Small intestinal bacterial overgrowth

Overview

Small Intestinal Bacterial Overgrowth (SIBO) is a gastrointestinal disorder characterized by an excessive growth of bacteria in the small intestine. Normally, the small intestine contains relatively low levels of bacteria compared to the colon. In SIBO, the bacterial population increases abnormally, leading to interference with nutrient absorption and digestive processes.

This condition is often associated with chronic digestive symptoms and can be a secondary complication of various underlying gastrointestinal diseases. SIBO may result in malnutrition, vitamin deficiencies, and damage to the intestinal lining if left untreated.

Causes

Multiple factors can disrupt the normal balance of bacteria in the small intestine and contribute to the development of SIBO. Common causes include:

• Decreased intestinal motility: Conditions like irritable bowel syndrome (IBS), scleroderma, or diabetes can impair peristalsis, allowing bacteria to remain and multiply in the small intestine.

• Structural abnormalities: Surgical alterations (e.g., gastric bypass), diverticula, or strictures can create stagnant areas where bacteria thrive.

• Reduced gastric acid secretion: Proton pump inhibitors (PPIs) or conditions like atrophic gastritis reduce acid that normally limits bacterial growth.

• Immunodeficiency: Weakened immune systems may fail to regulate bacterial populations effectively.

• Ileocecal valve dysfunction: This valve separates the small intestine from the colon. Its failure can allow colonic bacteria to migrate backward into the small intestine.

Symptoms

SIBO can present with a variety of gastrointestinal and systemic symptoms, which may range in severity. Common signs and symptoms include:

• Abdominal bloating and distention

• Excessive flatulence and belching

• Abdominal pain or cramping

• Diarrhea or, less commonly, constipation

• Fatigue and weakness

• Unintentional weight loss

• Steatorrhea (fatty stools) in severe cases

• Malabsorption of nutrients such as vitamin B12, iron, and fat-soluble vitamins

Some patients may also experience non-digestive symptoms such as brain fog, joint pain, or skin issues, which are believed to be due to systemic inflammation or nutrient deficiencies.

Diagnosis

Diagnosing SIBO requires a careful evaluation of symptoms and confirmation through testing. Common diagnostic methods include:

• Breath testing: The most widely used non-invasive method. Patients ingest a sugar solution (lactulose or glucose), and exhaled gases (hydrogen and methane) are measured over time. Elevated levels suggest bacterial fermentation in the small intestine.

• Small bowel aspirate and culture: A direct but invasive test that involves sampling fluid from the small intestine to measure bacterial counts. It is considered the gold standard but is rarely used due to its complexity.

• Blood and stool tests: These may be used to assess for nutritional deficiencies, inflammation, or other contributing conditions.

• Imaging studies: Sometimes used to detect structural abnormalities such as strictures, adhesions, or diverticula.

Treatment

The management of SIBO focuses on reducing bacterial overgrowth, alleviating symptoms, and addressing underlying causes. Treatment options include:

• Antibiotics: Rifaximin is the most commonly prescribed antibiotic for SIBO. Others may include metronidazole or ciprofloxacin. These reduce bacterial load and relieve symptoms.

• Dietary modifications: A low FODMAP or elemental diet may help starve the bacteria and reduce fermentation-related symptoms.

• Prokinetic agents: Medications that enhance gut motility (e.g., low-dose erythromycin) may prevent recurrence by improving intestinal clearance.

• Probiotics: The role of probiotics in SIBO is controversial. Some strains may help restore balance, but others may exacerbate symptoms in certain individuals.

• Addressing underlying conditions: Management of associated disorders such as IBS, diabetes, or structural problems is crucial for long-term control.

Prognosis

The outlook for individuals with SIBO depends on the underlying cause, severity, and response to treatment. Many patients experience significant symptom improvement with appropriate therapy, but relapse is common, especially if the root cause is not addressed.

With continued monitoring, lifestyle adjustments, and medical management, many patients can maintain control over symptoms and avoid complications such as malnutrition or intestinal damage. Early detection and a comprehensive treatment strategy are key to improving quality of life and minimizing recurrence.