Why SIBO Breath Tests Are Fundamentally Unreliable: And How to Actually Identify What's Happening in Your Gut — Part 1: Why SIBO Breath Tests Fail—The Mechanistic Reality

What the Breath Test Actually Measures A SIBO breath test measures one thing: the concentration of hydrogen and methane gases you exhale over 3 hours after drinking a substrate solution, usually lactulose or glucose. What it&#8217;s supposed to tell you is whether you have bacterial overgrowth in your small intestine. What it actually tells you is whether measurable quantities of hydrogen and methane appeared in your breath during those three hours, under those specific conditions. These are not the same thing. The distance between them is where thousands of people fall through the cracks. The First Problem: Bacterial Diversity and Gas Production Heterogeneity Most SIBO explanations skip a critical point: not all bacteria produce hydrogen or methane in equal amounts. Many don&#8217;t produce these gases at all under the substrate conditions used in the test. The small intestine can be colonized by hundreds of bacterial species. The classical SIBO narrative focuses on Methanobrevibacter smithii, which produces methane, and various hydrogen-producing bacteria like Klebsiella and Clostridia. But this oversimplifies reality. The actual microbial communities that overgrow in a compromised small intestine are far more diverse and metabolically heterogeneous than the simple hydrogen plus methane model suggests. When you ingest the test substrate, whether lactulose or glucose, the test