Cobalamin deficiency is defined in terms of the serum values of cobalamin and of homocysteine and methylmalonic acid, 2 components of the cobalamin metabolic pathway. High homocysteine levels (hyperhomocysteinemia) may also be caused by folate or vitamin B₆ deficiencies, and these should be excluded as causes of cobalamin deficiency before a diagnosis is made. To obtain cutoff points of cobalamin serum levels, patients with known complications are compared with age-matched control patients without complications. Because different patient populations have been studied, several serum concentration definitions have emerged.5,6,7 Varying test sensitivities and specificities result from the lack of a precise “gold standard.” The definitions of cobalamin deficiency used in this review are shown in Box 1 Based in part on the work of Klee7 and in part on our own work,8 they are calculated for elderly patients. The first definition is simpler to interpret, but it requires that blood samples be drawn on 2 separate days.

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

First described by Carmel in 1995,22 food-cobalamin malabsorption syndrome is characterized by the inability to release cobalamin from food or from intestinal transport proteins (Table 1), particularly in the presence of hypochlorhydria, where the absorption of “unbound” cobalamin remains normal. As various studies have shown,14,22,23 this syndrome is defined by cobalamin deficiency in the presence of sufficient food-cobalamin intake and a negative Schilling test, where the latter rules out malabsorption or pernicious anemia (Box 2). In theory — because the test is rarely practical in clinical settings — the indisputable evidence of food-cobalamin malabsorption comes from using a modified Schilling test, which uses radioactive cobalamin bound to animal proteins (e.g., salmon, trout) and reveals malabsorption when the results of a standard Schilling test are normal.4,22

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