SIBO is a common diagnosis that medicine still struggles to define
The colon teems with microbes, but the small intestine normally hosts far fewer. When too many settle there, the result is called small intestinal bacterial overgrowth. Yet as of 2020 its definition remained imprecise, its main tests were disputed, and one sampling method repeated its own result as little as 38 percent of the time.
The condition is blamed for a familiar list of complaints: bloating, nausea, wind, pain, diarrhoea or constipation, fatigue and, in more severe cases, fatty stools. Many stem from malabsorption, as bacteria either consume nutrients first or inflame the gut lining. Long-running cases can produce an unusual blood pattern of high folate with low vitamin B12, and anaemia of two kinds depending on where absorption fails: iron is taken up near the top of the small bowel, B12 at its far end in the ileum.
Risk factors fall into four groups. The gut may move too sluggishly, often through loss of the migrating motor complex, a cleansing wave of contractions; anatomy may trap contents, as in pouches called diverticula or the blind loop left after some stomach operations; immune or digestive defences such as acid, enzymes and bile may weaken, and the use of acid-suppressing proton pump inhibitors carries a higher risk; or colonic bacteria may flood upward, for example after surgery removes the valve between the small and large bowel.
Diagnosis is where the arguments begin. The reference standard is culturing fluid drawn from the jejunum, with more than 100,000 bacteria per millilitre pointing to overgrowth, but overgrowth can be patchy and results inconsistent. Breath tests, which track hydrogen or methane after a sugar drink, are more practical but indirect. Methane is often produced by an archaeon, Methanobrevibacter smithii, rather than a bacterium, and not necessarily in the small bowel, prompting a proposal to rename that pattern intestinal methanogen overgrowth.
Some studies reported that up to 80 percent of people with irritable bowel syndrome had SIBO by breath testing, and that their symptoms eased after treatment. Other researchers counter that abnormal breath results in IBS reflect unusual bacterial distribution rather than true overgrowth. Treatment in the literature involves antibiotics, sometimes cycled, or an elemental diet, occasionally followed by drugs that stimulate gut movement. Better ways of sampling and counting gut microbes are expected to settle much of the debate.
Source: Small intestinal bacterial overgrowth
