Vitamin B12's primary functions are in the formation of red blood cells and the maintenance 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. 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 maintenance of myelin. Prolonged B12 deficiency can lead to nerve degeneration and irreversible neurological damage.
Vitamin B12 can be stored in small amounts by the body. Total body store is 2-5mg in adults. Around 80% of this is stored in the liver. A standard daily intake of 10-25mcg is generally sufficient
Vitamin B12 is excreted in the bile and is effectively reabsorbed. This is known as enterohepatic circulation. The amount of B12 excreted in the bile can vary from 1 to 10ug (micrograms) a day. People on diets low in B12, including vegans and some vegetarians, may be obtaining more B12 from reabsorption than from dietary sources. Reabsorption is the reason it can take over 20 years for deficiency disease to develop in people changing to diets absent in B12. In comparison, if B12 deficiency is due to a failure in absorption it can take only 3 years for deficiency disease to occur.
Although quite rare, serious allergic reactions to vitamin B12 (sometimes even life-threatening) have been reported. Whether these reactions are to the vitamin itself, or to preservatives or other substances in the injectable vitamin B12 solution, remains somewhat unclear. Most, but not all, injectable vitamin B12 contains preservatives.
If a person is deficient in vitamin B12 and takes 1,000 mcg or more of folic acid per day, the folic acid supplementation can improve the anemia caused by vitamin B12. The effect of folic acid on vitamin B12 deficiency-induced anemia is not a folic acid toxicity. Rather, the folic acid supplementation is acting to correct one of the problems caused by B12 deficiency. The other problems caused by a lack of vitamin B12 (mostly neurological) do not improve with folic acid supplements, and can become irreversible if vitamin B12 is not provided to someone who is vitamin B12 deficient.
There is no known lethal dose of B12, what you do not use you urinate out
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