Understanding the Connection Between IBS and SIBO

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Irritable Bowel Syndrome (IBS) and Small Intestinal Bacterial Overgrowth (SIBO) are two intricately linked conditions, with recent studies indicating that as many as 84% of IBS cases involve SIBO. Understanding the relationship between these two conditions is crucial for effective diagnosis and management.

Irritable Bowel Syndrome (IBS)

IBS is a prevalent disorder affecting the large intestine, marked by irregular muscle contractions that can be more intense and frequent than normal. The symptoms of IBS vary widely but generally include chronic pain and discomfort in the digestive tract.

Common symptoms of IBS

  • Abdominal pain and cramping: Often relieved by passing a bowel movement.
  • Altered bowel habits: Including constipation, diarrhoea, or a combination of both.
  • Bloating and gas: Leading to discomfort and social embarrassment.
  • Food intolerance: Certain foods trigger symptoms in many individuals with IBS.

Causes and mechanisms

The exact causes of IBS are complex and not fully understood, but the disorder involves several interrelated factors:

  • Abnormal gastrointestinal motility: The speed at which food moves through the digestive system can be irregular in people with IBS, either too fast, leading to diarrhoea, or too slow, resulting in constipation. This irregularity is a core feature of the condition.
  • Increased gut sensitivity: People with IBS often have a more sensitive enteric nervous system (ENS), the network of neurons that governs the gut's functions. This sensitivity can cause them to experience intense pain from mild irritation or normal digestive activity. The neurons may send exaggerated pain signals to the brain, which interprets them as signs of severe distress when there is no actual danger. This phenomenon, known as visceral hypersensitivity, makes routine digestive processes unusually painful.
  • Psychological factors: There is a significant psychological component to IBS. Stress and mental health issues do not cause IBS but can exacerbate its symptoms. The interaction between the brain and the gut, known as the brain-gut axis, means that emotional and psychological stress can impact physical gut functions, influencing both motility and sensitivity.

Management strategies

Managing IBS typically requires a holistic approach, focusing on both physical and psychological aspects:

  • Dietary changes: Identifying and avoiding trigger foods can help manage symptoms. A low FODMAP diet is often recommended as it reduces foods that are known to cause bloating and gas.
  • Stress management: Techniques such as cognitive behavioural therapy, mindfulness, and yoga can help reduce the impact of stress on the gastrointestinal system.
  • Medication: Depending on the symptoms, medications may include fibre supplements, laxatives, anti-diarrheal agents, antispasmodics, and sometimes antidepressants for their gut-relaxing properties.

The complexity of IBS means that treatment can vary significantly from one person to another. Given that recent studies suggest up to 84% of IBS cases actually involve SIBO, it is crucial to deepen our understanding of this condition to tailor more effective treatments.

Small Intestinal Bacterial Overgrowth (SIBO)

SIBO occurs when excessive bacterial populations grow in the small intestine, a region typically low in bacteria. This overgrowth disrupts the normal digestive and absorption processes, often damaging the mucosal lining and leading to several health complications:

  • Nutrient malabsorption: The damage to the mucosal lining can hinder the absorption of essential vitamins and minerals, leading to symptoms like fatigue and weakness.
  • Compromised gut barrier: Chronic low-grade inflammation, a hallmark of SIBO, may impair the gut barrier—commonly known as "leaky gut." This condition increases intestinal permeability, allowing toxins and pathogens to seep into the bloodstream, potentially triggering immune responses and systemic health issues.
  • Systemic symptoms: The subtle yet persistent inflammation caused by SIBO often escapes detection in standard diagnostic tests such as coloscopies, which do not typically reveal 'infection' or visible inflammation. Over time, this can cause widespread effects, including skin problems like rosacea, a general feeling of unwellness, and autoimmune diseases.
  • Exacerbation of IBS symptoms: For individuals with IBS, the inflammation and motility changes caused by SIBO can worsen symptoms such as diarrhoea, constipation, and abdominal pain.
  • Bloating and gas: These are caused by the rapid fermentation of food by the bacteria in the small intestine, rather than in the large intestine where it normally occurs.
  • Abdominal pain and cramps: Especially after eating, as bacteria produce gas and other byproducts.
  • Altered bowel movements: Including both constipation and diarrhoea, depending on the bacterial species predominant.
The considerable symptom overlap between IBS and SIBO can complicate diagnoses, as SIBO may exacerbate or mimic IBS symptoms. Factors such as slow gut transit, structural abnormalities, immune dysfunction, or pH shifts in the small intestine that promote bacterial overgrowth complicate the clinical picture.

Recognising the potential impacts of SIBO is critical for accurate diagnosis and the development of targeted treatment strategies that address the root causes of bacterial overgrowth and its systemic effects.

Understanding the distinction and connection

IBS primarily affects the large intestine (colon) with symptoms like altered bowel habits, abdominal pain, and bloating. It is largely a functional disorder, meaning the symptoms arise from how the bowel functions, like abnormal motility, heightened sensitivity to pain, and perhaps an overactive immune response that affects the colon, rather than from structural or biochemical changes.

SIBO occurs in the small intestine when there is an excessive growth of bacteria that are typically found in the colon. The small intestine should have relatively low levels of bacteria compared to the colon. It can cause symptoms similar to IBS, such as bloating, pain, diarrhoea, and constipation due to the interference of bacteria with normal gut function. This overlap is due to the bacteria interfering with normal digestion and food absorption, leading to food fermentation that causes gas and bloating, and altering the gut’s motility.

Why the confusion?

The high prevalence of SIBO in IBS patients suggests that bacterial overgrowth in the small intestine might contribute significantly to the symptoms associated with IBS, particularly bloating and diarrhoea. This has led some researchers to propose that some cases of IBS might actually be misdiagnosed SIBO.


IBS is diagnosed based on symptom presentation and exclusion of other conditions, often using the Rome IV criteria.
SIBO is diagnosed through breath testing, which measures hydrogen and methane produced by bacteria. The overlap in symptoms and lack of universally accepted diagnostic standards for SIBO can complicate diagnosis.

Combined understanding and management

Given the symptom overlap and the high prevalence of SIBO in individuals with IBS, it is recommended to evaluate patients with typical IBS symptoms for SIBO. Management typically involves:

  • Dietary modifications: A low FODMAP diet is commonly recommended to reduce symptoms in both IBS and SIBO.
  • Medications: (Natural) antibiotics may be used to manage SIBO, while antispasmodics and laxatives may be used to control symptoms of IBS.
  • Probiotics and prebiotics: These can help restore gut flora balance, though they may initially worsen symptoms in SIBO cases.
  • Stress management: As psychological stress can exacerbate both IBS and SIBO, techniques such as mindfulness and cognitive-behavioural therapy can be beneficial.


While traditionally IBS has been viewed as a disorder of the large intestine, the frequent occurrence of SIBO in IBS patients suggests a significant interplay involving the small intestine. This underscores the need for careful diagnostic assessments and possibly reevaluating treatment approaches, considering SIBO's potential role.