DR. THOMAS BACHARACH

Specialist in General Medicine

🕑 Reading time: 10–12 minutes

👆 The most important points in brief

• SIBO stands for Small Intestinal Bacterial Overgrowth. In German, it is often called Dünndarmfehlbesiedlung, abbreviated as DDFB. This means that too many or unfavorable bacterial species are present in the small intestine.^1,2
• Typical symptoms include bloating, gas, abdominal pain, diarrhea, constipation, a feeling of fullness and sometimes fatigue or brain fog. Many affected people are initially diagnosed with irritable bowel syndrome because the symptoms can overlap strongly.^2,3
• A common SIBO test is the breath test using glucose or lactulose. It measures hydrogen and methane in the breath. Treatment should not only reduce bacteria, but also take causes such as motility disorders, digestive juices, medications or anatomical factors into account.^1,4

✔ Evidence-based:
This article is based on medical sources on SIBO, clinical guidelines and consensus recommendations on breath testing. SIBO can have many causes and should not be treated only symptomatically. What matters is proper diagnostics, careful interpretation of symptoms and a treatment concept that takes the underlying triggers into account.^1,2,4,5

SIBO / DDFB: What is small intestinal bacterial overgrowth? Symptoms, causes, breath test and treatment

1. Why SIBO is so often overlooked
2. What is SIBO or DDFB?
3. Bacteria in the gut: welcome in the colon, limited in the small intestine
4. Why the small intestine is normally not overgrown
5. Pregnancy-like bloating and brain fog
6. SIBO test: What is a breath test?
7. Causes: Why does small intestinal bacterial overgrowth develop?
8. Treatment: What can help with SIBO?
9. Checklist: When should you think of SIBO?
10. FAQ: Frequently asked questions about SIBO
11. Glossary
12. References

1. Why SIBO is so often overlooked

Many people with chronic digestive symptoms are eventually diagnosed with irritable bowel syndrome. That is understandable, because the symptoms can look very similar: bloating, abdominal cramps, diarrhea, constipation, food intolerances and the feeling that you can no longer rely on your own gut.^2,3

For affected people, this can be extremely stressful. Some plan their day around where the nearest toilet is. Others eat only a few foods because they are afraid of triggering symptoms. Still others appear healthy on the outside, but feel constantly exhausted, foggy or bloated on the inside.

One possible cause behind these symptoms is SIBO, a bacterial overgrowth in the small intestine. The good news: If small intestinal bacterial overgrowth is actually present, it can be investigated more specifically and often treated more specifically than a vague “IBS” label.^1,4

However, it is important to understand:

• SIBO is not the cause of every bloated abdomen.
• A positive test must always match the symptoms.
• Treatment should not only “fight” bacteria, but also clarify why they were able to multiply in the small intestine in the first place.

2. What is SIBO or DDFB?

SIBO stands for Small Intestinal Bacterial Overgrowth. In German, the term Dünndarmfehlbesiedlung is often used. The German abbreviation is DDFB.^1,2

In SIBO, the small intestine contains either:

• too many bacteria,
• unfavorable bacterial species,
• or bacteria in a place where they should not be present in this amount.

This does not automatically mean that these bacteria are “bad” or classic pathogens. Often, they are bacteria that are completely normal in the colon. It becomes problematic when they become too active in the small intestine.¹

The small intestine is mainly responsible for the breakdown and absorption of nutrients. If too many bacteria are feeding there as well, they can ferment carbohydrates, produce gases, alter bile acids and consume nutrients that were actually intended for the body.^1,2

3. Bacteria in the gut: welcome in the colon, limited in the small intestine

Bacteria are not fundamentally bad. On the contrary: they are an important part of our microbiome. Most gut bacteria are found in the colon. There, they perform important functions:

• They ferment dietary fiber.
• They produce short-chain fatty acids such as butyrate.
• They support the intestinal lining.
• They produce certain vitamins.
• They train our immune system.

The colon is designed for this bacterial activity. Gases and bacterial metabolic products are produced there regularly. To a certain extent, this is normal.

The small intestine, however, has a different task. This is where proteins, fats and carbohydrates are broken down and nutrients are absorbed. That is why the number of bacteria in the upper small intestine is normally significantly lower than in the colon.¹

In short:

• Many bacteria in the colon: normal and important.
• Too many bacteria in the small intestine: potentially problematic.

Which bacterial species may be involved in SIBO?

