What Is SIBO? (Small Intestinal Bacterial Overgrowth)
SIBO stands for small intestinal bacterial overgrowth, and in my online nutrition practice it's one of the most common things I find behind stubborn IBS symptoms in Australian clients.
Here's the thing that surprises most people: the bacteria involved aren't foreign invaders. They're the same bacteria that belong in your large intestine. The problem is that they've set up in the wrong place. Your small intestine should hold only about 100,000 bacteria per quarter-teaspoon of content. Your large intestine holds as many as 20 billion in the same amount — and needs them, to finish off digestion.
So it isn't the bacteria that are the problem. It's the postcode.
If you've been told you have IBS, cut out food after food, and still can't work out why some days are fine and others leave you unable to do up your jeans — this page is for you. It isn't that you've done something wrong. It's that nobody has looked at what's actually driving it.
What's the problem with these bacteria being in the small intestine?
Bacteria in the wrong place compete with you for your own food. They get to the carbohydrates and sugars first, ferment them, and take the nutrients you were meant to absorb.
That fermentation produces gas — hydrogen and methane. Your body doesn't make these gases; only the bacteria do. That gas is what causes the bloating, the burping, the wind and the pain. Which gas is dominant also shapes your symptoms: it's part of why some people swing towards constipation and others towards diarrhoea.
There's a second problem underneath the first. Over time, this activity damages the microvilli — the tiny finger-like projections lining your small intestine that do the actual work of absorbing nutrients. That damage contributes to increased intestinal permeability, better known as leaky gut.
A gut working properly acts as a filter: nutrients in, toxins out. When it becomes leaky, larger undigested food particles and bacterial by-products cross into the bloodstream. Depending on where you're individually susceptible, that can show up nowhere near your gut — joint pain, headaches, skin flare-ups, brain fog.
The third piece is motility. Your migrating motor complex, or MMC, is your gut's housekeeper: a wave of muscular contractions that sweeps through the small intestine between meals, clearing out debris and bacteria. SIBO impairs the MMC — and an impaired MMC allows more overgrowth. Stress and eating too frequently interrupt that same cleaning wave, which is why constant grazing so often makes symptoms worse.
Types of SIBO: hydrogen, methane (IMO) and hydrogen sulfide
Not all SIBO is the same, and the gas being produced matters — for your symptoms, and for what to do about it.
Hydrogen-dominant SIBO is the most common type. Hydrogen is produced by bacteria fermenting carbohydrates in the small intestine, and it tends to be associated with diarrhoea-predominant symptoms.
Methane — or more accurately IMO, intestinal methanogen overgrowth. This one is worth getting right. Methane isn't produced by bacteria at all, but by single-celled organisms called archaea, specifically Methanobrevibacter smithii. That's why the condition was renamed. Methane slows gut motility, which is why IMO tends to bring constipation and the kind of bloating that is slow to shift.
Hydrogen sulfide (ISO, intestinal sulfide overproduction) is the third type, now recognised alongside the other two. It's produced by sulfur-metabolising bacteria such as Bilophila wadsworthia and Desulfovibrio species, and can contribute to diarrhoea, urgency, and a characteristic rotten-egg smell.
The hydrogen sulfide breath test isn't yet available in Australia. When I suspect ISO in a client, I use a comprehensive stool test to look for overgrowth in those sulfur-metabolising strains instead. If your hydrogen and methane results come back normal but you're still symptomatic, this is usually the next piece to investigate.
Plenty of people have a combination of gas types. That's exactly why testing beats guessing.
How these gases cause your symptoms
The bloating, the distension, the cramping — that's fermentation happening somewhere it shouldn't. The gas is the direct cause of most day-to-day SIBO symptoms.
But the reason SIBO reaches so far beyond the gut comes down to three knock-on effects:
Malabsorption. Once the microvilli are damaged, you absorb nutrients less efficiently. This is why people with longstanding SIBO often turn out to be low in B12, iron, and the fat-soluble vitamins A, D, E and K — deficiencies that look out of proportion to how well they're eating.
