Vitamin B-12 (cobalamin) is a cofactor for 2 enzymes and is important for DNA synthesis, regenerating methionine for protein synthesis and methylation, and preventing homocysteine accumulation (implicated in Alzheimer disease). Vitamin B-12 is found naturally only in animal source foods such as dairy products, meat, eggs, fish, and shellfish. These contain 1–10 μg/100 g wet weight. Liver and kidney contain >10 μg/100 g, but the efficiency of vitamin B-12 absorption decreases markedly when a meal contains more than 2 μg, so the effective content of these sources becomes only 20% of that listed in food composition tables. Lacto-ovo vegetarians, who avoid meat in their diet, have lower intakes and serum concentrations of the vitamin than omnivores. People who strictly avoid all animal source foods will definitely become deficient unless they take supplements or fortified foods. Fortified foods, such as breakfast cereals, often supply an important proportion of the usually daily intake.4 Lifelong high B12 intake will result in sustained elevated serum B12.
Cobalamin has important metabolic functions. Decreased serum cobalamin concentrations (hypocobalaminemia ) can cause neurological, hematological (pernicious anemia), cardiovascular and catabolic disorders in man. High folic acid intake adversely influences the vitamin B-12 status, which affects many elderly individuals.3 Hypocobalaminemia in cats leads to encephalopathy, myelopathy, anemia, loss of appetitie, cold-intolerance and failure to thrive. Hypocobalaminemia has been seen in cats with gastrointestinal disease (decreased serum cobalamin can help to localise gastrointestinal disease), exocrine pancreatic insufficiency and hyperthyroidism.
When increased blood cobalamin concentrations (hypercobalaminemia) are encountered clinically they are usually attributed to supplementation, dietary factors, or otherwise ignored. However, recently, exessive concentration has been associated with numerous diseases in humans, most notably neoplastic and hepatic disorders (liver tumors and metastases) and chronic myeloid leukemia. Increased serum cobalamin concentrations in cats suggests the presence of either solid tumor or liver disease. Veterinarians should pay greater attention to when investigating sick cats and should consider thorough evaluation for tumor or hepatic disease where there is no evidence of prior supplementation.1