Dr. Nicholas Dodman
Vitamin B12 (cyanocobalamin and hydroxocobalamin) is essential to growth, cell reproduction, hematopoesis, and nucleoprotein and myelin synthesis.
It works in close concert with folate in the synthesis of the building blocks for DNA and RNA synthesis. It is also essential for the maintenance of the integrity of the nervous system and for the synthesis of molecules involved in fatty acid biosynthesis, amino acid metabolism, and the production of energy.
The cobalt-containing B12 molecule is the largest molecule to be transferred across the intestinal mucosa and transfer occurs only in the ileum. B12 is generally absent from plant and vegetable food sources, unless the food is contaminated by microbes. Ingested B12 is bound to R-proteins and must be liberated by calcium, proteases, and bicarbonate in the duodenum before it can be absorbed. Free B12 then attaches to intrinsic factor (IF) to be transported across ileal cells as a dimer. It is then transported to the liver in the bloodstream bound to transcobalamin-2 (TC-2). The liver is a rich repository for B12 and releases it, as needed into the systemic blood stream. Frank deficiency of B12 or gastric, intestinal, or pancreatic disorders that affect the absorption of B12 will lead to B12 deficiency. Deficiency of B12 causes pernicious (megaloblastic) anemia by affecting DNA production. Also, since B12 is involved in the synthesis of lipid and protein components of myelin, demyelination and nerve degeneration will occur in severe B12 deficiency.
The most important reactions in the body in which B12 is involved are:
a) rearrangement of methylmalonyl-CoA to succinyl-CoA for use in gluconeogenesis b) transfer of a methyl group from N5-methyl tetrahydrofolate to homocysteine in the formation of methionine [under the influence of methlycobalamin synthase] with resultant formation of tetrahydrofolate. Tetrahydrofolate is involved in purine, pyrimidine, and nucleic acid biosynthesis.
Clinical signs of B12 deficiency include inappetance, lethargy, and failure to thrive. Hematological signs include non-regenerative megaloblastic anemia, anisocytosis, poikilocytosis, and neutropenia with hypersegmentation, Biochemical signs of B12 deficiency include reduced serum cobalamin levels, increased serum methylmalonic acid levels, homocystinuria and methylmalonuria.
A congenital B12 malabsorption syndrome has been reported in giant schnauzers, border collies, beagles, and in cats.
Brand Names and Other Names
This vitamin is registered for use in humans.
Human formulations: Big Shot-12® (Naturally), Crystamine® (Dunhall), Cyanojet® (Mayrand), Cyomin® (Forest), Rubesol® (Central), Hydro-Crsyti-12® (Roberts Hauck), LA-12® (Hyrex), Nascobal® (Nastech), and generic Vitamin B12 (injectable and oral preparations).
Veterinary formulations: None (except as part of multivitamin formulations)
Uses of B12
Treatment of primary or secondary B12 deficiency.
Precautions and Side Effects
Vitamin B12 is essentially non-toxic but it should not be given to animals with known hypersensitivity to it or to cobalt. Anaphylactic shock and death have been reported in humans following administration of vitamin B12 by injection.
A number of drugs are known to affect the absorption of vitamin B12, including neomycin, potassium chloride, p-aminosalicylic acid, and colchicine.
How B12 is Supplied
Vitamin B12 is supplied in a varity of methods, including:
100 mcg, 250 mcg, 500 mcg, 1000 mcg, and 5000 mcg tablets
Gel, intranasal: 500 mcg/0.1 mL in 5 mL bottles
500 mcg and 1000 mcg tablets; 100 mcg/mL injection (30ml vial); 1000 mcg/mL injection (10 and 30 mL vials).
1000 mcg/mL injection (30mL vial)
By injection: 50 mcg/kg every 2 weeks (effective in one study)
Oral treatment of B12 deficiency is not always effective, depending on the cause. Intrinsic factor (IF) may have to be supplied to facilitate B12 absorption. It can be dosed at 100 to 200 mcg/kg once daily.
No good data are available
Oral treatment unlikely to be of much benefit in malabsorption states (e.g. pancreatic disease interfering with the production and release of pancreatic IF). Vitamin B12 can be dosed at 50 to 100 mcg/kg once daily.
Note: the human RDA of B12 is approximately 2 mcg per day.