Vitamin B-12 is an essential nutrient that cannot be synthesized by humans. The human liver is the major site of vitamin B-12 storage, and most stores can last approximately 3 years without replacement. The terminal ileum is the exclusive site of vitamin B-12 absorption in humans. Most urinary diversions in which ileum is used are performed in a way to compensate for the physiology of vitamin B-12 absorption; however, more than 50 cm of resection appears to be the critical length at which abnormal B-12 absorption may be expected.
With the addition of intrinsic factor, Pannek et al demonstrated that 20 of 25 (80%) patients with more than 50 cm of ileum who underwent resection for continent diversion demonstrated abnormal Schilling test results.3 Patients with shorter limbs of ileum are not immune to vitamin B-12 deficiency and/or megaloblastic anemia; symptoms may take longer to manifest. Long-term follow-up studies in patients with shorter ileal segments demonstrate a 25-28% incidence of complications from vitamin B-12 deficiency. Patients in whom more than 50 cm ileum is resected are at increased risk for vitamin B-12 deficiency, which usually does not become clinically apparent for at least 2 years.
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