Vitamin B₁₂ (cobalamin) deficiency in elderly patients

Box 1.

Fig. 1: Cobalamin metabolism and corresponding causes of deficiency. Causes of cobalamin deficiency are shown in blue. The metabolic pathway starts when (1) dietary cobalamin (Cbl), obtained through animal foods, enters the stomach bound to animal proteins (P). (2) Pepsin and hydrochloric acid (HCl) in the stomach sever the animal protein, releasing free cobalamin. Most of the free cobalamin is then bound to R-protein (R), which is released from the parietal and salivary cells. Intrinsic factor (IF) is also secreted in the stomach, but its binding to cobalamin is weak in the presence of gastric and salivary R-protein. (3) In the duodenum, dietary cobalamin bound to R-protein is joined by cobalamin–R-protein complexes that have been secreted in the bile. Pancreatic enzymes degrade both biliary and dietary cobalamin–R-protein complexes, releasing free cobalamin. (4) The cobalamin then binds with intrinsic factor. The cobalamin–intrinsic factor complex remains undisturbed until the distal 80 cm of the ileum, where (5) it attaches to mucosal cell receptors (cubilin) and the cobalamin is bound to transport proteins known as transcobalamin I, II and III (TCI, TCII and TCIII). Transcobalamin II, although it represents only a small fraction (about 10%) of the transcobalamins, is the most important because it is able to deliver cobalamin to all cells in the body. The cobalamin is subsequently transported systemically via the portal system. (6) Within each cell, the transcobalamin II–cobalamin complex is taken up by means of endocytosis and the cobalamin is liberated and then converted enzymatically into its 2 coenzyme forms, methylcobalamin and adenosylcobalamin (this process is shown in greater detail in Fig. 2).

*Nitrous oxide, a general anesthetic, causes multiple defects in cobalamin use, most of which are intracellular and clinically relevant only in people who have low or borderline-low serum cobalamin levels. Photo: Christine Kenney

Fig. 2: A. Cellular uptake and processing of cobalamin. Cobalamin (Cbl) bound to the transport protein transcobalamin II (TCII) enters cells by means of transcobalamin II receptor-mediated endocytosis. Lysosomal enzymes degrade the transcobalamin II, thereby freeing the cobalamin. Cob(III)alamin (CBL^III) represents the most oxidized form of cobalamin, and cob(II)alamin (CBL^II) and cob(I)alamin (CBL^I) represent reduced forms. In the mitochondria, cobalamin is converted to adenosylcobalamin (AdoCbl), a coenzyme involved in the conversion of methylmalonyl-CoA (MM-CoA) to succinyl-CoA. In the cytoplasm, cobalamin functions as a coenzyme for the reaction catalyzed by methionine synthase. PteGlu = folic acid, MeCbl = methylcobalamin. B. Intracytoplasmic biochemical pathways involving cobalamin. BHMT = betaine-homocysteine S-methyltransferase, NADP = nicotinamide adenine dinucleotide phosphate, NADPH = reduced form of NADP. Photo: Christine Kenney

Box 2.

Fig. 3: Diagnostic process for cobalamin deficiency. Photo: Christine Kenney