Secondary SIBO is caused by which factor?

• A. Excess substrate in the lumen
• B. Mucosal disease
• C. Abnormal anatomy
• D. High fiber diet

Answer: (C)

Secondary SIBO arises when something disrupts normal small-intestinal flow or structure, creating stasis that lets bacteria accumulate and multiply in the small intestine. Abnormal anatomy is a classic driver because surgical changes (like blind loops, adhesions, strictures, diverticula, or fistulas) alter the path or speed of contents, providing a niche where the intestinal bacteria can overgrow. This slowed transit shifts the bacterial population toward organisms normally found in the colon, leading to overgrowth and malabsorption.

Excess substrate in the lumen or a high-fiber diet can feed bacteria and worsen symptoms, but they don’t by themselves establish the overgrowth. Mucosal disease can affect motility or absorption but isn’t the primary factor defining secondary SIBO; the defining driver is the presence of an anatomical or motility-related disruption that causes stasis.