:48] Jonathan Wolf: Well that sounds very clever, and I think given a choice between a doctor sticking a tube all the way down my throat and through my stomach or having a drink of a sugary liquid, I'm going for the sugary liquid every time. 

[00:09:58] Dr Will Bulsiewicz: Yes, understandable, but at the same time, there is a trade-off that we take in the process of getting the convenience of this non-invasive test.  

[00:10:03] Jonathan Wolf: I somehow knew you were going to say that. Will, go on, help us to understand it.  

[00:10:11] Dr Will Bulsiewicz: Yes. So if you know me, then you know that I'm not a huge fan of these tests. I understand that we do need them, but unfortunately, I don't have tremendous confidence in them. And the reason why is because there's a very large number of false results. It could be a false positive and if it's a false positive, then you're going to be treated with antibiotics when you're not supposed to. It could be a false negative and if it's a false negative, then we're going to be confused and not treat you for SIBO when in fact that is the diagnosis that we're supposed to be treating. 

And some of the things that can affect the results on this test include antibiotic use, abnormal gut motility, which can come from a number of different reasons. Fiber intake and how much fiber you've been consuming in recent days. Whether or not you're using a laxative or an antidiarrheal drug. Exercise, how much you exercise can actually affect the results that we get on this test. 

And so, you know, what you see here between these different things is that there's many different ways in which we can actually get tripped up on this test result. And this is a bit problematic when it comes to using this test clinically to treat our patients. It's also problematic when it comes to the research because this is typically the test that's being done in these research studies. And so once again, it affects our ability to be very confident in how we approach SIBO in these studies. 

[00:12:56] Jonathan Wolf: And so just to make sure that we've got this, you're saying this is sort of the best test that anyone uses, but it's not a super accurate test. So in comparison to, you know, I didn't like a blood test where, you know, you're getting this really good score of something, this has like a lot of times where maybe it says Hey, it's positive, but you're saying it's actually for some other reason, you know, or it's negative and it's missed. So it's not highly sort of accurate?  

[00:13:21] Dr Will Bulsiewicz: Yeah, I think the bottom line is that we need better testing for SIBO in order to really refine our process, both in terms of the research and what we do on a clinical level. 

[00:13:30] Jonathan Wolf: So let's step into like your practice, Will, so what would you do if someone that you're treating returns with a, with a positive SIBO breath test? 

[00:13:41] Dr Will Bulsiewicz: The important thing from my perspective is the process and I want to start by first asking the question, why would this person have SIBO? And there should be an answer to this question. 
This is not something that just comes out of nowhere. 

[00:13:59] Jonathan Wolf: So can you give us some examples? 

[00:14:01] Dr Will Bulsiewicz: Definitely. So SIBO can be brought on by abnormal bowel motility. So I would be asking the question, does this person have chronic constipation, diabetes, which massively affects our bowel motility, low thyroid, gastroparesis which is a condition where a stomach is not properly emptying, or chronic opiate use, which among all the drugs that exist on the planet, those are the most powerful drugs in terms of affecting bowel motility. 

SIBO can also be caused by other things like abnormal gut anatomy. For example, if the patient has had an abdominal surgery, a removal of the last part of the small intestine, or if they have Crohn's disease. Some people that have Crohn's disease, they develop strictures within their small intestine, scar tissue where it gets tight and narrow. This can create a pocket where SIBO forms. 

And last but not least is low stomach acid. Stomach acid is there for a reason, when there's low stomach acid, then this allows bacteria to build up and this can create SIBO. 

[00:15:05] Jonathan Wolf: Because in all these cases, you're sort of saying like, why are these bacteria hanging around in here? And you say one of the reasons is maybe like food is hanging around in here, whereas actually it's supposed to be sort of pushed through this first part of your intestine and then it sits around in the large intestine afterwards is that my hazy understanding of what's going on?

[00:15:27] Dr Will Bulsiewicz: So, you know, if you think about stagnant water, Jonathan. It's stagnant water where the mosquitoes form, right? And in the same way, it's stagnant intestinal fluid where the microbes build up. And that stagnant water could come from Abnormal motility where things are just not moving through and they're just pooling and sitting there. 

[00:15:48] Jonathan Wolf: And that's when you say abnormal motility because there's not a word I think we use all the time. You're saying it's not being sort of flushed through, but actually, this part of our gut is sort of supposed to be flushing this through sort of quite fast, Will?  

[00:16:01] Dr Will Bulsiewicz: Well, you know, not necessarily fast, but at least things are moving. And if things are not moving, then there's just sitting there. And when they're just sitting there, then you create this sort of pool where the bacteria can start to multiply and grow and you can create SIBO. 

