Small Intestinal Bacterial Overgrowth (SIBO)

Small intestinal bacterial overgrowth (SIBO) is a common condition which causes a range of digestive symptoms.

In this article, we will explain what SIBO is and what the symptoms of SIBO are. 

We will also review popular diets that are promoted in treating SIBO and if there is any evidence behind them.

WHAT IS SIBO?

SIBO occurs when you have an abnormally high level of bacteria in your small bowel (1). This is the part of your gut which does the majority of your digesting. 

Normally, bacteria lives in your colon and not your small bowel. Bacteria overgrows in the small intestine when one (or more) defence mechanism that protects us, fails (2). 

Defence mechanisms that protect us include (2):

• The normal motility of the gut
• Gastric acid
• Pancreatic enzymes
• Bile salts

WHAT ARE THE SYMPTOMS OF SIBO?

If you have this condition then you will typically experience (1):

• Diarrhoea 
• Flatulence
• Stomach pain 
• Bloating 
• Discomfort
• Constipation

In severe cases, SIBO can cause malabsorption and nutritional deficiencies in fat-soluble vitamins and some water-soluble vitamins, like B12 (1).

SIBO can also cause elevated levels of folate as it is a byproduct of bacterial metabolism (3).

HOW TO GET A SIBO DIAGNOSIS

The gold standard for SIBO diagnosis is by endoscopy. An endoscopist takes a sample of small bowel fluid which is also called small bowel aspirate. The sample is sent to the laboratory for culture (4).

If the concentration of the bacteria in aspirate is ≥ 10³ CFU in 1 mL, the SIBO diagnosis is confirmed (4).

But this is rarely seen outside of the research setting as it is invasive, expensive, and lacks standardised protocol (4).

A more common method of testing for SIBO involves a breath test (1). Doctors will use both a hydrogen and methane breath test to both diagnose and then guide their choice of antibiotic (5).

SIBO is difficult to diagnose so we don’t know exactly how common it is (6). 

HOW IS SIBO TREATED?

There is a 3 step approach to SIBO treatment (7):

1. Diagnosis
2. Treatment
3. Prevention of a further occurrence

SIBO is treated using antibiotics (5). The exact regime may depend on:

• The type of gas which was high
• What type of probiotic you have tried previously
• What is licensed for use in your country 

The success of antibiotic treatment for SIBO may be improved through the use of both hydrogen and methane testing which can help indicate what type of bacteria is in the small bowel. This then allows the doctor to match a specific antibiotic (7).

Research shows that 68% of people have had symptom improvement or relief with antibiotic therapy (8).

CAN DIET TREAT SIBO? 

There is limited research on diet in SIBO treatment. However, there is some evidence that dietary modification can help you manage your symptoms.

We found different dietary advice to treat SIBO and now we are going to review and check the evidence behind them.

Dietary recommendations according to the American College of Gastroenterology Guidelines 2020

According to the American College of Gastroenterology Guidelines 2020, it is recommended to follow a low fibre diet, to avoid alcohol sugars and fermentable sweeteners (4).

The main focus of the guidelines suggests the avoidance of fermentable carbohydrates (FODMAPs) (4).

Currently, we don’t know how FODMAPs react as the research is done looking at the large bowel rather than the small bowel (9). 

It is predicted that the bacteria from SIBO may eat these FODMAPs and release gas (the same as they do in the large bowel in IBS) but there isn’t a lot of research. However, avoidance of FODMAPs wouldn’t cure it, it would only reduce symptoms (9).

Another recommendation is to follow a low fibre diet (4). Again, there is no research to back that guideline up. But we know that good gut bacteria comes from a diverse diet, high in fibre so if you decide to follow that route, then only do it for a short period of time.

The specific carbohydrate diet 

The specific carbohydrate diet claims that by consuming only carbohydrates which feed good bacteria, the bad ‘overgrowth’ bacteria will die off. 

Unfortunately, the diet is extremely difficult to follow and has no scientific proof to work in SIBO at all.

The Elemental Diet

The elemental diet is a diet which provides all your food components broken down into their simplest forms. The diet is liquid in form and you can only consume this and water whilst following the regime. 