A stalled housekeeper. The MMC disruption described above becomes self-perpetuating: overgrowth impairs motility, and impaired motility permits more overgrowth. Breaking that loop is a large part of the work.
Systemic effects. With a more permeable gut barrier, undigested food particles and bacterial by-products reach the bloodstream. Where that lands depends on your individual susceptibilities — joint pain, skin flares, brain fog, headaches, fatigue.
Symptoms of SIBO
The digestive symptoms most people notice first:
- Bloating and abdominal distension
- Excess wind, flatulence or burping
- Cramping and abdominal pain
- Constipation, diarrhoea, or alternating between the two
- Symptoms that get worse with fibre
- Indigestion and nausea
- Fatigue
Because SIBO affects how well you absorb nutrients, the effects rarely stay in your digestive system. Many of my clients also have at least one of these:
- Reflux or GERD
- Headaches or migraines
- Fat malabsorption and nutritional deficiencies
- Food intolerances that keep multiplying
- Leaky gut
- Low mood or mood swings
- Hormonal imbalance
- Joint pain
- Skin issues — eczema, breakouts, rosacea
- Brain fog and difficulty concentrating
Two patterns are worth watching for. Symptoms tend to worsen after sugary and starchy foods — bread, pasta, potatoes. And the longer SIBO has been present, the more foods tend to become a problem, with gluten, dairy and fructose usually joining the list.
If you'd like the full symptom picture, I've written about it in more detail in 10 warning signs of SIBO
How common is SIBO?
More common than most people realise — and considerably more common than it's diagnosed.
Estimates vary widely between studies depending on the testing method used, with some putting SIBO in as many as 80% of IBS cases. Its prevalence among people without gut symptoms isn't well established.
What I see in practice is the underdiagnosis. People spend years managing symptoms without anyone investigating what's underneath them. As an online clinical nutritionist working across Australia, I see clients from Sydney, Canberra, Brisbane, Perth and everywhere in between who have simply never been offered a look at the cause.
What causes SIBO?
There's rarely a single cause. For most people it's a combination of factors that have gradually reduced the gut's ability to keep bacteria where they belong — and working out which ones apply to you is what stops it coming back.
The common contributors:
- A gut infection — food poisoning or gastroenteritis is one of the most common triggering events, and people often date the start of their symptoms to it. Campylobacter jejuni in particular can set off an autoimmune response that damages the MMC.
- Chronic stress — lowers stomach acid and impairs the MMC, both of which let bacteria overgrow.
- Low stomach acid (hypochlorhydria) — including from pylori infection, or prolonged use of proton pump inhibitors and antacids.
- Abdominal surgery — appendectomy, hysterectomy, or any surgery that leaves adhesions can alter gut motility and structure.
- Hypothyroidism and poor blood sugar regulation — both slow gut transit.
- Other gut conditions — coeliac disease (especially where a gluten-free diet hasn't fully resolved things), Crohn's disease and ulcerative colitis all raise the risk.
- Ileocecal valve dysfunction — this valve sits between the small and large intestine; when it isn't closing properly, bacteria can migrate upward.
- Certain medications — antibiotics, narcotics, and the oral contraceptive pill.
- Gastroparesis — delayed stomach emptying creates ideal conditions for overgrowth.
- Structural issues — small bowel diverticulosis, adhesions, obstructions, or previous gastric bypass surgery.
- Diet — heavily processed food, refined flours and excess sugar feed the overgrowth.
- Alcohol — even moderate intake (one drink a day for women, two for men) has been linked with SIBO.
- Herbicide exposure — glyphosate has antimicrobial properties, so dietary exposure is worth considering for some people, though the direct human link to SIBO is still emerging.
If you're reading that list and recognising three or four, that's normal. It's usually the accumulation that tips things over, not any single item.
Is SIBO contagious?
No. There's no evidence that exposure to any single microorganism causes SIBO, and it can't be passed between people the way an infection can. It develops from the interplay of the factors above.
It's a question I'm asked often enough that I've answered it properly over on is SIBO contagious?
What foods trigger SIBO symptoms?
