What Causes SIBO?
- Ola Krawczyk, Clinical Nutritionist
- 2 days ago
- 5 min read
ATMS Accredited Member | SIBO-Certified Nutritionist | Microba Certified Nutritionist
Published: 03 September 2026
Small intestinal bacterial overgrowth, commonly known as SIBO, occurs when excessive numbers or an altered balance of bacteria are present in the small intestine and contribute to gastrointestinal symptoms.
Common symptoms may include:
Bloating and abdominal distension
Excessive gas or belching
Abdominal discomfort
Diarrhoea or constipation
Nausea
Food-related digestive symptoms
However, SIBO usually does not appear without a reason. It often develops when something disrupts the small intestine’s normal ability to move food and microorganisms forward.
Ola’s clinical perspective: “A positive SIBO breath test may indicate abnormal fermentation, but it does not explain why the problem developed. Looking at motility, constipation, medical history, previous surgery and medications is an important part of creating a personalised and sustainable plan.” — Ola Krawczyk, Clinical Nutritionist, ATMS Member and SIBO-Certified Practitioner
What normally protects the small intestine?
The body has several natural mechanisms that help prevent microorganisms from accumulating in the small intestine:
Normal intestinal movement
The migrating motor complex between meals
Stomach acid
Bile and pancreatic enzymes
The ileocecal valve between the small and large intestines
Intestinal immune defences
SIBO may become more likely when one or more of these protective mechanisms are impaired.
The American College of Gastroenterology explains that normal intestinal movement, timely emptying, digestive enzymes and immune factors all help maintain a balanced small-intestinal environment. American College of Gastroenterology: SIBO
1. Slower intestinal motility
Impaired intestinal motility is one of the best-recognised contributors to SIBO.
Between meals, coordinated muscular contractions—known as the migrating motor complex—help move residual food and microorganisms through the small intestine.
If this clearing activity is reduced, bacteria may remain in the small intestine for longer and have an opportunity to multiply.
Slower motility may be associated with:
Diabetes affecting the intestinal nerves
Hypothyroidism that is not adequately managed
Scleroderma and other connective-tissue conditions
Parkinson’s disease and certain neurological conditions
Chronic constipation
Gastroparesis
Medications that slow intestinal movement
Persistent slow motility may also help explain why SIBO sometimes returns after treatment.
2. Previous surgery or structural changes
Changes to the normal structure of the digestive tract can create areas where intestinal contents move slowly or become trapped.
Examples include:
Previous gastrointestinal surgery
Gastric bypass surgery
Surgical blind loops
Small-intestinal diverticula
Strictures or narrowed sections of the intestine
Partial bowel obstruction
Adhesions following surgery
Changes associated with Crohn’s disease
Removal of the ileocecal valve
A 2024 systematic review involving 407 people with intestinal failure found that approximately
, although the researchers noted considerable differences between the included studies. This finding illustrates how altered intestinal anatomy and impaired function can substantially increase risk in particular medical populations. Read the systematic review
This statistic does not mean that 57.5% of the general population has SIBO. It applies to a specific high-risk group.
3. Problems with the ileocecal valve
The ileocecal valve separates the small intestine from the large intestine. It helps control the forward movement of intestinal contents and limits backward flow from the colon.
If the valve has been surgically removed or significantly impaired, microorganisms from the large intestine may move backwards into the small intestine.
This can increase susceptibility to bacterial-overgrowth syndromes.
4. Reduced stomach acid
Stomach acid supports digestion and helps limit the number of microorganisms entering the intestinal tract with food.
Very low stomach-acid production may weaken this protective barrier. It may occur with:
Atrophic or autoimmune gastritis
Previous stomach surgery
Increasing age
Certain medical conditions
Acid-suppressing medication in some circumstances
The relationship between proton-pump inhibitors and SIBO is not straightforward, and research results are mixed. These medications may be medically necessary and should never be stopped without speaking with the prescribing doctor.
5. Reduced bile or pancreatic enzymes
Bile and pancreatic enzymes support digestion and help maintain the intestinal environment.
Conditions that affect these secretions may increase the likelihood of bacterial overgrowth, including:
Pancreatic insufficiency
Chronic pancreatitis
Certain liver or biliary conditions
Structural changes affecting the delivery of bile or enzymes
These concerns require medical assessment. Digestive enzymes or bile-related supplements should not be recommended from symptoms alone.
