Introduction Vitamin B12

Populations at risk for, and consequences of, vitamin B12 deficiency

Vegetarians
Because plants do not synthesize vitamin B12, individuals who consume diets completely free of animal products (vegan diets) are at risk of vitamin B12 deficiency. This is not true of lacto-ovo vegetarians, who consume the vitamin in eggs, milk, and other dairy products.

Pernicious anaemia
Malabsorption of vitamin B12 can occur at several points during digestion (1, 4). By far the most important condition resulting in vitamin B12 malabsorption is the autoimmune disease called pernicious anaemia (PA). In most cases of PA, antibodies are produced against the parietal cells causing them to atrophy, and lose their ability to produce intrinsic factor and secrete hydrochloric acid. In some forms of PA, the parietal cells remain intact but autoantibodies are produced against the intrinsic factor itself and attach to it, thus preventing it from binding vitamin B12. In another less common form of PA, the antibodies allow vitamin B12 to bind to the intrinsic factor but prevent the absorption of the intrinsic factor–vitamin B12 complex by the ileal receptors. As is the case with most autoimmune diseases, the incidence of PA increases markedly with age. In most ethnic groups, it is virtually unknown to occur before the age of 50, with a progressive rise in incidence thereafter (4). However, African American populations are known to have an earlier age of presentation (4). In addition to causing malabsorption of dietary vitamin B12, PA also results in an inability to reabsorb the vitamin B12 which is secreted in the bile. Biliary secretion of vitamin B12 is estimated to be between 0.3 and 0.5mg/day. Interruption of this so-called enterohepatic circulation of vitamin B12 causes the body to go into a significant negative balance for the vitamin. Although the body typically has sufficient vitamin B12 stores to last 3–5 years, once PA has been established, the lack of absorption of new vitamin B12 is compounded by the loss of the vitamin because of negative balance. When the stores have been depleted, the final stages of deficiency are often quite rapid, resulting in death in a period of months if left untreated.

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