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Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

What Is It?

Vitamin B12 is needed to produce an adequate amount of healthy red blood cells in the bone marrow. Vitamin B12 is available only in animal foods (meat and dairy products) or yeast extracts (such as brewer's yeast). Vitamin B12 deficiency is defined by low levels of stored B12 in the body that can result in anemia, a lower-than-normal number of red blood cells.

Vitamin B12 deficiency can develop for the following reasons:

  • Absence of intrinsic factor, also called pernicious anemia Intrinsic factor is a protein secreted by cells of the stomach lining. Intrinsic factor attaches to vitamin B12 and takes it to the intestines to be absorbed. An absence of intrinsic factor is the most common cause of pernicious anemia. Absent intrinsic factor is often associated with a condition called atrophic gastritis, a thinning of the lining of the stomach. Atrophic gastritis is more common in elderly people of African-American or Northern-European descent. In these people, pernicious anemia develops at about age 60.



In children, decreased levels of intrinsic factor can be an inherited (genetic) condition. When this happens, low levels of intrinsic factor produce symptoms of juvenile pernicious anemia in patients younger than age 10.

Pernicious anemia occurs more commonly in people who already have diseases that are linked to immune-system abnormalities, such as Graves' disease, hypothyroidism (under-functioning thyroid gland), thyroiditis (inflammation of the thyroid), vitiligo and Addison's disease (adrenocortical insufficiency).

  • Removal or destruction of the stomach Vitamin B12 deficiency can develop in people who have had surgery to remove part or all of the stomach.

  • Overgrowth of bacteria Some people develop vitamin B12 deficiency as a result of conditions that slow the movement of food through the intestines (diabetes, scleroderma, strictures, diverticula), allowing intestinal bacteria to multiply and overgrow in the upper part of the small intestine. These bacteria steal B12 for their own use, rather than allowing it to be absorbed by the body.

  • Dietary deficiency Vegans (strict vegetarians who do not eat any meat, fish, egg or dairy products) can develop vitamin B12 deficiency because they lack vitamin B12 in their diets. In patients with bulimia or anorexia nervosa, vitamin B12 deficiency also can be related to diet. However, your liver can store vitamin B12 for up to five years, so it's rare for diet to cause this anemia.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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