The Ileum is the Major Site of Absorption of Vitamin B₁₂ Analogues

Non‐cobalamin vitamin B₁₂ analogues constitute a significant percentage of total corrinoids in human serum. The source and means of absorption of analogues and their significance are largely unknown. We studied the sites of production and absorption of IV analogues by measuring serum vitamin B₁₂ and analogues in 93 patients with various gastrointestinal diseases: pernicious anemia (PA), ileal resections, ileitis, Crohn's colitis, ulcerative colitis, and irritable bowel syndrome (IBS). Patients with PA had normal analogue levels that were unchanged or that rose during cessation of B₁₂, administration. Patients with IBS, Crohn's colitis, ulcerative colitis, and total colectomies had B₁₂ analogues in the normal range. Patients with diseased or resected ileums had low B₁₂ and analogues. These data suggest that serum B₁₂ analogues are absorbed in the ileum by a mechanism independent of intrinsic factor, and that colonic bacteria and endogenous metabolism of vitamin B₁₂ do not contribute significantly to their level.