Small intestinal bacterial overgrowth (SIBO)
A condition in which too many bacteria grow in the small intestine, where numbers are normally low. The bacteria ferment food and interfere with absorption, causing bloating, wind, discomfort and sometimes diarrhoea and nutrient deficiencies. It is treated mainly with antibiotics and by tackling the cause.
What it is
The small intestine normally contains relatively few bacteria, most of the gut's bacteria living in the large bowel. In small intestinal bacterial overgrowth (SIBO), excessive numbers of bacteria colonise the small intestine, where they ferment carbohydrates before the body has absorbed them and interfere with normal digestion. This produces gas and can damage the absorptive surface, leading to bloating, excessive wind, abdominal discomfort, and sometimes diarrhoea, greasy stools and weight loss. Over time it can cause specific nutrient problems: the bacteria consume vitamin B12 (causing a deficiency) while sometimes producing extra folate, and fat malabsorption can lower fat-soluble vitamins. SIBO usually develops because something has disturbed the mechanisms that normally keep the small bowel relatively sterile — slowed gut movement, structural changes after surgery, adhesions or pouches, or reduced stomach acid. It commonly overlaps with, and can be mistaken for, irritable bowel syndrome. It is typically diagnosed with a breath test (measuring gases produced by the bacteria after a sugar drink) and treated with a course of antibiotics, along with addressing the underlying cause and any nutritional deficiencies. Recurrence is common if the underlying predisposition remains.
Key lab markers
- Vitamin B12 — often low, because the bacteria consume it.
- Folate — sometimes high, as some bacteria produce it (a suggestive pattern with low B12).
- Iron and ferritin, fat-soluble vitamins — may be low from malabsorption.
- Breath test (hydrogen/methane) — the usual diagnostic test after a sugar drink.
- Tests for an underlying cause — such as slowed gut motility or prior surgery.
Symptoms
- Bloating, often worse after meals
- Excessive wind (flatulence)
- Abdominal discomfort or pain
- Diarrhoea, sometimes with greasy stools
- A feeling of fullness
- Fatigue and, over time, features of nutrient deficiency
- Unintended weight loss in more severe cases
Related lab panels
When to discuss with a doctor
SIBO is worth considering when bloating, wind and abdominal discomfort are persistent — particularly if there is diarrhoea, greasy stools, unexplained weight loss, or a vitamin B12 deficiency, and especially in someone with a reason for it such as previous bowel surgery or a condition that slows gut movement. Because its symptoms overlap heavily with irritable bowel syndrome, it is easy to miss, and a low B12 (sometimes with a high folate) can be a useful clue. It is diagnosed with a breath test and treated with antibiotics, alongside correcting deficiencies and, importantly, addressing the underlying cause to reduce recurrence. See a doctor for persistent symptoms, and seek assessment promptly if there is weight loss or signs of malnutrition. Mediora.AI can highlight the B12/folate and malabsorption pattern that raises suspicion of SIBO; the diagnosis and treatment belong with your doctor.