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Vitamin B12

Vitamin B12, also called cobalamin, is a water-soluble vitamin needed for the normal functioning of the brain and nervous system, and for the formation of blood. 
Only bacteria can synthesise vitamin B12, and if present in the human intestine, can be a source of endogenous vitamin B12. Vitamin B12 is present in animal products, such as meat, poultry, fish (including shellfish), and to a lesser extent dairy products and eggs as a result of bacterial fermentation in the animal's digestive tract, or consumption of other animals or algae containing B12. Vegans may need supplemental vitamin B12 to meet their requirements, although some plant-source foods, such as certain fermented beans and vegetables and edible algae and mushrooms contain substantial amounts of bioactive vitamin B12. Individuals over the age of 50 may also need to obtain their vitamin B12 in supplements or fortified foods (e.g., fortified cereals) due to the increased likelihood of vitamin B12 malabsorption with increasing age.

A number of drugs reduce the absorption of vitamin B12: 
  • Proton-pump inhibitors (e.g., omeprazole and lansoprazole), used for therapy of  gastroesophageal reflux disease, markedly decrease stomach acid secretion required for the release of vitamin B12 from food but not from supplements. Long-term use of proton-pump inhibitors has been found to decrease blood vitamin B12 levels. However, vitamin B12 deficiency does not typically develop until after at least three years of continuous therapy. 
  • Gastric acid inhibitors known as Histamine2 (H2)-receptor antagonists (cimetidine and ranitidine), often used to treat peptic ulcer disease, have also been found to decrease the absorption of vitamin B12 from food. Individuals taking drugs that inhibit gastric acid secretion should consider taking vitamin B12 in the form of a supplement because gastric acid is not required for its absorption. 
  • Cholestyramine (a bile acid-binding resin used in the treatment of high cholesterol), chloramphenicol and neomycin (antibiotics), and colchicine (medicine for gout treatment) inhibit vitamin B12 absorption from food
  • Metformin, an oral medication for individuals with type 2 diabetes, was found to decrease vitamin B12 absorption by binding to free calcium required for absorption of the IF-B12 complex.
  • Nitrous oxide, a commonly used anesthetic, oxidizes and inactivates vitamin B12, thus inhibiting both of the vitamin B12-dependent enzymes, and can produce many of the clinical features of vitamin B12 deficiency, such as megaloblastic anemia or neuropathy. Nitrous oxide is commonly used for surgery and childbirth.
High-dose folic acid given to an individual with an undiagnosed vitamin B12 deficiency may correct megaloblastic anemia without correcting the underlying vitamin B12 deficiency, leaving the individual at risk of developing irreversible neurologic damage.

The commonest form of vitamin B12 supplementation is cyanocobalamin. Some people have genetic polymorphisms of B12 metabolism or methylation, and may need to use different types of vitamin B12 supplements, such as methylcobalamin or hydroxycobalamin. Vitamin B12 can be taken orally or given by intramuscular injection, and is a component of the intravenously administered Myers cocktail.

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