If you've been dealing with ongoing gut symptoms and still don't have a clear answer for what's causing them, you're not alone. It's one of the most confusing places to be.

Maybe you've been to your GP and been told everything looks normal. Maybe you've been given an IBS diagnosis but that hasn't actually led anywhere useful in terms of getting better. Or maybe you've spent time in the forums and the Facebook groups and now you have a list of five different things you might have, and no idea where to start.

The problem with not knowing what you're actually dealing with is that you can't treat it properly. And that's a big reason why so many people end up trying things for years without getting the results they're looking for. You can be putting in a lot of effort, spending a lot of money, and still not move forward, simply because the starting point wasn't right.

So let's try and bring some clarity to this.

What SIBO Actually Is (And What IMO Is, And Why Neither Term Is Perfect)

SIBO stands for Small Intestinal Bacterial Overgrowth. I know that sounds clinical, but the concept is actually quite simple once you understand the basics of how the digestive system is supposed to work.

Your small intestine is where most of your nutrient absorption happens. It's meant to be a relatively quiet environment with only small amounts of bacteria present. Your large intestine, further down, is where the majority of your gut bacteria are supposed to live. That's their home.

SIBO happens when bacteria migrate into the small intestine where they don't belong. And when you eat, instead of your body being able to absorb the nutrition from your food properly, those bacteria get to it first. They ferment it, produce gas, and cause all the symptoms that come along with that. Bloating, discomfort, unpredictable bowel movements, and a long list of other things we'll get to in a moment.

Now, you may also have come across the term IMO, which stands for Intestinal Methanogen Overgrowth. This term exists because not all overgrowth in the gut involves bacteria. Some people have an overgrowth of methane-producing organisms called archaea, and because archaea are technically not bacteria, using the term SIBO for them is not quite accurate. IMO was introduced to make that distinction, and I understand why.

However, I don't love IMO as a diagnosis either, and here is why. The word "intestinal" doesn't tell you where in the intestine the overgrowth is happening. It could be in the small intestine, the large intestine, or both. And that location matters a great deal, because the treatment is completely different depending on where it is.

If the overgrowth is in the small intestine, we almost always need to treat it with targeted antimicrobials to clear it. If it's happening in the large intestine, the approach is quite different. We focus on correcting the environment, rebuilding the good bacteria, and allowing a healthy microbiome to crowd out what we don't want there in large amounts. Same organisms, very different treatment. Knowing where you're dealing with it changes everything about how you approach it.

This is why someone with SIBO, whether bacterial or archaeal, can eat a completely healthy meal and still feel terrible afterwards. It's not the food itself that's the problem. It's what's happening to that food before your body can properly use it.

The Symptoms That Might Point to SIBO

SIBO can show up in a lot of different ways, which is part of what makes it tricky to identify without testing.

The most common symptom is bloating. For a lot of people this starts within an hour or two of eating, or it builds gradually through the day so that by mid-afternoon they're uncomfortable in their clothes and just want to lie down. Some people describe looking visibly pregnant by the evening. It can be that significant.

Beyond bloating, symptoms can include excessive burping after meals, constipation, diarrhea, or alternating between the two with no clear pattern. Brain fog is a really common one, that heavy, cotton-wool feeling in your head, especially after eating. Fatigue that isn't explained by how much sleep you're getting. Food reactions that seem to keep multiplying, where the list of things you can tolerate keeps shrinking over time.

And then there are the symptoms people don't always connect to their gut. Skin breakouts that flare at the same time as digestive symptoms. Anxiety that seems to come from nowhere. Low iron or B12 levels despite eating well, because when bacteria or archaea are interfering with absorption in the small intestine, your body simply isn't getting what it needs from your food, no matter how well you eat.

If several of these resonate with you, that's worth paying attention to.

Why an IBS Diagnosis Is Just the Beginning

A lot of people who suspect they might have SIBO have already been told they have IBS. And I want to be clear that this isn't a wrong diagnosis, it's just an incomplete one.

IBS stands for Irritable Bowel Syndrome. Any time you see the word syndrome in a diagnosis, it means you're being given a label for a collection of symptoms, not an explanation for why those symptoms are happening. It's a bit like going to a doctor with a painful knee and being told you have Painful Knee Syndrome. That tells you what's happening, but nothing about what's actually wrong or what to do about it.

Up to 84% of people diagnosed with IBS have SIBO as the underlying driver of their symptoms. Many others have something else going on entirely. The IBS label isn't the destination, it's really just the beginning of the investigation. What matters is finding out what's causing it.