Food didn't cause your SIBO — but food does feed it. Give the overgrowth its preferred fuel and it grows, and your symptoms follow. Take that fuel away and you can reduce the overgrowth's food supply, which is the reasoning behind the various SIBO diets.
The usual culprits:
- High-FODMAP foods
- Sugar alcohols
- Wheat and gluten
- Lactose
- Fermentable oligosaccharides
The diets vary, and so do individual responses — which is the whole argument for working from your own test results rather than a generic list off the internet.
Is SIBO linked to other health conditions?
Yes. SIBO is associated with a number of conditions that can either contribute to it or be made worse by it:
- Irritable bowel syndrome — a significant proportion of people with IBS are found to have SIBO
- Inflammatory bowel disease — Crohn's and ulcerative colitis, where altered motility and some medications play a part
- Coeliac disease — particularly where symptoms persist despite a strict gluten-free diet
- Type 2 diabetes — where neuropathy affects the nerves controlling gut transit
- Gastroparesis — delayed stomach emptying
- Chronic pancreatitis — through malabsorption
- Scleroderma and other connective tissue disorders — through their effect on motility
- Hypochlorhydria — low stomach acid, including from acid-suppressing medication
- Previous intestinal surgery or structural differences in the gut
Having one of these doesn't mean you'll develop SIBO. It does raise the odds, and it makes investigating worthwhile. This list isn't exhaustive either.
SIBO testing
There's a simple, non-invasive breath test that can identify SIBO. Using either lactulose or glucose, it measures the hydrogen and methane in your breath across a series of samples taken over about three hours. The results show whether SIBO is present, roughly where in the small intestine it sits, and which gas is driving your symptoms. You do it at home, after a one-day preparation diet.
There's no blood test for SIBO itself, but bloods matter alongside it — they pick up the vitamin and mineral deficiencies that can follow, B12 in particular, and can check thyroid function.
Full details are on my SIBO testing page
SIBO treatment: my 7-step process
My seven-step process was built around SIBO as the most common driver of IBS, and the order genuinely matters — most people who come to me have tried the right things in the wrong sequence.
- Stop putting fuel on the fire — remove dietary triggers, using either an elimination or elemental approach
- Test where needed, to confirm SIBO, where it sits, and which gas is involved
- Use the appropriate antimicrobial herbs for your results and your situation
- Address the stress and lifestyle factors feeding into your symptoms
- Repair leaky gut, settle inflammation, and reintroduce foods
- Rebalance the microbiome and reduce the risk factors for relapse
- Work with someone who can keep the steps in the right order
You can read more about how this works in the Gut Fix Program, and about the approach itself on my SIBO treatment page
Working with a SIBO specialist
After 16 years working with gut health clients, the pattern I see most consistently is this: people who've been told "it's just IBS" have often had SIBO for years without anyone naming it. They've done the elimination diets, the probiotics, the fibre supplements — and nothing has held, because nothing has addressed what's actually driving it.
That isn't a failure of willpower. It's a plan that was never built for your body.
SIBO responds to a structured approach, and the sequence matters more than any single intervention. If what you've read here sounds like your experience — bloating that builds through the day, bowel habits you can't predict, foods that used to be fine and now aren't — it's worth investigating properly.
If you'd like to talk through your options, book a free Gut Fix strategy session and we'll go through them together. Or call 1300 16 75 72 and leave a message with reception — I will call you back.
FAQs
How do you tell if you have SIBO?
The gold-standard test is a hydrogen/methane breath test: you take a sugar solution, then provide breath samples at regular intervals over two to three hours. If bacteria in the small intestine ferment that sugar, the gases they produce show up in your breath, and the timing and pattern indicate both the presence and the type of SIBO.
Clinically, the signs that make me think of SIBO are: bloating that worsens through the day and after meals; excessive wind or belching; alternating diarrhoea and constipation; brain fog, fatigue and deficiencies out of proportion to someone's diet; and IBS symptoms that haven't budged with standard dietary management. A history of food poisoning marking the onset of gut trouble is a strong pointer too.
I can arrange breath testing as part of an assessment — see my SIBO testing page for how it works.