Or alternatively, if you have one of these abnormal anatomical issues, then that also, you know, for example, if you have a stricture, then fluid will build up in front of that stricture because things are slowing down in that specific location. And that's really what you're seeing here is whether it be pooling of the fluid or if it's low stomach acid, it's the fact that we're just not keeping control over the bacterial numbers because of the stomach acid issue.  

[00:16:40] Jonathan Wolf: And so are there certain types of medicine that are common that could bring on SIBO? 

[00:16:44] Dr Will Bulsiewicz: Yeah, so we were just talking about the stomach acid. And if we were to say, what is the number one cause of low stomach acid in the U.K. and the U.S., it's very clear. And that is the use of proton pump inhibitor drugs, which actually cut stomach acid production. And these drugs, it's not really a surprise, have been linked to increased risk of developing SIBO. 

So if you are a person who's using a proton pump inhibitor and you have developed SIBO, then one of the things that we would really want to do is build strategies to try to get you off the proton pump inhibitor, which allows your body to fall back into balance.  

[00:17:17] Jonathan Wolf: And so I think you're saying if you can, it's best to understand what the root cause of the problem is and treat it. 
What are the other treatment options?  

[00:17:26] Dr Will Bulsiewicz: The typical approach is for someone to say, take this antibiotic, stop eating fiber. And this actually is problematic. Although this might work in the short term, the treatments have the potential for long-term collateral harm to the gut microbiome. There was a study published in Nature Communications that I love, where they found two major risk factors for dysbiosis, which basically means the SIBO. 

The two major risk factors were reducing fiber intake and recent treatment with antibiotics. And so when our conventional therapy is just antibiotics and reducing fiber intake, perhaps we're actually creating problems for ourselves. And this may explain why SIBO relapse rates are so high.  

[00:18:21] Jonathan Wolf: So what approach would you take? 

[00:18:22] Dr Will Bulsiewicz: Well, so it's not to discredit antibiotics. There's clearly a role for antibiotics in the treatment of SIBO, but I think that they have to be measured and controlled. We have to be very thoughtful about this. And we have to remember that, and this is, I think, one of the big lessons when it comes to ZOE, you don't fix the microbiome by destroying it. You fix it by building it back up. 

So going back to this prior study in nature communications, they found that consuming fiber actually protects the microbiome. Now, this can be hard to hear if you're a person who's had SIBO because you're probably yelling at your podcast right now saying, Uh, Dr. B., fiber hurts my stomach. 
I don't feel well when I consume fiber. And so we have to be smart about this. 

And the way that we do it is to start with a low FODMAP diet because low FODMAP sources of fiber, they produce less gas, they will make you more comfortable. And then what you can do is ramp up over time as you start to reintroduce those FODMAP foods eventually. 

[00:19:25] Jonathan Wolf: And we discussed that really in detail, didn't we? On another podcast on FODMAP. So I think if that's relevant for you or anyone, you know, I would actually, I would really recommend that because I think Will did a great job and sort of. talking through at a high level how that works and why that's relevant for people with these particular problems. 

Our team that was researching this also found one study that reported a 29% drop in hydrogen from bacteria after only two weeks on a low FODMAP diet. So that sounds like pretty promising stuff, Will.  

[00:19:55] Dr Will Bulsiewicz: Yeah, that is promising. And there was a little bit more in that study that was interesting. They actually used a probiotic called Saccharomyces boulardii. In this particular study, the evidence indicates that this probiotic Saccharomyces boulardii when used alongside a low FODMAP diet actually increases the reduction. So Jonathan you mentioned a 29% drop in bacterial hydrogen excretion and actually you can crank that up to 41% by doing both. So both low FODMAP with the probiotic together.

Now It's really important for people to understand that it needs to be the right probiotic. Probiotics are not universally and categorically the approach to healing from SIBO and if you choose the wrong one you can actually make your symptoms worse. 

Which actually brings me to my third and final myth that I want to bust Jonathan. Antimicrobials.  There are many people out there who believe in herbal antimicrobial protocols and will elevate them as if it's safer than using an antibiotic. And I say, no, I don't think that that is true at all. Antimicrobial is synonymous with antibiotic.  

[00:21:19] Jonathan Wolf: These are ways to kill bacteria inside your gut.

[00:21:23] Dr Will Bulsiewicz: Right. We're destroying the bacteria. Either way, that's what we're trying to do. The difference is that one is in supplement form and the other is a tightly regulated medication. Now, the problem is that the one that's in supplement form, not only is it less regulated, but there's less research to demonstrate the efficacy or the risks. 

