01 · IBSWhat is IBS?
Irritable Bowel Syndrome (IBS) is a functional digestive disorder. This means there are real and significant symptoms, but conventional tests (colonoscopy, blood tests, ultrasound) show no structural or biochemical abnormalities.
It affects approximately 10-15% of the world's population and is more common in women. It is classified into subtypes based on the predominant stool pattern:
- IBS-D: diarrhoea-predominant.
- IBS-C: constipation-predominant.
- IBS-M: mixed (alternating between diarrhoea and constipation).
- IBS-U: unclassified.
Diagnosis is based on the Rome IV criteria, which require recurrent abdominal pain at least 1 day per week over the last 3 months, associated with two or more of the following:
- Related to defecation.
- A change in stool frequency.
- A change in stool form or appearance.
IBS is considered a diagnosis of exclusion: it is reached after ruling out other organic causes.
02 · SIBOWhat is SIBO?
SIBO (Small Intestinal Bacterial Overgrowth) is a specific diagnosis: there is a measurable overgrowth of bacteria in the small intestine, where the bacterial population should normally be low.
Unlike IBS, SIBO has an identifiable cause — excess bacteria — and can be objectively measured with a breath test or, in some cases, with aspiration and culture of small intestinal contents (considered the gold standard, though rarely used in practice).
There are three main types:
- Hydrogen-dominant SIBO: bacteria produce excess hydrogen. Mainly associated with diarrhoea.
- IMO (Intestinal Methanogen Overgrowth): methanogenic archaea produce methane. Associated with constipation.
- Hydrogen sulphide SIBO: a less well-known third type, produced by sulphate-reducing bacteria. Associated with diarrhoea and "rotten egg" symptoms.
03 · SharedShared symptoms.
This is the main reason for the confusion: IBS and SIBO share an impressive list of symptoms:
- Bloating and abdominal distension.
- Excessive gas.
- Abdominal pain and cramps.
- Diarrhoea, constipation or both.
- Early satiety.
- Urgency to use the bathroom.
- General discomfort after eating.
- Fatigue.
In the consulting room, a patient with SIBO and one with "pure" IBS can present practically identical complaints. That is why it is crucial to go beyond symptoms.
04 · DifferencesSymptoms that set them apart.
Although there is significant overlap, certain patterns can point towards one or the other:
| Feature | More suggestive of IBS | More suggestive of SIBO |
|---|---|---|
| Bloating | Variable, often stress-related | Progressive throughout the day, very marked after meals |
| Symptom onset | After stress, travel or gastroenteritis | After prolonged PPI use, abdominal surgery, or post-gastroenteritis |
| Belching | Occasional | Frequent and prominent |
| Weight loss | Uncommon | Possible (due to malabsorption) |
| Nutritional deficiencies | Rare | Common (iron, B12, fat-soluble vitamins) |
| Response to antibiotics | Partial, temporary improvement with rifaximin | Significant improvement (though may recur) |
| Response to probiotics | Variable | May initially worsen |
| Brain fog | Possible | More frequent and pronounced |
| Pain | May improve or worsen with defecation | More related to food intake |
| Steatorrhoea (fatty stools) | Rare | Possible if malabsorption is present |
05 · IBS diagnosisHow IBS is diagnosed.
IBS is diagnosed through:
- Detailed clinical history: assessment of Rome IV criteria.
- Exclusion of other conditions: blood tests (full blood count, CRP, ESR, coeliac antibodies, faecal calprotectin); in those over 50 or with alarm signs, colonoscopy; thyroid function; in some cases lactose intolerance test or inflammatory bowel disease screening.
- Positive criteria: it is not just "we did not find anything", but rather the symptoms fit the Rome IV pattern.
Common problem: many patients receive an IBS diagnosis without ever having had a SIBO breath test, which can lead to years of symptomatic treatment without resolving the underlying cause.
06 · SIBO diagnosisHow SIBO is diagnosed.
Breath test
The most widely used and accessible test:
- Preparation: restrictive diet 24 hours before + 12-hour fast.
- Procedure: you drink a lactulose or glucose solution. Breath samples are collected every 15-20 minutes for 2-3 hours.
