Fat-soluble vitamins A, D, E, and K are absorbed in micelle form in the small intestine. A patient with ileal resection of >100 cm is at MOST risk for deficiency of which vitamin, and why?
- A Vitamin K, because bile salt reabsorption at the ileum is impaired, reducing micellar solubilization of vitamin K
- B Vitamin B12, because the terminal ileum is the exclusive site of intrinsic factor-B12 complex (cubilin receptor) absorption ✓
- C Vitamin D, because 1α-hydroxylase in the ileal enterocyte activates 25-OH vitamin D before absorption
- D Vitamin A, because retinol esterification enzymes are concentrated in the ileal mucosa
Explanation
The terminal ileum (distal 60 cm) is the only intestinal site with cubilin receptors that bind the intrinsic factor-cobalamin (IF-B12) complex and mediate its endocytosis. No other intestinal segment can compensate for this loss. Therefore, ileal resection of >60–100 cm invariably leads to vitamin B12 malabsorption and megaloblastic anemia within 3–5 years (the time to deplete hepatic B12 stores). Additionally, bile salt reabsorption also occurs in the terminal ileum (enterohepatic circulation), and ileal loss also impairs fat-soluble vitamin absorption secondary to bile salt depletion, but B12 deficiency is the most predictable and universal consequence of large ileal resection.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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