SIBO: symptoms, diagnosis and what to eat in the meantime

What is SIBO?

SIBO (from the English Small Intestinal Bacterial Overgrowth) is an overgrowth of bacteria in the small intestine: there are more than there should be [1]. Most of the microbiota lives in the colon. There are far fewer bacteria in the first sections of the small intestine, the duodenum and jejunum, which allows nutrients to be digested and absorbed properly.

The intestine has defences that maintain this balance. These include stomach acid, pancreatic enzymes and the ileocecal valve, the gateway between the small intestine and the colon. The most important defence is the movement of the small intestine itself [1]. When these defences fail, bacteria ferment carbohydrates too early. They release hydrogen and methane, which distend the intestinal wall and can make it harder to absorb fats and nutrients [1].

There is a distinct variant with its own name. When the excess consists of methane-producing archaea, the American College of Gastroenterology guideline refers to IMO (Intestinal Methanogen Overgrowth) [1]. The microorganism most commonly involved, Methanobrevibacter smithii, produces methane from hydrogen. It is associated with slower transit and constipation [1].

What are the symptoms of SIBO, and what can it be confused with?

The most common symptoms are abdominal bloating that gets worse as the day goes on, gas, and abdominal pain or discomfort [1]. Bowel habits may also change, with diarrhoea, soft stools or constipation [1]. As the condition progresses, bacteria alter bile salts and fats are absorbed less effectively. Steatorrhoea may then occur—greasy stools that float—and there may be deficiencies of fat-soluble vitamins (A, D, E) and vitamin B12 [1].

None of these symptoms is unique to SIBO. They overlap with irritable bowel syndrome, coeliac disease, lactose or fructose intolerance, functional dyspepsia and inflammatory bowel disease. Medical guidelines do not agree on when to investigate SIBO in someone with irritable bowel syndrome, so that decision belongs to your doctor. A list of symptoms, therefore, is never the same as a diagnosis.

How is SIBO diagnosed?

The reference test is a culture of a jejunal aspirate, obtained by endoscopy. A result is considered positive at 1,000 colony-forming units per millilitre (10³ CFU/ml) or above [1]. Because this is an invasive and costly technique, in practice the hydrogen and methane breath test is used, after drinking a glucose or lactulose solution [1].

The criteria for interpreting the test are standardised [1, 2]. Your doctor interprets these figures together with your symptoms:

• Hydrogen SIBO: a rise in hydrogen of at least 20 parts per million (ppm) above the baseline value within the first 90 minutes [1, 2].
• Methane overgrowth (IMO): methane at or above 10 ppm at any point during the test, often with constipation [1, 2].

How to prepare for the breath test

The reliability of the test depends on preparation. A mistake can lead to false positives or false negatives. The centre carrying out your test will give you its instructions in writing, and the North American consensus on breath testing includes the following [2]:

• The test is usually done after four weeks without antibiotics. Agree on the date with your doctor [2].
• Ask your doctor whether you should stop laxatives and prokinetics (medicines that speed up transit) at least one week beforehand [2].
• On the day before, avoid fermentable foods such as pulses, bread, pasta, fruit, vegetables and dairy products. White rice, grilled meat or fish, egg and water are usually sufficient [2].
• Fast for 8 to 12 hours before the measurements begin [2].
• Do not smoke on the day of the test and limit physical activity during it, because breathing faster lowers hydrogen readings [2].

The same consensus considers it unnecessary to stop stomach-protective medicines before the test [2]. Even so, ask your doctor before changing any medication.

What to eat while you wait for the test

While you wait for your appointment or test, food can help reduce fermentation and ease abdominal bloating. This can help you eat with peace of mind. Food during this phase is varied, simple and easy to digest.

Build your plate with white rice, boiled potatoes or oats, a lean protein and vegetables that ferment less, such as peeled courgette or cooked carrot. For example, rice with grilled chicken and courgette, or a plain omelette with steamed potatoes. Dress it with extra-virgin olive oil and divide your food into portions that feel comfortable, avoiding large meals.

You can temporarily reduce foods that contain more fermentable sugars, which fuel gas-producing bacteria. This reduction is temporary. It may reduce gas without compromising your nutritional status or removing too many foods from your diet.

Do you need a low-FODMAP diet?

The low-FODMAP diet (short-chain fermentable sugars) is used for irritable bowel syndrome, not to treat SIBO on its own. If you also have irritable bowel syndrome and are prescribed this diet, the elimination phase lasts from 2 to 6 weeks. Set the end date from the very first day. This is followed by guided reintroduction by food group and personalisation. Each phase is explained in our guide to the low-FODMAP diet.

The British guideline says it should be prescribed by a dietitian with specific training. It also says that deficiencies and overly restrictive eating habits should be monitored [3]. A diet that continues without review narrows what you eat and impoverishes the colonic microbiota, increasing the risk of deficiencies.

Food can support you and ease symptoms, but it does not eliminate the overgrowth or replace medical treatment. If you have active SIBO with symptoms, your gastroenterologist will consider an antibiotic. Rifaximin is the most studied [1].

Do you need supplements or probiotics?

According to the American College of Gastroenterology guideline [1], there is no consistent evidence to recommend probiotics for SIBO. Many products promise to “cleanse” the intestine or rebalance the flora quickly without this supporting evidence.

Prebiotics are fermentable fibres. With an excess of bacteria in the small intestine, gas and bloating can easily increase. High-dose antimicrobial plant extracts taken on your own may irritate the digestive tract without you knowing whether they do anything.

A supplement makes sense when it addresses a confirmed need, such as a vitamin B12 or iron deficiency found in a blood test. Your doctor or dietitian will prescribe it.

What mistakes do we see in consultations?

Cutting out dozens of foods for months. This is what we see most often. After reading about SIBO, people start removing foods and never try them again. Their diet becomes increasingly restrictive, and every meal ends up being a source of fear. If this happens to you, bring it up in a consultation: this distress is also addressed with the Psychology team.

Assuming that all bloating is SIBO. This leads to the wrong treatments and delays the detection of other digestive problems that need medical attention.

Trusting tests sold online. Stool “dysbiosis” tests, intolerance tests based on IgG or a drop of blood, and bioresonance cannot diagnose SIBO. No medical society endorses them.

Trying to starve the bacteria. Eating less and less to eliminate the overgrowth only leads to malnutrition and a more sensitive intestine.