IBS vs SIBO: How to Tell the Difference (And Why Treatment Differs Entirely)
- Aug 11
- 6 min read
IBS and SIBO look almost identical. Both cause bloating, abdominal pain, and unpredictable digestion. Millions of people manage one when they actually have the other — and that distinction matters, because treating them the same way rarely works.
Here is how to tell them apart, how each is diagnosed, and why the treatment path is completely different.
What Is IBS?
IBS (Irritable Bowel Syndrome) is a functional digestive disorder — meaning the structure of the gut appears normal, but how it functions is disrupted. It affects an estimated 10–15% of adults worldwide and is one of the most common diagnoses in gastroenterology.
IBS is diagnosed using the Rome IV criteria: recurrent abdominal pain at least one day per week over the past three months, associated with a change in stool frequency or form. Crucially, IBS is a diagnosis of exclusion — it is confirmed after conditions such as coeliac disease, inflammatory bowel disease, and infection are ruled out.
There is no single test for IBS. Its causes are multifactorial: gut-brain axis dysfunction, visceral hypersensitivity (where normal gut sensations feel painful), altered gut motility, microbiome disruption, and in some cases a history of gut infection or significant stress.
What Is SIBO?
SIBO (Small Intestinal Bacterial Overgrowth) is a measurable condition — not a symptom cluster. It occurs when bacteria that belong in the large intestine migrate into and colonise the small intestine in excessive numbers.
The small intestine is meant to have relatively few bacteria. When that changes, those bacteria ferment carbohydrates before they can be properly absorbed, producing gas and disrupting nutrient uptake — which is why SIBO often causes malabsorption alongside digestive symptoms.
SIBO is diagnosed with a breath test measuring hydrogen and methane gas. It presents in three main types:
Hydrogen-dominant SIBO — associated with diarrhoea
Methane-dominant SIBO (also called Intestinal Methanogen Overgrowth, IMO) — associated with constipation
Hydrogen sulphide-dominant SIBO — associated with loose stools and a sulphurous odour
The Symptoms Overlap — But There Are Clues
Both conditions share a core symptom set: bloating, abdominal pain or cramping, gas, and altered bowel habits. But certain patterns point more strongly toward SIBO:
Bloating that appears within 30–90 minutes of eating — the bacteria ferment food rapidly in the small intestine
Symptoms that worsen with "healthy" foods — high-fibre foods, prebiotics, onion, garlic, legumes, and fermented foods feed the bacteria and amplify symptoms
Brain fog and fatigue — particularly in methane-dominant SIBO, where gases enter systemic circulation
Nutritional deficiencies — especially B12, iron, and fat-soluble vitamins (A, D, E, K), due to malabsorption in the small intestine
Symptoms that worsen after taking probiotics — adding bacteria to an already-overgrown environment can intensify fermentation
IBS, by contrast, is more closely linked to stress, emotional state, and nervous system regulation. Symptoms fluctuate significantly with psychological factors and tend not to cause nutritional deficiencies.
The Critical Connection: SIBO May Drive Many IBS Cases
Research suggests that between 30% and 85% of people diagnosed with IBS test positive for SIBO when formally assessed with a breath test. This does not mean all IBS is SIBO — but it means a significant proportion of people managing an IBS diagnosis may have an underlying bacterial overgrowth driving their symptoms.
This overlap has significant clinical consequences. Many standard IBS management approaches — increasing dietary fibre, taking prebiotic supplements, adding fermented foods — actively worsen SIBO by feeding the bacteria. If SIBO is the underlying issue, treating it as IBS will not produce lasting improvement.
How Each Is Tested
IBS: There is no definitive diagnostic test. Diagnosis is clinical, based on the Rome IV symptom criteria, after blood tests (to rule out coeliac disease, thyroid dysfunction, and inflammatory markers), stool tests, and in some cases colonoscopy have excluded other conditions.
SIBO: A hydrogen and methane breath test is the standard diagnostic method. You fast for 12–24 hours, then drink a lactulose or glucose solution. You exhale into tubes at timed intervals over 2–3 hours. A rise in hydrogen gas indicates bacterial fermentation in the small intestine; a rise in methane indicates methane-producing archaea (IMO). Results are interpreted alongside symptoms to reach a diagnosis.
Treatment — Why They Are Completely Different
This is where the distinction matters most.
