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What is Vitamin B12 or Cobalamin + Food Sources of Vitamin B12 + Roles in the Body, Requirements and Deficiency and Toxicity

What is vitamin B12 or Cobalamin?

Tucked away in the central part of the vitamin B12 molecule is an atom of cobalt. Therefore molecules that have vitamin B12 activity have been named the cobalamins. Because vitamin B12 plays a role in activities that process energy nutrients, it holds its place on the roster of B-complex vitamins.


What foods provide this vitamin?

In the human diet, vitamin B12 is only found in foods of animal origin. Unlike animals, plants do not have a functional role for vitamin B12 and therefore do not make it. Interestingly, animals do not seem to make vitamin B12 either and rely instead upon their intestinal bacteria to make it. Vitamin B12 is then absorbed into that animal’s body from its digestive tract. The best sources of vitamin B12 are meats, fish, poultry, shellfish, eggs, milk, and milk products (see Vitamin B12 in Food Table). The vitamin B12 content in these foods is modest but compatible with our needs.

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How much vitamin B12 do we need?

The RDA for adults is 2.4 micrograms regardless of age and gender. Meanwhile during pregnancy and lactation the recommendation increases for women to 2.6 and 2.8 micrograms respectively.See the DRI/RDA Table for recommended levels of Vitamin B12 for children and teens and across the entire lifespan.

Are there special factors involved in the absorption of vitamin B12?

The absorption of vitamin B12 needs a little help. Special proteins called R proteins and intrinsic factor produced by the stomach must interact with vitamin B12 both in the stomach and small intestine and facilitate its absorption. A lack of these proteins can reduce vitamin B12 absorption dramatically. This might be a concern for people who lack a properly functioning stomach, such as people who have had their stomach stapled or part (or all) of the stomach removed (gastroplasty).

How much vitamin B12 is lost from the body daily?

Once vitamin B12 is in the body it stays there for a while. Very little amounts of this vitamin are actually lost from the body on a daily basis, barring abnormalities. Contrary to the other water-soluble vitamins, the primary route of vitamin B12 loss from the body is not by way of the urine but rather in feces. The liver mixes a little vitamin B12 in with bile, which carries it to the digestive tract. A small portion of this vitamin B12 is not reabsorbed and becomes part of feces.


What does vitamin B12 do in the body?

Vitamin B12 is directly involved in the proper metabolism of folate. In fact, a deficiency of vitamin B12 can impact folate metabolism to the point that signs of a folate deficiency appear. When folate is used to make molecules it is rendered “unusable,” for lack of a better word. Vitamin B12 is involved in converting folate back to a usable form. Said another way, vitamin B12 is involved in folate recycling. This dramatically reduces the amount of folate we need to eat daily to have optimal levels of usable folate in our cells.

Vitamin B12 is also required for the breakdown of certain amino acids and fatty acids that have an odd chain length (e.g. 3 carbons) for ATP production. Finally, vitamin B12 appears vital in maintaining the special insulating wrapping around nerve cells called myelin. Myelin serves as insulation, which increases the velocity of a nerve impulse traveling from one part of the body to another.


What happens if too little vitamin B12 is consumed?

Contrary to other water-soluble vitamins, vitamin B12 losses from the body are small and occur primarily through the feces. Small quantities of vitamin B12 enter the digestive tract daily as part of bile released during meals. Most of this vitamin B12 is reabsorbed from the digestive tract while some is lost through feces. It has been estimated that we lose only about 0.1 percent of vitamin B12 stores daily through this process. Therefore, provided that there is optimal vitamin B12 reabsorption from the digestive tract, a person with good vitamin B12 stores could eat a diet lacking vitamin B12 for years before showing signs of deficiency—at least in theory.


Deficiency of vitamin B12 will result in a form of anemia in which red blood cells appear large and immature (macrocytic megalo­blastic anemia). About 150 years ago, English physicians recognized that people with this type of anemia often died. They called this illness pernicious anemia, as pernicious means “leading to death.” This anemia is usually related to the involvement of vitamin B12 in folate metabolism and DNA production. People who are vitamin B12 deficient also show destruction of nerve myelin, which can lead to nerve impulse conduction disturbances, paralysis, and ultimately death.


What situations can result in vitamin B12 deficiency?

There are a couple of situations that can lead to a deficiency of vitamin B12. These include:

  • Strict vegetarian - Eating only plant-derived foods (vegan) without vitamin B12 supplementation will eventually lead to deficiency. For individuals who became a vegetarian later in life may not show signs of a vitamin B12 deficiency for a long time but for children the onset of deficiency will be shorter. In either case the time to deficiency depends on the level of body B12 stores prior to conversion.

  • Absorption/Digestion Conditions - Factors that affect vitamin B12 digestion and absorption are more likely to cause vitamin B12 deficiency than insufficient dietary intake. Diseases and surgical manipulation of the stomach (i.e., removal and stapling) can affect its ability to make and release adequate intrinsic factor and R proteins. This can result in a dramatic decrease in vitamin B12 absorption. These proteins are very important in absorbing vitamin B12 in food and also reabsorbing the vitamin B12 entering the small intestine as part of bile.

  • Aging - Older people are at increased risk of a vitamin B12 deficiency as their stomachs lose the ability to make sufficient acid with age. Stomach acid helps liberate the vitamin B12 in food so that it can interact with R proteins and intrinsic factor. Beyond anemia, other signs of a vitamin B12 deficiency include weakness, back pain, apathy, and a tingling in the extremities. These signs and symptoms usually appear before significant nerve damage occurs.

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