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Nutrient Details For : Vitamin B 12

A nutrient is any element or compound necessary for or contributing to an organism's metabolism, growth, or other functioning. Click name for details Arsenic Biotin Boron Calcium Carbohydrates Carnitine Carotenoids Chloride Choline Chromium Copper Energy Fiber Fluoride Folate Iodine Iron Lipids Magnesium Manganese Molybdenum Niacin Nickel Pantothenic acid Phosphorus Phyto-chemicals Potassium Protein Riboflavin Selenium Silicon Sodium Thiamin Vanadium Vitamin A Vitamin B 12 Vitamin B 6 Vitamin C Vitamin D Vitamin E Vitamin K Zinc Further Trace Elements

Vitamin B12 is a member of the vitamin B complex. It contains cobalt, and so is also known as cobalamin. It is exclusively synthesised by bacteria

Vitamin B-12 (cobalamin), stored in the liver, is a cofactor for two coenzymes.

Methyl-cobalamin catalyzes methyl group transfer from a folic acid cofactor to form methionine; the unmethylated folate cofactor then participates in single carbon reactions for nucleic acid synthesis. Thus some B-12 and folic deficiency symptoms are similar.

The B-12 coenzyme deoxyadenosylcobalamin catalyzes amino acid and fatty acid breakdown.

Vitamin B12's primary functions are in the formation of red blood cells and the maintenence of a healthy nervous system. B12 is necessary for the rapid synthesis of DNA during cell division. This is especially important in tissues where cells are dividing rapidly, particularly the bone marrow tissues responsible for red blood cell formation.

Deficiencies:

If B12 deficiency occurs, DNA production is disrupted and abnormal cells called megaloblasts occur. This results in anaemia. Symptoms include excessive tiredness, breathlessness, listlessness, pallor, and poor resistance to infection. Other symptoms can include a smooth, sore tongue and menstrual disorders. Anaemia may also be due to folic acid deficiency, folic acid also being necessary for DNA synthesis.

B12 is also important in maintaining the nervous system. Nerves are surrounded by an insulating fatty sheath comprised of a complex protein called myelin. B12 plays a vital role in the metabolism of fatty acids essential for the maintainence of myelin. Prolonged B12 deficiency can lead to nerve degeneration and irreversible neurological damage.

Stages of deficiency symptoms include Stage I (early deficiency), lower serum holoTC II (<60 pg/mL); Stage II, lower serum vitamin B-12 (<300 pg/mL) and holoTC II (<40 pg/mL); Stage III, serum B-12 <200 and holoTC II <40 pg/mL, neutrophil hypersegmentation, elevated serum and urinary methylmalonic acid and homocysteine; and Stage IV (severest deficiency), also megaloblastic, macrocytic anemia. Around Stage III (before anemia), potentially irreversible demyelination of spinal cord, brain, and optic and peripheral nerves produces peripheral neuropathy progressing to subacute combined degeneration. Dementia, poor attention span, and depression may be early symptoms. The stomach secretes intrinsic factor that binds B-12 and mediates its absorption at receptor sites in the ileum. Inadequate intrinsic factor secretion occurs in pernicious anemia, an autoimmune disease.

In the elderly, atrophic gastritis is commonly associated with B-12 malabsorption and deficiency. Because the absorbed vitamin is secreted in bile and subsequently reabsorbed, deficiency symptoms can take 20 years to develop from low intakes, e.g., in strict vegetarians. However, in malabsorption, deficiency occurs in months or a few years because absorption from both the diet and enterohepatic circulation is impaired.

Recommendations:

The Recommended Daily Allowances (RDAs) are (µg/day): 0.3 at age 0-6 months, 0.5 for 6-12 months, 0.7 for 1-3 years, 1.0 for 4-6 years, 1.4 for 7-10 years, 2.0 for adolescents and adults, 2.2 in pregnancy and 2.6 in lactation. Usual intakes are about 4-8 µg/d. Pregnant, lactating, and long-term strict vegetarians should take supplements providing the RDA.

Food sources:

Vitamin B-12 is found only in animal products. Excellent sources (>10 µg/100g) include organ meats and bivalve mollusks such as clams and oysters. Moderate amounts (1-10 µg/100g) are contained in egg yolks, muscle meats and poultry, fish, fermented cheeses and dry milk. Milk and milk products contain <1 µg/100g. There is no human-active form of B-12 in algae such as nori and spirulina; the forms are all analogues.

Toxicity:

No toxic effects have been reported when up to 100 µg/day are consumed. Intramuscular injections of 100 µg are usually given once/month to individuals who cannot absorb the vitamin through their intestine, because of pernicious anemia or other problems.

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