Symptoms of Small Intestinal Bacterial Overgrowth (SIBO)
The most common symptoms of SIBO are bloating, abdominal discomfort, flatulence, and changes in bowel habits, with bloating and abdominal distention being the predominant presenting complaints that typically worsen after meals. 1
Core Gastrointestinal Symptoms
Primary Symptoms
- Bloating and abdominal distention are the hallmark symptoms, characteristically worsening post-prandially (after eating) 1, 2
- Flatulence and excessive gas production result directly from bacterial fermentation of carbohydrates in the small intestine 1
- Diarrhea is a common presenting symptom, though the pattern varies by SIBO subtype 2
- Abdominal pain and discomfort occur frequently, particularly shortly after eating when chyme triggers severe painful non-propulsive contractions in patients with enteric neuropathies 3, 2
Bowel Pattern Changes
- Constipation is particularly associated with methane-dominant SIBO and may be the first symptom in patients with gut stasis 3, 1
- Changes in stool consistency and frequency are common across all SIBO subtypes 1
Severe Manifestations (Advanced Disease)
When SIBO progresses beyond mild presentations, additional symptoms emerge:
- Steatorrhea (fatty stools) develops when bacterial overgrowth causes bile salt deconjugation and pancreatic enzyme degradation 3, 2
- Large volume vomiting may occur when significant fluid accumulates from gut stasis, potentially containing feculent material or food debris from days prior 3
- Weight loss and malnutrition result from malabsorption, though these are uncommon in mild SIBO presentations 3, 1
- Nutritional deficiencies including fat-soluble vitamins (A, E, D, K) causing night blindness, poor color vision, dry flaky skin, and ataxia 3
- Vitamin B12 deficiency may occur, while folate and vitamin K levels can paradoxically be elevated due to bacterial production 3, 2
Important Clinical Distinctions
Overlap with IBS
The symptoms of SIBO overlap significantly with irritable bowel syndrome (IBS), making clinical distinction challenging without diagnostic testing (breath tests or small bowel aspiration). 1, 2 This overlap explains why SIBO is increasingly recognized in patients previously diagnosed with IBS alone.
Inflammatory Markers
- Normal inflammatory markers such as fecal calprotectin are expected in SIBO—elevated levels should prompt investigation for alternative diagnoses like inflammatory bowel disease rather than attributing them to bacterial overgrowth 1, 4
- Laboratory findings may show elevated folate levels, but inflammatory biomarkers remain normal in uncomplicated SIBO 4, 2
Risk Factor Considerations
Patients with underlying conditions are more likely to develop SIBO and may present with mild symptoms even without severe disease: 1
- Prior gastrointestinal surgery
- Pancreatic exocrine insufficiency
- Diabetes mellitus
- Motility disorders (impaired migrating motor complex)
Pathophysiologic Symptom Mechanisms
Understanding why symptoms occur helps recognize SIBO clinically:
- Impaired migrating motor complex (MMC) prevents clearance of intestinal debris, predisposing to bacterial overgrowth and gut stasis 3
- Bacterial fermentation of unabsorbed carbohydrates produces hydrogen and methane gas, causing bloating and flatulence 1
- Bile salt deconjugation by anaerobic bacteria leads to fat malabsorption and steatorrhea in advanced cases 3
- Gut distension from accumulated fluid and gas causes visible abdominal distention 3
Common Pitfalls to Avoid
- Do not assume elevated inflammatory markers indicate SIBO—they suggest concurrent inflammatory conditions requiring separate evaluation 4
- Do not dismiss SIBO based on absence of weight loss or malabsorption—mild presentations lack these features 1
- Do not confuse colonic dysbiosis (qualitative bacterial changes in IBS) with true small intestinal bacterial overgrowth (quantitative increase) 5
- Recognize that breath test abnormalities may reflect rapid colonic transit rather than true small intestinal overgrowth 5