When colonic bacteria overgrow the small intestine
The small intestine normally holds far fewer bacteria than the colon, kept low by stomach acid, bile, digestive enzymes, fast transit, and a sweeping motility wave between meals called the migrating motor complex. In small intestinal bacterial overgrowth (SIBO), these defenses falter and bacteria more typical of the colon proliferate higher up.
Because the bacteria arrive before nutrients are fully absorbed, they ferment carbohydrates prematurely, producing gases such as hydrogen and, via archaea, methane.
Symptoms, causes, and diagnostic caveats
Premature fermentation can drive bloating, distension, gas, and altered bowel habits, and can impair absorption of some nutrients. Risk rises with conditions that slow motility or alter anatomy, including prior surgery, diabetes, and structural abnormalities.
SIBO is commonly diagnosed with breath testing, which is imperfect and prone to false results, and the topic is debated among specialists. This is general educational information, not medical advice; suspected SIBO should be assessed by a qualified clinician rather than self-treated.