What's actually behind chronic bloating, and why is "IBS" usually not the real answer?

What is IBS, and what does the diagnosis actually tell you?

IBS is a symptom-based diagnosis: it records that a recognized pattern of abdominal pain and altered bowel habit is present, not what is causing it. IBS is defined by the Rome IV criteria as recurrent abdominal pain, on average at least one day per week in the last three months, associated with two or more of: defecation, change in stool frequency, or change in stool form. There are four subtypes:

• IBS-D (diarrhea-predominant)
• IBS-C (constipation-predominant)
• IBS-M (mixed)
• IBS-U (unclassified)

The diagnosis describes a pattern. It doesn't explain the cause. Two patients with the same IBS subtype may have completely different underlying drivers and need different treatment approaches.

What is SIBO, and how often is it the cause of IBS?

SIBO is an overgrowth of bacteria in the small intestine, and it is meaningfully more common in people with IBS than in healthy people, though it is not the explanation for every case. SIBO stands for small intestinal bacterial overgrowth. Bacteria that belong in the colon migrate to or overgrow in the small intestine, where they ferment food prematurely, producing gas, bloating, distension, and disrupted absorption.

A meta-analysis of 25 case-control studies found the odds of SIBO were 3.7 times higher in IBS than in controls; by breath testing, prevalence was 35.5% in IBS versus 29.7% in controls [PMID: 31913194]. Other meta-analyses have shown similar ranges, with SIBO prevalence in IBS varying based on diagnostic methods and study populations. The relationship is significant: SIBO is meaningfully more common in IBS than in healthy controls, and treating SIBO often resolves IBS symptoms.

That said, SIBO isn't the answer in every IBS case. Some studies have questioned how strongly SIBO drives IBS symptoms specifically, with the relationship more complex than initially thought. The clinical takeaway: SIBO testing is worth doing in IBS workups, and treating SIBO when present often helps, but SIBO isn't the sole explanation for all IBS.

What's the difference between hydrogen and methane SIBO?

The difference is the gas the organisms produce and the symptoms that follow: hydrogen-dominant overgrowth tends toward diarrhea and urgency, while methane-dominant overgrowth tends toward constipation and slow transit. SIBO has subtypes based on which gases the bacteria produce:

• Hydrogen-dominant SIBO. Often presents with diarrhea-predominant symptoms, urgency, and bloating that worsens with carbohydrates and fermentable foods
• Methane-dominant SIBO (more accurately called intestinal methanogen overgrowth, or IMO, since methanogens are archaea, not bacteria). Often presents with constipation-predominant symptoms, slower transit, and bloating that builds through the day. Methane positivity is significantly more common in IBS-C than IBS-D [PMID: 34190027]
• Hydrogen sulfide SIBO. A less common form with distinctive symptoms (rotten egg gas smell, often associated with diarrhea)

Different subtypes have different treatment approaches. Identifying the subtype affects which antimicrobials are most likely to work.

How is SIBO actually tested and diagnosed?

The standard test is a breath test (lactulose or glucose), where the patient consumes a test substrate and breath samples are collected over 2-3 hours. Bacteria fermenting the substrate produce hydrogen, methane, or both, which appear in the breath.

Different breath test substrates have different sensitivity profiles:

• Lactulose breath test is the more commonly used in clinical practice but can have false positives
• Glucose breath test has higher specificity but may miss distal small intestine overgrowth

Breath testing isn't perfect. It's the practical option (small intestinal aspiration, the more direct test, requires endoscopy and culture). The clinical workup combines breath test results with symptom patterns, dietary triggers, and response to treatment.

What is low stomach acid, and why does it cause bloating?

Low stomach acid is too little acid to digest protein and sterilize food, and it causes bloating because undigested protein and surviving bacteria ferment further down the gut. Low stomach acid (hypochlorhydria) is a counterintuitive but common cause of bloating and reflux. Without adequate acid:

• Protein digestion is impaired
• The lower esophageal sphincter doesn't close properly (paradoxically causing reflux because pressure backs up rather than because acid is excessive)
• Bacteria aren't killed before reaching the small intestine (contributing to SIBO)
• Minerals like iron, calcium, magnesium, and B12 aren't absorbed efficiently

Low stomach acid is common in aging, in chronic stress, and in patients on long-term proton pump inhibitors (PPIs, the acid-blocking drugs like omeprazole). Many patients diagnosed with reflux and put on long-term PPIs actually have low acid that's being further suppressed.

What does the bloating timeline tell you?

The timing of bloating points to where in the gut the problem sits: early bloating suggests the stomach, a slower build suggests the small intestine, and bloating that worsens across the day suggests slow motility. The pattern and timing of bloating carries diagnostic information:

• Bloating within 30 minutes of eating often suggests upper GI involvement (low stomach acid, food sensitivities, gastroparesis)
• Bloating that builds over 1 to 3 hours often suggests small intestinal involvement (SIBO, IMO, fermentable carbohydrate intolerance)
• Bloating that worsens through the day often suggests motility issues or methane-dominant SIBO

These patterns aren't diagnostic by themselves but they help focus the investigation.

What's the right way to use a low-FODMAP diet?

The right way to use a low-FODMAP diet is as a short diagnostic elimination followed by structured reintroduction, not as a permanent way of eating. The low-FODMAP diet (which restricts fermentable carbohydrates, the FODMAPs in dairy, certain fruits and vegetables, wheat, and others) is a useful diagnostic tool. It identifies which fermentable foods drive symptoms in a given patient.

The right use: temporary elimination for 2-6 weeks to reduce symptoms, then systematic reintroduction of food groups one at a time to identify specific triggers. The goal is to identify the underlying SIBO or microbiome dysfunction so that food tolerance can be restored, not to stay on the diet indefinitely.

Indefinite restriction of FODMAPs deprives the microbiome of beneficial fibers and worsens dysbiosis over time. The diet is a tool, not a destination.

What treatments actually work for SIBO?

The treatments with evidence are targeted antimicrobials matched to the gas type, prokinetic support to keep the small intestine clearing between meals, and correction of whatever allowed the overgrowth in the first place. SIBO treatment typically involves:

• Targeted antimicrobials. Rifaximin is the most studied for hydrogen-dominant SIBO. For methane-dominant SIBO/IMO, rifaximin combined with neomycin or metronidazole has more evidence. Herbal antimicrobial protocols have comparable evidence to rifaximin in some studies
• Prokinetic agents. Drugs or supplements that support the migrating motor complex (the gut's "housekeeping" wave that clears the small intestine between meals). Helps prevent recurrence
• Addressing root causes. Low stomach acid, structural issues (adhesions, anatomical variants), motility disorders, vagal nerve dysfunction, chronic stress

SIBO has a high recurrence rate (20-40% within 9-12 months), which is why addressing root causes alongside the antimicrobial treatment matters.

The deeper picture

IBS isn't an explanation; it's a description of a pattern. The actual driver is usually identifiable, often through a combination of breath testing, microbiome assessment, and clinical pattern recognition. Once identified, IBS-pattern symptoms are typically responsive to targeted treatment. Extend approaches gut symptoms with this level of investigation rather than chronic symptom management.