Articles of Weight loss,Vitamin B12

Viamin B₁₂

Vitamin B₁₂  (Cobalamin) is an essential nutrient in our diet. Vitamin B₁₂ is required for development of red blood Cells and is used to make the protective coating surrounding nerves. It exists in different chemical forms.  All the Vitamin B₁₂ have a cobalt atom at the centre of a corrin ring. The major natural vitamin B₁₂ (cobalamin) is methylcobalamin, found in human plasma and in cell cytoplasm.  It is the cofactor for methionine synthase.   In nature, the Vitamin b12 is mainly in the 2-deoxyadenosyl (ado) form, which is located in mitochondria.  It is the cofactor for the enzyme methylmalonyl CoA mutase. There are also minor amounts of hydroxocobalamin to which methyl and adocobalamin are rapidly converted by exposure to light. The Vitamin B₁₂ is synthesized solely by microorganisms. The only source of Vitamin B₁₂ for humans is food of animal origin. i.e. meat, fish and dairy products.  Vegetables, fruits, and other foods of non animal origin are free from Vitamin B₁₂ unless they are contaminated by bacteria.  A normal Western diet contains between 5 and 30µg of Vitamin B₁₂ daily.  An adult daily losses between 1 and 3µg (-0.1% of body stores) of Vitamin B₁₂ by urine and feces, and as the body does not have the ability to degrade Vitamin B₁₂, daily requirements are also about 1-3 µg.  Body stores are of the order of 2-3mg, sufficient for 3-4 years if supplies are completely cut off. 

Out of the two mechanisms existing for absorption of Vitamin B₁₂, one is passive, occurring equally through buccal, duodenal and ileal mucosa, is rapid but extremely inefficient, less than 1% of an oral dose is being absorbed by this process. The normal physiologic mechanism is active; it occurs through the ileum and is efficient for small oral doses of Vitamin B₁₂ and is medicated by gastric intrinsic factor. Dietary Vitamin B₁₂ is released from protein complexes by enzymes in the stomach, duodenum and jejunum; it combines rapidly with a salivary glycoprotein that belongs to the family of cobalamin-binding proteins known as haptocorrins.  In the intestine, the haptocorrin is digested by pancreatic trypsin and the Vitamin B12 is transferred to intrinsic factor. The intrinsic factor-cobalamin complex passes to the ileum, where intrinsic factor attaches to a specific receptor(cubilin) on the microvillus membrane of the enterocytes.  The Vitamin B₁₂ –intrinsic factor complex enters the ileal cell where intrinsic factor is destroyed.  After about 6 hours, the Vitamin B₁₂ appears in portal blood attached to transcobalamin (TC)II.  A major portion of biliary Vitamin B₁₂ is normally reabsorbed together with Vitamin B₁₂ derived from sloughed intestine cells.

There are two main Vitamin B₁₂ transport proteins exists in human plasma; they both bind Vitamin B₁₂ . One haptocorrin, known as transcobalamin I, is closely related to other cobalamin-binding haptocorrins in milk, gastric juice, bile, saliva, and other fluids.  It is about two-thirds saturated with cobalamin.  The other major Vitamin B₁₂ transport protein in plasma is transcobalamin II.  This is synthesized by liver and other tissues.  It gives up cobalamin to marrow, placenta, and other tissues.