6. Certain gastrointestinal conditions
SIBO may occur alongside other digestive conditions, including:
Coeliac disease
Crohn’s disease
Small-intestinal diverticulosis
Chronic intestinal pseudo-obstruction
Short-bowel syndrome
Radiation enteritis
Gastroparesis
Severe constipation
IBS and SIBO can also overlap, but IBS does not automatically cause SIBO, and not everyone with IBS needs SIBO treatment.
7. Previous gastrointestinal infection
Some people develop ongoing digestive symptoms following food poisoning or infectious gastroenteritis.
One proposed explanation is that an infection may affect the nerves involved in intestinal motility, disrupting the small intestine’s natural clearing activity.
However, previous food poisoning does not prove that current bloating or bowel changes are caused by SIBO. Post-infectious IBS and other digestive conditions may cause similar symptoms.
8. Medications that affect intestinal movement
Certain medications may contribute indirectly by slowing the movement of food through the digestive tract or causing significant constipation.
Examples can include:
Opioid pain medications
Some anticholinergic medicines
Certain neurological or psychiatric medications
Other medicines that slow gastrointestinal movement
Medication effects should always be considered in the context of the person’s health and treatment needs. Never discontinue prescribed medication without medical advice.
Do sugar, carbohydrates or stress cause SIBO?
Not directly.
Fermentable carbohydrates may worsen gas, bloating or diarrhoea when SIBO is present because intestinal microorganisms ferment them. This does not mean carbohydrates caused the overgrowth.
Stress may influence gut–brain communication, intestinal sensitivity and motility, but it should not be presented as the sole cause of SIBO.
Restricting food may temporarily reduce symptoms without addressing the underlying reason SIBO developed.
Why does SIBO keep coming back?
SIBO may return when treatment reduces the overgrowth but the underlying contributor remains.
Possible reasons include:
Persistent slow motility
Ongoing constipation
Structural or surgical changes
An underlying digestive or metabolic condition
Medication-related changes
Incomplete or inaccurate diagnosis
Intestinal methanogen overgrowth rather than bacterial overgrowth
Symptoms caused by another digestive condition
The American Gastroenterological Association advises that SIBO management should focus on identifying and correcting underlying causes where possible, addressing nutritional deficiencies and using antibiotics appropriately. AGA Clinical Practice Update
Does bloating always mean SIBO?
No. Bloating may also be associated with:
IBS
Constipation
Coeliac disease
Lactose or fructose malabsorption
Pelvic-floor dysfunction
Food intolerances
Gastroparesis
Gynaecological conditions
Other gastrointestinal concerns
Symptoms alone cannot confirm SIBO. A clinical assessment and appropriately selected testing may be required.
The bottom line
The most recognised factors contributing to SIBO include:
Impaired intestinal motility
Chronic constipation
Previous surgery or structural changes
Reduced stomach-acid protection
Conditions affecting bile or pancreatic enzymes
An impaired or removed ileocecal valve
Medical conditions or medications that slow digestion
Understanding the possible cause matters because reducing bacterial overgrowth without addressing the underlying contributor may increase the likelihood of recurring symptoms.
Do you need help investigating possible SIBO?
If you experience persistent bloating, gas, abdominal discomfort, constipation, diarrhoea or food-related digestive symptoms, I can help you review your health history, diet, bowel patterns and previous investigations.
Where appropriate, we can discuss SIBO breath testing, medical referral and a personalised nutrition approach.
This article is for educational purposes only and does not replace medical diagnosis, treatment or advice. Speak with your doctor about persistent, worsening or concerning gastrointestinal symptoms.
Book a SIBO Breath Test
If you are considering SIBO breath testing, you can learn more and book a test through SIBO Test.
Breath-test results should be interpreted alongside your symptoms, medical history and other relevant health information.
About the author
Ola Krawczyk is a Polish-Australian Clinical Nutritionist, accredited ATMS member, SIBO-Certified Practitioner and Microba Certified Practitioner specialising in SIBO, digestive health, microbiome interpretation, thyroid health and perimenopause nutrition.
Her professional approach is also informed by her personal experience with SIBO, thyroid and hormonal health challenges. Ola provides personalised online nutrition consultations across Australia through Clinical Nutrition by Ola.
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