IBS Treatment Focuses On:
Low FODMAP dietary protocol — structured elimination and reintroduction of fermentable carbohydrates
Gut-brain axis interventions — stress management, nervous system regulation, gut-directed hypnotherapy
Specific probiotic strains — Lactobacillus and Bifidobacterium species chosen for the symptom pattern
Soluble fibre and motility support — psyllium husk, magnesium
Addressing lifestyle triggers — sleep quality, stress load, food sensitivities
SIBO Treatment Requires a Staged Protocol:
Antimicrobial phase — clearing the overgrowth with pharmaceutical antibiotics (rifaximin) or herbal antimicrobials (berberine, allicin, oregano oil, neem). Herbal protocols typically run 4–8 weeks.
Dietary support during treatment — a low-fermentation or low-FODMAP diet to reduce bacterial food supply and ease symptoms
Prokinetics after treatment — the migrating motor complex (MMC), the gut's natural housekeeping wave, is often impaired in SIBO. Prokinetics help prevent recurrence.
Microbiome rebuild — carefully reintroducing probiotics and prebiotic foods after the overgrowth is cleared
Root cause resolution — addressing what allowed SIBO to develop: low stomach acid, impaired motility, history of antibiotic use, or underlying conditions
Important: adding probiotics or increasing fibre intake before clearing SIBO can worsen symptoms by feeding the bacteria already present.
When to Work with a Specialist
If you have been managing digestive symptoms for more than 3–6 months without lasting improvement — or if your symptoms consistently worsen with fibre, prebiotics, or fermented foods — it is worth a formal SIBO assessment alongside a root-cause evaluation.
A functional nutritionist can coordinate breath testing, interpret results in the context of your full symptom picture, and build a protocol that addresses root causes rather than symptoms alone. Whether the underlying issue is IBS, SIBO, or both, the approach should be specific to your case.
Frequently Asked Questions
What is the main difference between IBS and SIBO?
IBS is a functional disorder diagnosed by symptoms; SIBO is a measurable condition confirmed by breath test, where bacteria overgrow in the small intestine. They share similar symptoms but have different root causes and require different treatments.
Can you have both IBS and SIBO at the same time?
Yes. Research suggests SIBO may underlie 30–85% of IBS cases. Many people carry an IBS diagnosis while SIBO is the actual driver of their symptoms. Treating the SIBO often significantly improves or resolves the digestive symptoms previously attributed to IBS.
How is SIBO diagnosed?
With a hydrogen and methane breath test. After a fast, you drink a lactulose or glucose solution and exhale into tubes at timed intervals over 2–3 hours. A rise in hydrogen or methane gas indicates bacteria fermenting the sugar in the small intestine.
How is IBS diagnosed?
IBS is diagnosed using the Rome IV criteria — recurrent abdominal pain at least once a week for the past three months, associated with changes in bowel frequency or stool form — after conditions like coeliac disease, inflammatory bowel disease, and infection have been excluded.
Is the low FODMAP diet good for both IBS and SIBO?
Low-FODMAP reduces symptoms in both conditions by limiting fermentable carbohydrates. However, for SIBO it should be paired with an antimicrobial protocol — dietary changes alone do not clear the bacterial overgrowth, and symptoms typically return when FODMAPs are reintroduced.
Does SIBO cause brain fog?
Yes. Methane and hydrogen gas produced by bacteria in the small intestine can enter systemic circulation and affect cognitive function. Brain fog, fatigue, and mood changes are common in SIBO — particularly in methane-dominant (IMO) cases.
What foods make SIBO worse?
High-FODMAP foods (onion, garlic, legumes, wheat), prebiotic fibres (inulin, FOS), and fermented foods can feed small intestinal bacteria and worsen symptoms during active SIBO. Paradoxically, eating high-fibre while having SIBO often amplifies bloating.
When should I see a functional nutritionist about IBS or SIBO?
If you have had digestive symptoms for more than 3–6 months, if symptoms worsen with healthy foods like fibre, prebiotics, or fermented foods, or if you have had an IBS diagnosis for years without lasting improvement — a root-cause assessment including SIBO testing is worth pursuing.
Rita Soares is a functional nutritionist (Ordem dos Nutricionistas 2604N) and IFM Certified Practitioner based in Lisbon. She specialises in gut health, SIBO, hormones, and longevity, working with clients online and in Lisbon through a root-cause approach to complex digestive and hormonal conditions.
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