The diet works as the broken down liquid food is absorbed in the start of your digestive system. It avoids making its way down towards that bad gut bacteria, providing it with more food. The bad bacteria then starves and dies away (7).

One study showed that 85% of participants had normal breath tests after 22 days on the elemental diet (10). Although the results were in favour of the diet, the type of breath test used for diagnosis isn’t recommended for SIBO diagnosis so we can’t know for sure if the diet was effective.

Moreover, the elemental diet is generally used for individuals that do not tolerate antibiotics first as there is lack of research to prove if it works or not. The diet is also very difficult to stick to and costly (7).

At present, we do not recommend this diet. 

The Low FODMAP Diet

The low FODMAP diet is a 3 stage process consisting of: 

• the restriction phase
• the reintroduction phase
• the modified diet  

It is thought that the low FODMAP diet reduces SIBO symptoms likewise as in IBS. But currently, there are no scientific studies that confirm those predictions as all the studies are focused on the large bowel instead of the small bowel (9). 

SIBO patients may feel better during the restriction phase of the low FODMAP diet as it is assumed that FODMAPs may feed bacteria in the small bowel. Bacterial metabolism of FODMAPs results in a lot of gas. 

However, a low FODMAP diet doesn’t get rid of SIBO, it just reduces some of the symptoms as we take away the food source of the bacteria. SIBO patients also don’t need the FODMAP reintroduction, as they aren’t trying to find intolerances or triggers, but just trying to ease the symptoms.

However, a low FODMAP approach may be beneficial as a proportion of patients with IBS have SIBO as well (11). 

The low fermentation diet

The low fermentation diet has been designed specifically for treating SIBO. The diet takes into account meal timing and avoidance of fermentable products (12). 

However, there is no evidence that the diet is effective. Moreover, the low fermentation diet excludes a lot of prebiotic foods and can long-term lead to nutritional deficiencies. 

Gluten-free diet

Gluten is a protein, found in grains like wheat, rye, barley, and spelt. You may have heard that a gluten-free diet can help treat SIBO. But there is no evidence behind the gluten-free diet for SIBO (4). 

DO PROBIOTICS HELP IN SIBO TREATMENT?

The evidence about probiotics in SIBO treatment is mixed. 

A meta-analysis showed that taking probiotics is associated with a reduction in abdominal pain, but it has no significant effect on the frequency of bowel movements (13). 

Another research compared antibiotic treatment and use of probiotics to only antibiotic treatment. Greater improvement in symptoms were in the group that was taking probiotics among antibiotics compared to the group in only antibiotic treatment (14). 

Yet, some other study showed that taking probiotics induced SIBO-related symptoms, such as bloating, gas, and brain fog (15).

We need further research to confirm if probiotics can help in SIBO treatment as the results are not consistent so far. Also, we need more research to confirm what type of probiotics to use.

DO PREBIOTICS HELP IN SIBO TREATMENT?

So far, we don’t have evidence that prebiotics help in SIBO treatment. 

According to the American Journal of Gastroenterology Guidelines 2020, it is advised to avoid certain prebiotics such as inulin, as they can worsen your symptoms (4). 

Prebiotics like inulin are fermentable types of fibre (or high FODMAP). That means that bacteria in your gut metabolise it and produce a lot of gas, which can cause bloating.

DO HERBAL TREATMENTS WORK FOR SIBO?

Herbal treatments for treating SIBO are showing promising results. One study showed herbal treatments to be as effective as antibiotic treatment (16). However, we have limited data on herbal treatments so further research is needed.

SUMMARY

SIBO is a common condition which causes a range of digestive symptoms. If you have symptoms of SIBO then it is advised to speak to your Doctor. 

It is important that you get a diagnosis and to address the underlying cause. Your doctor will discuss treatment options with you. 

There are dietary approaches for SIBO, however, these should be done with the support of a specialist registered dietitian. Remember, a dietitian can also help to address and manage any underlying causes e.g. IBS.

Written by Barbara Lešnik Student Dietitian 2022, reviewed by Serena Bansal Registered Dietitian BSc Hons, and Kirsten Jackson Consultant Dietitian BSc Hons, RD, PG Cert