SIBO (small intestinal bacterial overgrowth): symptoms, diet and treatment

Guts UK describes the symptoms as vague and much like those of other digestive conditions. They include belly pain, bloating or distention, diarrhoea that may be pale, fatty or greasy (steatorrhoea), weight loss and deficiencies in vitamins or minerals such as iron, vitamin B12 or calcium. Because these features overlap with irritable bowel syndrome, coeliac disease and inflammatory bowel disease, the BNSSG guidance says common causes should be excluded first and that red flags should be checked for and acted on.

Unplanned weight loss, bleeding, persistent changes in bowel habit or anaemia are not features to put down to SIBO without an assessment. Speak to a GP promptly about them.

Most of the bacteria in the digestive tract live in the large bowel. The small bowel, which sits higher up, normally holds far fewer. Guts UK explains that after surgery or in some diseases the structure of the small bowel or the speed at which its contents move can change, and more bacteria than normal can then grow there. This is small intestinal bacterial overgrowth, or SIBO.

BNSSG ICB, a local NHS referral and formulary guide for GPs, describes the normal defences that keep bacterial counts low in the upper small bowel: stomach acid, bile and pancreatic secretions, a competent valve at the end of the small bowel and normal movement. If these fail, for example after surgery, with obstruction or a motility disorder, overgrowth can occur. The consequences it lists include fat malabsorption, competition for nutrients such as vitamin B12, gas and bloating, and in some cases malnutrition.

Guts UK lists antibiotics as the main treatment, which may be prescribed by a hospital consultant who will decide the best option, with a trial of treatment sometimes suggested if SIBO is strongly suspected. The BNSSG guidance for GPs says antibiotic treatment for SIBO is not recommended for use in primary care and often will not lead to sustained improvement in symptoms. University College London Hospitals (UCLH) says the initial treatment is a course of antibiotics, that most patients remain symptom free, but that some people require a repeat dose.

Treating the underlying problem matters as well. Guts UK suggests that people who have had pancreatic surgery ask to be tested for pancreatic exocrine insufficiency and have their enzyme prescription reviewed, and that diabetes control may need optimising.

Guts UK says that the evidence for several popular self-help and diet approaches is limited or absent. It makes these points, summarised in the table. If you are struggling with diet or your body weight is below a healthy level, it says to talk to your doctor, who could refer you to a dietitian.

Guts UK explains that the best way to diagnose SIBO is to take a sample directly from the small bowel through a flexible tube, but this method is used only in research. In practice a clinician reviews your history for previous surgeries or conditions that could cause SIBO and may use a breath test, in which you drink a sugary solution and then blow into a device that measures gas produced by bacterial fermentation.

The breath test is not a direct measure. Guts UK says it can show SIBO when it is not there or miss genuine cases, that the UK IBS guidelines for gastroenterologists say to avoid it in IBS, and that private adverts for testing may be from companies with a vested interest. BNSSG ICB likewise says breath testing is very limited on the NHS and often performed privately, and that gastroenterologists may consider empirical treatment rather than testing if SIBO is deemed likely. Where a hospital does offer a hydrogen or methane breath test, UCLH says it carries no risks, and that you must avoid antibiotics and probiotics for 4 weeks beforehand, avoid high fibre foods the day before and fast for 13 hours.

Guts UK says SIBO happens when the anatomy of the digestive tract changes, allowing bacteria to move easily from the large bowel into the small bowel, or when bacteria do not move through the small bowel as they should. Surgery is a common cause, and certain existing conditions make it more likely.

The symptoms of SIBO are much like those of other digestive conditions, so a GP needs to look for alternative explanations first. The local NHS guidance for GPs from BNSSG ICB says it is important to check for alarm symptoms or red flags and refer appropriately. NICE Clinical Knowledge Summaries says an IBS diagnosis should only be suspected in the absence of alarm symptoms or signs.

The NHS lists symptoms that should be checked by a GP because they can be a sign of bowel cancer: a change in your poo that is not usual for you, blood in your poo, losing weight without trying, a lump in your tummy, and feeling very tired or short of breath, which can be signs of anaemia. It says to ask for an urgent GP appointment or call NHS 111 if your poo is black or dark red or you have bloody diarrhoea, and to call 999 or go to A&E for non-stop or heavy bleeding from the bottom.

BNSSG ICB guidance says breath testing is very limited on the NHS and often performed privately, and that a hospital referral may not lead to testing for SIBO or treatment with antibiotics. Gastroenterologists may consider empirical treatment, which means treating without a test, rather than testing if SIBO is deemed likely.

Private gastroenterology clinics and direct-to-consumer breath tests are available. Guts UK warns that adverts for testing often come from private companies with a vested interest, and that there is little proof that the breath test effectively diagnoses SIBO in people with IBS. If you pay for a test, ask a clinician to interpret it alongside your history rather than relying on the result alone.

Guts UK says it is not always possible to get rid of the causes of SIBO, so symptoms may come back after treatment at different times and may need more antibiotics or a longer course. Questions it suggests asking your doctor include whether a dietitian referral is appropriate, whether other medicines could be tried, whether returning symptoms need another or different treatment, and whether other tests are needed to rule out other conditions.