And you know, when it comes to being evidence-based and also the way that I approach these issues clinically as a medical doctor is I'm thinking about risk versus benefit. I want to know that the benefit outweighs the risk. How can I possibly do that if I don't know what the benefit or the risk is in this particular case? 

So from my perspective, I would rather go with the thing that I actually know what I'm getting myself into than the thing that sounds attractive, but we have no evidence to prove it.  

[00:22:15] Jonathan Wolf: So Will, having pulling all of that together, what's your verdict on SIBO?  

[00:22:20] Dr Will Bulsiewicz: Here's my takeaway. SIBO is real. I am not trying to sit here and make it sound like SIBO is not a real thing, it is. It's an overgrowth of bacteria in the small intestine. 

I want people to understand that there should be a cause for SIBO. Things like abdominal surgery, medication, altered bowel motility, the way your intestines move. We need to find what that is. We need to understand the root cause of the problem. New research is suggesting a link between SIBO and numerous other conditions in the body. But our knowledge at this point is still quite limited. 

So I'm not quite ready to sit here and say SIBO is causing this or this is causing SIBO. What I want is to see more research, more evidence emerge so that we can understand this more completely. 

Nonetheless, if you are being diagnosed with SIBO, the way that we approach this is yes, we treat with antibiotics, but no, we don't cut our fiber. Instead, we transition to a low FODMAP diet and we gently start to reintroduce FODMAPs as tolerated and we ramp up over time. 

And the approach, by the way, Jonathan, I should mention, so you gave a shout-out to our prior episode on the low FODMAP diet. Absolutely, you should listen to that episode. If this is something that you're dealing with. 
The second episode that I would recommend is actually the protocols after antibiotics that we did between you, Tim, and I. And the reason why is because the approach is the same here. When you take the antibiotic after you're done, we want to rebuild the microbiome. That's what we talked about in that episode. 

[00:24:08] Jonathan Wolf: And so I'm guessing fundamentally there's a similar story here, which is that you don't want to throw the baby out with the bath water. And this, I guess, is your, you know, your fear of ending up eating no fiber forever and having this really damaged and restricted microbiome, you know, where it should be. 
Well, we know how important that is now for our health. 

And so somehow, I think, well, if I understand rightly, you're saying like, you've got this problem, so you've got to understand how to deal with it. But in the long run, you want to be. getting onto a diet that is going to be able to support your health and rather similarly to the conversation we've had about, you know, my living with food intolerances in the past, that potentially you can make changes over time that actually can get you to a place where you are eating a lot of fiber. 

You are getting a much better microbiome with a lot more of those. you know, there's good bugs inside it. And that is sort of where ultimately you're suggesting people want to end up.

[00:25:10] Dr Will Bulsiewicz: My perspective on this comes from, I want us to be thoughtful about the way that we approach this. I don't want our approach, which is what is currently being done, is to destroy the microbiome and continue to destroy the microbiome with the hope that somehow we will be better. 

It can make you feel better for a short period of time, but it actually is not getting you away from this problem. The way that we get you away from this problem is through healing. Healing the microbiome, building it up, nurturing the good bugs, and supporting them. And when we do this, we are actually reversing dysbiosis. 

So I want people to think about this as a dysbiosis or a damaged gut microbiome. And when we reverse it, we can take it to a place where not only is SIBO not present, but we are elevating our health. We are raising the tide on our health and we get the benefit of a healthy gut microbiome throughout our entire body. 
And that's a big part of what ZOE is all about. We want to help you heal and nurture. your microbiome and take it to a better place.  

[00:26:16] Jonathan Wolf: Will, thank you so much for that. I think that was a brilliant whistle-stop tour of something that is I think new for many of us and I think as always sort of shows you, you know, some of the opportunities and challenges as there's like this new area of research and not all the answers are clear. 

Now, if anyone listening to this, has ended up being more interested in knowing more about their own gut microbiome, having heard Will so eloquently explain, you know, why it's so important for all of our health, then you may well be interested in learning more about becoming a ZOE member. 

As part of that, you get to test your own gut microbiome and understand how many of these good and bad bacteria you have. This is something I do regularly, and most importantly, get this personalized program to help you to understand how can you adjust what you're eating for you and your body in order to support a healthy microbiome and in order to help you feel better now and hopefully live for many more healthy years in the future. 

Now you can learn more about this, simply go to and you can also get 10% off your membership there. As always, the ZOE Science & Nutrition podcast is not medical advice. It's for general informational purposes only. If you have any medical concerns, please consult your doctor. I'm Jonathan Wolf.

[00:27:35] Dr Will Bulsiewicz: I’m Will Bulsiewicz

[00:23:37] Jonathan Wolf: Join us next week for another ZOE podcast.