- Interpretation: hydrogen increase ≥20 ppm above baseline in the first 90 minutes — positive for hydrogen SIBO. Methane ≥10 ppm at any point — positive for IMO.
Limitations
- Considerable false negative rate (especially with glucose, which only evaluates the proximal small intestine).
- The lactulose test can give false positives if lactulose reaches the colon earlier than expected.
- Does not detect hydrogen sulphide SIBO well.
Duodenal/jejunal aspirate
- Theoretical gold standard: a direct sample from the small intestine is taken and bacteria are cultured.
- In practice, rarely used because it is invasive, expensive and not available in many centres.
07 · ConnectionThe IBS-SIBO connection.
The relationship between IBS and SIBO is close and bidirectional:
- SIBO as a cause of IBS: studies suggest that 60-84% of IBS patients have a positive SIBO test (figures vary depending on the diagnostic method and study population).
- Infectious gastroenteritis → post-infectious IBS → SIBO: a gastroenteritis episode (food poisoning) can damage the interstitial cells of Cajal and enteric plexus neurons through an autoimmune mechanism (anti-vinculin and anti-CdtB antibodies), permanently altering the MMC.
- Pimentel's model: Dr Mark Pimentel (Cedars-Sinai, Los Angeles) has proposed that many cases of IBS are, in fact, undiagnosed SIBO, and that gastroenteritis is the most common trigger.
What if I have IBS and my SIBO test is negative?
Several possibilities:
- The test may be a false negative.
- There may be undetected hydrogen sulphide SIBO.
- It may be IBS without SIBO, mediated by other mechanisms (visceral hypersensitivity, colonic dysbiosis, gut-brain axis, etc.).
- There could be other causes: SIFO (fungal overgrowth), histamine intolerance, exocrine pancreatic insufficiency, etc.
08 · TreatmentIBS treatment vs SIBO treatment.
| Aspect | IBS treatment | SIBO treatment |
|---|---|---|
| Diet | Low FODMAP (very effective) | Low FODMAP + possible elemental or Bi-Phasic diet |
| Primary medication | Antispasmodics, laxatives/antidiarrhoeals, low-dose tricyclic antidepressants | Rifaximin +/- neomycin/metronidazole, or herbal antimicrobials |
| Supplements | Probiotics, soluble fibre, peppermint oil | Herbal antimicrobials, prokinetics, digestive enzymes |
| Prokinetics | Not always needed | Essential for preventing relapse |
| Psychological approach | CBT, gut-directed hypnotherapy | Useful as a complement |
| Goal | Symptom control | Eradicate overgrowth + prevent recurrence |
| Treatment duration | Long-term, chronic management | Treatment cycles + maintenance |
Key point: IBS treatment tends to be symptomatic and long-term, while SIBO treatment seeks to resolve an identifiable cause and prevent it from returning.
09 · BothCan you have both?
Yes, and in fact it is the most common scenario. Many patients have:
- SIBO as the primary cause — treating it resolves or significantly improves IBS symptoms.
- IBS with SIBO layered on top — SIBO worsens underlying IBS. Treating SIBO significantly improves symptoms, but some degree of visceral hypersensitivity or gut-brain axis dysregulation may persist.
- IBS without SIBO — symptoms are real but there is no bacterial overgrowth. The approach is dietary, pharmacological and psychological.
The good news is that, regardless of the exact diagnosis, dietary strategies like the Low FODMAP diet work well in both scenarios.
10 · QuestionsQuestions to ask your doctor.
If you suspect you may have SIBO in addition to (or instead of) IBS, these questions can guide the conversation with your healthcare professional:
- "Can we rule out SIBO with a breath test?"
- "Could my symptoms be explained by bacterial overgrowth?"
- "Have nutritional deficiencies (iron, B12, vitamin D) been evaluated?"
- "Would it make sense to try treatment with rifaximin or herbal antimicrobials?"
- "Should I see a dietitian specialising in FODMAP?"
- "Is there an underlying cause we should investigate (motility, hypothyroidism, adhesions)?"
- "Should we measure anti-vinculin and anti-CdtB antibodies?" (IBS Smart test, available in some centres)
Disclaimer: this guide is informational and does not replace assessment by a healthcare professional. If you suspect you may have SIBO, consult your doctor for appropriate diagnostic testing.