Vitamin B-12

• Method of Action
• Consequences of Deficiency
• Deficiency Symptoms
• Recommended Dietary Allowances

• Properties and Uses
• Toxicity Levels
• Food Sources

(CYANOCOBALAMIN)
Description

Vitamin B-12 (Cyanocobalamin) is a component of the water-soluble vitamin B-Complex. It is essential for normal growth, nucleic acid synthesis, protein and lipid synthesis, and the maintenance of epithelial cells and the nervous system.

Vitamin B-12 was given the name cobalamin because of the presence of cobalt. The active form of this vitamin is hydroxycobalamin, and the vitamin is produced commercially as cyanocobalamin, the most stable form of the vitamin.

Method of Action

Calcium is necessary for the proper absorption of vitamin B-12. However, the vitamin is poorly absorbed from the intestinal tract unless a mucoprotein intrinsic factor is present. This intrinsic factor is part of the gastric juices and assists in absorption by binding to vitamin B-12, which helps attach the vitamin to a receptor in the membranes of the lower small intestine. The vitamin is then transported into the cells.

Vitamin B-12 is transported in the bloodstream bound to transcobalamin 1 and 11. In this way it circulates to the tissues, and is concentrated in the liver and, to some extent, the kidneys. Vitamin B-12 is then released as needed to other tissues of the body and to bone marrow.

Vitamin B-12 is involved in nucleic acid synthesis as well as protein, fat, and carbohydrate metabolism. Its primary function is the removal and transfer of methyl groups. In folacin metabolism a methyl group is removed by vitamin B-12 to form the active species of tetrahydrofolate. Vitamin B-12 is also essential for the normal metabolism and growth of the cells, especially those of the gastrointestinal tract, bone marrow, and nervous system tissues. It has been reported that vitamin B-12 assists in maintaining hormone balance.

Absorption appears to decrease with aging, iron deficiency, and vitamin B-6 deficiency. Tobacco, ethanol, coffee, and laxatives prevent absorption of the vitamin. Vitamin B-12 is destroyed by heating, acid, or alkali. Vitamin B-6, vitamin B-Complex, choline, inositol, vitamin C, potassium, and sodium assist absorption. Absorption increases during pregnancy.

Properties and Uses

Vitamin B-12 has been promoted as a treatment for night blindness, psoriasis, warts, problems related to menopause, and general malaise. However, no scientific evidence suggests that these problems result from a lack of vitamin B-12, or that they can be cured by its use. Currently the only clinical use for vitamin B-12 is to treat a metabolic defect in its absorption, such as pernicious anemia.

Vitamin B-12 supplements contain the active vitamin, as well as several analogs; the analogs have little or no vitamin activity. The presence of vitamin C, thiamine, and vitamin E in vitamin B-12 supplements may cause the creation of undesirable analogs, which can interfere with normal vitamin function.

Commercially produced vitamin B-12 is usually in the form of cyanocobalamin, which is produced by bacterial fermentation.

Consequences of Deficiency

If the normal vitamin B-12 levels of 100 to 1,000 micrograms per milliliter fall to below 100 micrograms, pernicious anemia occurs. Pernicious anemia can result from an inadequate dietary intake of the vitamin; a diminished secretion of intrinsic factor by the glands of the stomach; the partial or complete removal of the stomach; or from a lack of the transport proteins transcobalamin 1 and 11.

An additional cause of vitamin B-12 deficiency is the atrophy of the mucosal cells lining the stomach. This atrophy can result from genetic facts, alcoholism, iron deficiency, or thyroid gland disorders. An infection with the intestinal parasite Diphyllobothrium latum can produce pernicious anemia; the worm absorbs the available vitamin B-12, leaving little or none for the host's use. In the latter case, the anemia is easily cured by expulsion of the worm from the intestines.

Injections of vitamin B-12 can control pernicious anemia; however, the anemia must be detected at an early stage so that treatment starts prior to irreversible nerve degeneration. It is possible to diagnose vitamin B-12 deficiency on the basis of blood levels of the vitamin using microbiological techniques.

Since the body is efficient in storing vitamin B-12, it could take up to six years before vitamin B-12 deficiency symptoms develop in a diet devoid of the vitamin. Children of vegetarian parents can become vitamin B-12 deficient in two to three years, as they have no vitamin B-12 stores in their bodies.

Toxicity Levels

Vitamin B-12 is nontoxic to humans in doses exceeding the RDA by 10,000 times. Water-soluble vitamins in excess tend to be excreted in the urine, rather than stored in the tissues as are fat-soluble vitamins.

Recommended Dietary Allowances

• RDA for adult males 2.0 mcg
• RDA for adult females 2.0 mcg
• RDA for children 7 to 10 years 1.4 mcg
• RDA for infants: 0.5 mcg
• RDA for pregnant and lactating women 2.6 mcg

The amount of dietary vitamin B-12 needed for normal human metabolism appears to be very small. Reported minimum requirements have been from 0.6 to 1.2 mcg per day, with a range to approximately 2.8 mcg to allow for individual variance. The ordinary diet easily provides this amount. For example, one cup of milk, one egg, and four ounces of meat provide 2.4 mcg. The RDA standard recommends a daily intake of 2.0 mcg for adults.

Food Sources

Summary Deficiency Symptoms