Symptoms of Small Intestinal Bacterial Overgrowth (SIBO)
The hallmark symptoms of SIBO are bloating and abdominal distention that characteristically worsen after eating, accompanied by excessive flatulence, abdominal pain, and diarrhea. 1
Core Gastrointestinal Symptoms
Bloating and distention are the most characteristic features of SIBO, directly resulting from bacterial fermentation of carbohydrates in the small intestine where bacteria should not be present in high numbers. 1 These symptoms predictably intensify following meals as ingested carbohydrates reach the overgrown bacterial population. 1
Excessive gas production and flatulence occur as bacteria ferment unabsorbed carbohydrates, producing hydrogen and methane gas that causes both discomfort and embarrassment. 1, 2
Abdominal pain and discomfort develop particularly shortly after eating, distinguishing SIBO from other functional disorders where pain patterns may be more variable. 3, 1
Diarrhea is common and results from multiple mechanisms including bile salt deconjugation, direct mucosal irritation, and osmotic effects of bacterial metabolites. 3, 4
Advanced Disease Manifestations
Steatorrhea (fatty, foul-smelling stools) develops when bacterial overgrowth causes bile salt deconjugation and degradation of pancreatic enzymes, leading to fat malabsorption. 1 This represents more severe disease and indicates significant functional impairment. 1
Weight loss and malnutrition occur in advanced cases due to malabsorption of macronutrients, though these are uncommon in mild presentations. 1, 5, 4
Nutritional deficiencies, particularly fat-soluble vitamins (A, D, E, K) and vitamin B12, may manifest with symptoms such as night blindness, bone pain, neuropathy, or anemia. 1, 5 These deficiencies result from both bacterial consumption of nutrients and impaired absorption due to mucosal damage. 5
Critical Diagnostic Considerations
SIBO symptoms overlap extensively with irritable bowel syndrome (IBS), making clinical distinction impossible without diagnostic testing such as breath tests or small bowel aspiration. 1, 6 Up to 30% of patients with functional bowel symptoms may have underlying SIBO. 3, 2
Inflammatory markers should be normal in SIBO. Fecal calprotectin and other inflammatory markers remain within normal limits, and elevated levels should prompt investigation for inflammatory bowel disease or other pathology rather than SIBO. 1, 2
Symptom Severity Spectrum
The clinical presentation varies dramatically from mildly symptomatic patients with only occasional bloating to those suffering chronic diarrhea, significant weight loss, and severe malabsorption. 5, 4 This variability depends on the degree of bacterial overgrowth, specific bacterial species involved, and underlying predisposing conditions. 4, 6
High-Risk Clinical Contexts
Suspect SIBO particularly in patients with predisposing conditions including prior gastrointestinal surgery (especially gastric bypass or loss of ileocecal valve), chronic pancreatitis, diabetes mellitus with autonomic neuropathy, scleroderma, proton pump inhibitor use, opioid medications, or any condition causing intestinal dysmotility. 3, 1, 4, 7 These conditions impair the migrating motor complex that normally clears intestinal debris and prevents bacterial accumulation. 1
Common Diagnostic Pitfall
Do not assume bloating and gas alone confirm SIBO. These symptoms are nonspecific and occur in numerous gastrointestinal disorders including IBS, carbohydrate malabsorption (lactose, fructose), celiac disease, and functional dyspepsia. 3, 4, 7 Objective testing with glucose or lactulose breath testing, or in select cases small bowel aspirate culture, is required for diagnosis. 3, 1, 2