Vitamin B12(cobalamin) is a compound that contains the mineral cobalt. It is slightly soluble in water and fairly stable to heat, but it is damaged by strong acids or alkalies and by light. It can be stored in the human body for 3 to 5 years.
Functions.Vitamin B12is involved in folate metabolism, maintenance ofthe myelin sheath, and healthy red blood cells. In order for vitamin B12 to be absorbed, it must bind with a glycoprotein (intrinsic factor) present in gastric secretions in the stomach and travel to the small intestine, where it combines with pancreatic proteases, then travels to the ileum, where it attaches to special recep-tor cells to complete the absorption process. A patient who has lost the ability to produce the gastric secretions, pancreatic proteases, intrinsic factor, or the special receptor cells because of disease or surgery will develop pernicious anemia.
Sources.The best food sources of B12are animal foods, especially organmeats, lean meat, seafood, eggs, and dairy products.
Requirements.Vitamin B12is measured in micrograms. The DRI foradults is 2 to 4μg a day, but it increases during pregnancy and lactation. The amount absorbed will depend on current needs.
Deficiency.Fortunately, a vitamin B12deficiency is rare and is thought tobe caused by congenital problems of absorption, which inhibit the body’s abil-ity to absorb or synthesize sufficient amounts of vitamin B12. It may also be due to years of a strict vegetarian diet that contains no animal foods.
When the amount of B12 is insufficient, megaloblastic anemia may result. If the intrinsic factor is missing, pernicious anemia develops. Intrinsic factor could be missing because of surgical removal of the stomach, or a large portion of it, or because of disease or surgery affecting the ileum. Dietary treatment will be ineffective; the patient must be given intramuscular injections of B12, usu-ally on a monthly basis.
Vitamin B12 deficiency may also result in inadequate myelin synthesis. This deficiency causes damage to the nervous system. Signs of vitamin B12 defi-ciency include anorexia, glossitis, sore mouth and tongue, pallor, neurological upsets such as depression and dizziness, and weight loss.