Other Things That Can Look Like SIBO

This is where it gets a bit more complicated, and it's also where a lot of people go wrong when they try to treat themselves based on what they've read online.

Several other conditions can produce symptoms that look almost identical to SIBO. Slow motility on its own can cause bloating and constipation that mirrors SIBO exactly. Small intestinal fungal overgrowth, or SIFO, produces very similar symptoms to SIBO but needs a completely different treatment approach. Treating for SIBO when you actually have SIFO will not get you better.

Histamine intolerance is another one that gets missed frequently. It can cause bloating, skin reactions, headaches, and anxiety, and it doesn't show up on standard testing, which is part of why it's so often overlooked. Visceral hypersensitivity is a condition where the gut's pain signaling becomes heightened, so even normal amounts of gas feel very uncomfortable. And a microbiome imbalance in the large intestine can produce a whole range of overlapping symptoms too.

The reason this matters so much is that if you launch into a SIBO protocol when what you actually have is one of these other things, you're not going to get better. And in some cases, treating aggressively for the wrong thing can make things harder to sort out afterwards. This is why identifying what's actually going on before you start treating is so important.

So How Do You Actually Find Out?

The main tool for diagnosing SIBO is a breath test. It's non-invasive and can be done at home with a kit that gets sent to a lab.

The way it works is that you drink a sugar solution and then breathe into a collection device at set intervals over a couple of hours. The bacteria or archaea in your small intestine, if they're present, will ferment that sugar and produce gases that make their way into your breath. The two main gases measured are hydrogen and methane.

Hydrogen SIBO, which involves bacterial overgrowth, is typically associated with diarrhea-predominant symptoms. The diagnostic threshold I work with is a rise of 20 parts per million above the lowest preceding value within the first 90 to 120 minutes of the test.

Methane, which is produced by archaea rather than bacteria and is what some practitioners now refer to as IMO, is more commonly associated with constipation and slower gut motility. The threshold here is a rise of 12 parts per million above the lowest preceding value within that same 90 to 120 minute window. Methane-producing organisms tend to need a lower threshold to be clinically significant, which is why the criteria differs.

That 90 to 120 minute window is important because it corresponds to the time it takes for the sugar solution to travel through the small intestine. Any fermentation happening after that point is more likely to be occurring in the large intestine, where some fermentation is completely normal. So what happens in that early window is what tells us whether there's an overgrowth where it shouldn't be.

A positive result tells you overgrowth is present and gives you important information about what type you're dealing with and how to approach treatment. But a negative result doesn't always mean SIBO isn't there. The preparation before the test matters a lot, and if it isn't followed correctly the results won't be reliable. Interpreting results in context matters too. A number on a page doesn't tell the whole story on its own, and results always need to be read alongside symptoms and the full clinical picture.

This is one of the reasons that ordering a test yourself, without someone who can properly interpret the results and guide next steps, can sometimes leave you more confused than before you started.

If you already have results but aren't sure what they mean or what to do next, we offer professional interpretation sessions for $50. We'll go through your test, identify the type and pattern of overgrowth, and give you clear guidance on what your next steps should be: Order a test interpretation here

What to Do If You Suspect SIBO

If you're reading this and a lot of it sounds familiar, the most important thing I can say is this: please don't go and start a random protocol you found on a blog or in a Facebook group.

I know it's tempting. When you're not feeling well and you want answers, the instinct is to do something, anything, to try and get better. But treating the wrong thing, or treating the right thing in the wrong way, can make things significantly harder to resolve later. I see this regularly with clients who come to us having done multiple rounds of antimicrobials that weren't right for their situation, and their system is more depleted and more complicated to work with as a result.

What actually helps is working with someone who specializes in this specifically. Not a generalist, but someone who works with these cases day in and day out, because the nuance matters. The difference between bacterial and archaeal overgrowth, between small intestine and large intestine involvement, between SIBO and SIFO, between a motility issue and a microbiome issue - these distinctions change everything about how you approach treatment.

If you'd like to talk through what's been going on for you before deciding anything, that's exactly what our Discovery Calls are for. We'll go through your symptoms, what you've already tried, and what might actually be driving things. No treatment advice on the call, and no pressure. Just a conversation to figure out whether we're the right people to help, and what that would look like.

Book a free Discovery Call here

Lots of love, Kirsten

& The Gut Clinic Team