Absorption of vitamin B12 is normally complex, involves multiple carriers leading to uptake of this micronutrient in the distal small intestine. Vitamin B12 is mainly from animal sources and, after ingestion, becomes complexed to haptocorrin derived from salivary glands to prevent acid destruction in the stomach. In the duodenum, pancreatic proteases hydrolyze this haptocorrin permitting vitamin B12 binding to intrinsic factor, a protein derived from gastric parietal cells. Linkage to intrinsic factor permits trafficking to the cubulin receptor in the ileum allowing entry into the enterocyte. After uptake, vitamin B12 exits the cell linking to another carrier protein in the blood, transcobalamin II. This process allows the micronutrient to circulate systemically to other cells. In celiac disease, one or more steps in this intestinal absorptive process may be impaired leading to significant neurologic, hematologic and, often poorly appreciated, further superimposed gastrointestinal effects.
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