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not present in plants Daily requirement: ~ 23 g/day Body stores: 25 mg , mostly in the liver (sufficient for 25 years) Absorption Pathway : In the mouth/stomach: B12 is protein-bound in food Stomach acid and pepsin release B12 from food proteins B12 binds to haptocorrin (also called R-binder or transcobalamin I) secreted by salivary glands and gastric mucosa In the small intestine : In the duodenum , pancreatic proteases degrade haptocorrin Free B12 then binds to intrinsic factor (IF) , a glycoprotein secreted by gastric parietal cells The B12IF complex travels to the terminal ileum , where it binds to cubilin receptors and is absorbed via receptor-mediated endocytosis 1 In the enterocyte and bloodstream: Inside enterocytes, B12 dissociates from IF It binds to transcobalamin II (TCII) for systemic transport Only B12TCII complex (called holotranscobalamin ) is biologically active and taken up by tissues Storage and transport: B12 is stored in the liver (main site), but also in muscle Transcobalamin II transports active B12 to tissues
Methylcobalamin is one of the biologically active forms of B12
This also emphasizes that symptoms and not blood values should be used as a guideline
Its briskness should be proportional to the severity of the anemia
To further support the claim that the model is minimal, we performed a parameter identifiability analysis as detailed in the Methods - Uncertainty estimation section
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