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How Vitamin B12 Works

Vitamin B12 Deficiency

The RDA for vitamin B12 is 2.4 micrograms (mcg) daily for adults, 2.6 mcg daily for pregnant women, and 2.8 mcg for women who are breast-feeding. The average American diet provides 7 to 30 mcg of the vitamin. Unlike other vitamins, vitamin B12 is stored in the liver, so these suggested amounts refer to an average daily intake over a span of days.

When the supply of vitamin B12 in the body is low, it slows down the production of red blood cells (causing anemia) and the cells that line the intestine. This is similar to what happens as a result of insufficient folate. But unlike folate deficiency, a lack of vitamin B12 can also cause serious damage to the nervous system. If the condition persists for a long time, the damage is irreversible.

An injection of B12 may be necessary for patients with pernicious anemia.
© 2007 Publications International, Ltd.
An injection of B12 may be necessary
for patients with pernicious anemia.

A deficiency of vitamin B12 caused by insufficient intake is not common. The average well-fed person has a supply of the vitamin stored in the liver that can last five years or longer. Dietary deficiency of vitamin B12 is seen only in strict vegetarians (vegans) who don't eat foods of animal origin -- not even milk or eggs.

Such a restricted diet is a particular problem for pregnant or breast-feeding women since the baby can develop a vitamin B12 deficiency even if the mother remains healthy. For this reason, vegan mothers should eat foods fortified with vitamin B12. Vegetarians who regularly eat eggs or drink milk usually get all the vitamin B12 they need.

Pernicious anemia is usually an inherited disease in which a deficiency of vitamin B12 occurs despite adequate amounts in the diet. People with this disease cannot produce intrinsic factor, the substance needed to absorb vitamin B12. They need to receive injections of vitamin B12 so the vitamin can bypass the stomach and intrinsic factor and enter the bloodstream directly.

Because intrinsic factor originates in the stomach, partial or total removal of the stomach reduces absorption of vitamin B12. Moreover, removal of the end of the small intestine (ileum) also creates a deficiency, because that's where absorption of the vitamin takes place. In these cases, pernicious anemia results from the surgery, not dietary deficiency.

Stomach acid frees vitamin B12 from the proteins it is bound to in foods, but for the one-third of adults who experience a decline in stomach acid as they age, this can be a problem. They risk a vitamin B12 deficiency later in life. If undetected, the problem can cause nerve damage. An unexplained unsteady gait and loss of coordination often signal this type of vitamin B12 deficiency.

Research around the world concludes that 10 to 12 percent of older adults are deficient in vitamin B12 to some degree. All agree that assessment of this nutrient is difficult because typical tests may not be sensitive enough and other nutrients and medications can interfere with results.

For those who don't have enough B12, huge amounts of supplements may be necessary. Find out why on the next page.

Vitamin B12 is just one of the many vitamins that are part of a healthy diet. Check out the following links to learn more:
  • Folate is necessary for healthy growth, because it makes DNA and RNA. Learn how it manufactures cells at How Folate Works.
  • When teamed with other B vitamins, B2 helps in metabolism. Find out what it does at How Vitamin B2 Works.
  • Vitamin B6 is effective against more than 100 ailments. How Vitamin B6 Works will explain the details and tell you how to get enough in your diet.
  • Vitamin A plays an essential role in eyesight. Learn how it helps us to see, even in the dark, at How Vitamin A Works.
  • Vitamin E is an antioxidant, which keeps the blood clean. Separate truth from fiction at How Vitamin E Works.
  • To learn about the many vitamins in our diet, how much you should be eating, and where to find them, go to our general Vitamins page.
  • If you're looking for the best prices on B vitamins, click here.
This information is solely for informational purposes. IT IS NOT INTENDED TO PROVIDE MEDICAL ADVICE. Neither the Editors of Consumer Guide (R), Publications International, Ltd., the author nor publisher take responsibility for any possible consequences from any treatment, procedure, exercise, dietary modification, action or application of medication which results from reading or following the information contained in this information. The publication of this information does not constitute the practice of medicine, and this information does not replace the advice of your physician or other health care provider. Before undertaking any course of treatment, the reader must seek the advice of their physician or other health care provider.