Different bacterial species may be involved in SIBO. Medical sources describe, among others, Streptococcus, Bacteroides, Escherichia, Lactobacillus, Klebsiella and Aeromonas.¹

The important point is: there is no single “SIBO bacterium.” What matters more is that the bacterial composition and activity in the small intestine no longer fit the function of this section of the gut.

4. Why the small intestine is normally not overgrown

The small intestine is not completely sterile. However, it has several protective mechanisms that are designed to prevent bacteria from multiplying too much there.¹

These include:

• Stomach acid: It reduces the number of germs entering the upper digestive tract with food.
• Digestive juices: Bile and pancreatic secretions not only help with digestion, but also influence the bacterial environment.
• Gut motility: The so-called migrating motor complex acts like a cleaning wave between meals.
• Mucus and secretory IgA: These factors support the local immune defense of the intestinal lining.
• Ileocecal valve: It is located between the small intestine and the colon and can help limit backflow from the colon.

If one or more of these protective mechanisms are disturbed, bacteria may remain in the small intestine more easily or migrate back from the colon. For many affected people, this is where the problem begins.

The migrating motor complex: the gut’s “cleaning wave”

The migrating motor complex, or MMC for short, is a wave-like movement of the small intestine. It occurs mainly during fasting periods and moves food residues, secretions and bacteria onward toward the colon.

If someone eats constantly or if gut motility is impaired, this mechanism can become weaker. As a result, food residues and bacteria remain in the small intestine for longer. For bacteria, this means more time, more food and better growth conditions.

5. Pregnancy-like bloating and brain fog

One particularly typical symptom of SIBO is a severely bloated abdomen. Many affected people describe it as pregnancy-like bloating: relatively flat in the morning, then increasingly tense, round and painful after eating.

The reason: bacteria in the small intestine ferment food components. This produces gases such as hydrogen or methane. However, the small intestine is not designed to store large amounts of gas. This can lead to pressure, pain, gas and visible abdominal distension.^1,2

Typical SIBO symptoms may include:

• bloating, especially after eating,
• gas and a feeling of pressure,
• abdominal cramps or abdominal pain,
• diarrhea,
• constipation,
• alternating diarrhea and constipation,
• a feeling of fullness, nausea or belching,
• fatigue, exhaustion or brain fog,
• in more pronounced cases, weight loss or nutrient deficiencies.^1,2

Why SIBO can also be noticeable outside the gut

Although SIBO affects the small intestine, it can put stress on the whole body. If bacteria consume nutrients, alter bile acids or impair fat digestion, deficiencies may develop over time. Problems with vitamin B12, fat-soluble vitamins and fat absorption have been described.^1,2

Some affected people also report concentration problems, lightheadedness or a flu-like feeling. These symptoms are nonspecific and can have many causes. They should therefore always be interpreted as part of the overall clinical picture.

6. SIBO test: What is a breath test?

A common SIBO test is the breath test. It measures how much hydrogen and methane are present in the breath. The test is non-invasive and is usually performed with glucose or lactulose.^1,4

The principle is simple:

• Humans themselves do not produce relevant amounts of hydrogen and methane.
• These gases are primarily produced by microbial fermentation.
• When bacteria metabolize sugar in the gut, gases are produced.
• These gases enter the bloodstream and are exhaled through the lungs.

During the breath test, you drink a test solution. Breath samples are then taken over several hours. If hydrogen or methane rise early, this may indicate that bacteria are already active in the small intestine.^1,4

Glucose or lactulose?

Two substrates are commonly used for SIBO breath testing:

• Glucose: normally absorbed early in the small intestine. An early rise in gas may therefore indicate bacterial activity in the upper small intestine.
• Lactulose: not digested by humans and travels farther through the gut. This means it may capture other sections, but it is also more difficult to interpret.

No test is perfect. That is why proper preparation, correct performance and careful interpretation are important. Antibiotics, laxatives, prokinetics, bowel cleanses and the diet before the test can all influence the result.^1,4

Hydrogen, methane and IMO

Many SIBO tests measure hydrogen and methane. A hydrogen rise is more suggestive of hydrogen-producing microorganisms. Methane is often associated with constipation and is now often referred to as IMO: Intestinal Methanogen Overgrowth. In this case, methane-producing archaea are at the center of the problem, not classic bacteria.^4,5

7. Causes: Why does small intestinal bacterial overgrowth develop?

SIBO rarely develops randomly. In most cases, there are factors that promote bacterial growth in the small intestine.

