Small intestinal bacterial overgrowth can impair absorption of which vitamin?

• A. Thiamine
• B. Vitamin C
• C. Vitamin D
• D. Vitamin K

Answer: (A)

Small intestinal bacterial overgrowth disrupts nutrient absorption by crowding the small intestine with bacteria that compete with the host for nutrients and can damage the mucosa. Thiamine (vitamin B1) is absorbed in the proximal small intestine, and when bacterial numbers are high, they can consume or inactivate thiamine, reducing its availability for absorption. This makes thiamine particularly susceptible to impairment in SIBO, leading to deficiency symptoms if the overgrowth is significant. While fat-soluble vitamins (A, D, E, K) can be affected by fat malabsorption caused by bile salt deconjugation, the strongest link in this context is the impact on thiamine due to bacterial competition for a water-soluble B vitamin.