Cobalamin (Vitamin B12)

B₁₂ is unique in that it is the first cobalt-containing substance found to be an essential nutrient. It is the only vitamin that contains essential mineral elements. Animal protein is almost the only place that contains B12, although vegetarians can get it from microbial synthesis and from legume nodules where it is synthesized by microbes.
    Vitamin B₁₂ is necessary for normal metabolism of nerve tissue and is involved in protein, fat and carbohydrate metabolism. B₁₂ aids folic acid in the synthesis of choline. It helps the placement of vitamin A into body tissues.
    B₁₂ is poorly absorbed unless intrinsic factor, a mucoprotein secreted in the stomach, is present. Autoimmune reactions in the body can bind intrinsic factor or can affect the cells that produce it. Absorption of B₁₂ appears to decrease with age, and with iron, calcium and B6 deficiencies. Absorption increases during pregnancy.
    Pernicious anemia develops insidiously and progressively as the large hepatic stores of B₁₂ are depleted. It may take five or six years to develop. Usually the problem is more profound than what would be expected from the symptoms. This is due to physiologic adaptation. Splenomegaly and hepatomegaly may occasionally be seen. GI problems may be present, including anorexia, intermittent constipation and diarrhea and poorly localized abdominal pain. Considerable weight loss is common. Peripherial nerves are commonly involved, even in the absence of anemia. Second to this is spinal cord involvement beginning in the dorsal column with loss of vibratory sensation in the lower extremities, loss of position sense and ataxia. Lateral column involvement follows with spasticity and hyperactive reflexes and a Babinski's sign. Some patients have irritability, mild depression or actual paranoia. Occasionally yellow-blue color blindness occurs.
    Rare signs are fever of unknown origin that responds promptly to B₁₂ therapy. Endocrine deficiencies, especially of the thyroid and adrenal glands, if they are associated with pernicious anemia, suggest an autoimmune basis for gastric mucosal atrophy. Hypogammaglobulinemia may be present.
    Anemia is macrocytic with an MCV>100, but some physicians will consider supplementing with B₁₂ when the MCV gets above 90. Measuring B₁₂ in the serum is the common way to test, but there is a possibility that some deficiencies may be missed. Many physicians test for methylmalonic acid to get a sense of B₁₂ status.