Possible causes and risk factors include:

• Impaired gut motility: If chyme moves too slowly through the intestine, bacteria have more time to multiply.
• Anatomical changes: Surgeries, strictures, diverticula or blind loops can cause stagnation.
• Low stomach acid: Too little stomach acid can weaken germ control in the upper digestive tract.
• Medications: Certain medications can influence stomach acid, motility or the microbiome.
• Disturbances in digestive juices: Problems with bile or pancreatic secretions can alter digestion and the bacterial environment.
• Underlying diseases: Diabetes, hypothyroidism, scleroderma, celiac disease, inflammatory bowel disease or other conditions can increase the risk.^1,2

Why treating symptoms alone is often not enough

If only the bacterial load is reduced but the underlying cause remains, SIBO can return. That is why the most important question is not only: “How do I get rid of the bacteria?” but also: “Why were they able to multiply there?”

This is exactly why diagnostics, nutrition, motility, digestive juices, medications and underlying diseases all belong together in one treatment concept.

8. Treatment: What can help with SIBO?

The treatment of SIBO should be planned individually. There is no one-size-fits-all solution. What matters is which symptoms are present, which gas pattern is abnormal in the breath test, which causes are likely and whether deficiencies or accompanying conditions are present.

In principle, there are three levels:

• 1. Reduce overgrowth: for example with antibiotics or, in some concepts, with botanical antimicrobial strategies.
• 2. Treat symptoms and deficiencies: for example through nutrition, nutrients, electrolytes or symptomatic support.
• 3. Address the cause: for example motility disorders, low stomach acid, anatomical problems, medications or disturbances in digestive juices.^1,2

Antibiotics and medical treatment

In conventional medicine, antibiotics are often used for SIBO. Depending on the situation, different active substances may be considered. Rifaximin is frequently mentioned; in methane-positive cases, it may sometimes be combined with other active substances.^1,5

Important: Antibiotics can be useful, but they are not a long-term solution if the underlying cause is not taken into account.

Nutrition in SIBO

Nutrition can significantly influence symptoms. Many affected people react to highly fermentable carbohydrates, large meals or frequent snacking. In the short term, adapted nutritional concepts can help reduce symptoms.

Possible approaches include:

• temporarily reducing fermentable carbohydrates,
• an individually adapted low-FODMAP diet,
• sufficient breaks between meals to support the migrating motor complex,
• a gradual rebuilding of the diet so that food variety does not remain unnecessarily restricted.

You can find more information about the FODMAP diet here.

Motility and relapse prevention

In recurrent SIBO, gut motility often plays a central role. If the small intestine does not “clean up” sufficiently between meals, symptoms can return after successful treatment.

Useful building blocks may include:

• regular breaks between meals,
• assessment of motility disorders,
• treatment of thyroid problems or diabetes, if relevant,
• prokinetics if appropriate after medical evaluation,
• treatment of structural causes, if present.

9. Checklist: When should you think of SIBO?

10. FAQ: Frequently asked questions about SIBO

11. Glossary

12. References

1. MSD Manual Professional Edition. Small Intestinal Bacterial Overgrowth (SIBO). Reviewed/Revised Mar 2025, Modified May 2025. Retrieved from
   https://www.msdmanuals.com/professional/gastrointestinal-disorders/malabsorption-syndromes/small-intestinal-bacterial-overgrowth-sibo
2. Cleveland Clinic. SIBO (Small Intestinal Bacterial Overgrowth): Symptoms, Diagnosis, Causes & Treatment. Last updated 08/09/2024. Retrieved from
   https://my.clevelandclinic.org/health/diseases/21820-small-intestinal-bacterial-overgrowth-sibo
3. Rao, S. S. C., & Bhagatwala, J. (2019). Small Intestinal Bacterial Overgrowth: Clinical Features and Therapeutic Management. Clinical and Translational Gastroenterology, 10(10), e00078. Retrieved from
   https://pubmed.ncbi.nlm.nih.gov/31584459/
4. Rezaie, A., Buresi, M., Lembo, A., et al. (2017). Hydrogen and Methane-Based Breath Testing in Gastrointestinal Disorders: The North American Consensus. American Journal of Gastroenterology, 112, 775–784. Retrieved from
   https://pubmed.ncbi.nlm.nih.gov/28323273/
5. Pimentel, M., Saad, R. J., Long, M. D., & Rao, S. S. C. (2020). ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. American Journal of Gastroenterology, 115(2), 165–178. Retrieved from
   https://pubmed.ncbi.nlm.nih.gov/32023228/