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Vitamin B12
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What Is It?

Vitamin B12 is a water-soluble vitamin which is linked up with small peptides, or protein-like compounds, when it occurs in foods. When food gets to the stomach, the B12 and peptides are split by enzymes, and the B12 joins up with a special protein called intrinsic factor (IF) that is manufactured in the stomach. Vitamin B12 from food cannot get into the body unless it is hooked up to IF. Pernicious anemia results when the body does not make enough IF. The chemical name for vitamin B12 is cobalamin. Free cobalamin is unstable outside the body, so it is usually added to supplements as a stable derivative called cyanocobalamin.

What Does It Do?

Vitamin B12 is needed to:
  • produce normal red blood cells
  • prevent megaloblastic anemia
  • maintain myelin coating in the spinal cord, peripheral nerves and optic nerve
  • synthesize RNA and DNA
  • promote normal growth in children
  • use fat and protein from foods effectively
  • help slow the progression from HIV-positive status to AIDS
  • prevent homocysteine accumulation, a risk factor in heart disease
Where Do You Get It?
Vitamin B12 is found in:
  • beef, poultry, fish and dairy products
  • fortified cereals, fruit juice, soy and bread products
  • organ meats, such as liver and kidney
  • egg yolks
  • crab, oyster, salmon and herring
Fruits and vegetables contain no vitamin B12. Anything that grows in soil does not contain B12. Bacteria, fungi and algae can produce it, but plants cannot. Animals cannot produce vitamin B12 either; we get it from animal sources where it was already produced by bacterial synthesis. Our own small intestines have bacteria that make vitamin B12, but it is not absorbed.

How Much Do We Need?

The Recommended Daily Allowance, or RDA, is 2.4 micrograms a day. The average adult diet in the U.S. provides about four to eight micrograms a day. Certain groups are especially at risk for vitamin B12 deficiency.
  • Some people over age 50 make less gastric acid and pepsin in their stomachs and therefore do not absorb enough vitamin B12 from food. This is called atrophic gastritis.
  • Older people have an overgrowth of bacteria in their stomachs. The bacteria that are usually killed by stomach acid compete for the vitamin B12 that is available and take it for their own use.
  • The elderly and strict vegetarians should eat fortified cereals and soy foods, or take a supplement, to get enough vitamin B12. If you are in either of these categories, be careful not to rely on label amounts of vitamin B12 in foods such as tempeh; this number represents not only vitamin B12, but also some analogs of it which are not absorbed.
If you have a deficiency of vitamin B12, or are at high risk for a deficiency because of your age or diet, the National Academy of Sciences recommends taking a supplement or eating fortified breakfast cereals to make sure you absorb enough vitamin B12. (The amounts added to fortified foods are not bound to protein and should be absorbed quite well.) Typical one-a-day vitamin supplements provide six micrograms in each pill, and those formulated for older people contain about 15-25 micrograms. People who lack if may need vitamin B12 injections; however, this is a very rare disorder. Sometimes, people may be given vitamin B12 injections as placebos to make them feel more energetic; this may not represent effective clinical care. There is no need to boost with extra vitamin B12 for energy.

Is It Safe?

Up to levels of about 100 micrograms, vitamin B12 seems to be safe when taken orally. It is only beneficial in larger amounts for people with deficiencies.
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