Nutrition Science and Everyday Application - beta v 0.1

VITAMINS AND MINERALS INVOLVED IN ENERGY METABOLISM 531
Figure 9.18. Dietary sources of vitamin B 12 . Examples of good sources pictured include clams,
salmon, nutritional yeast, red meat, and milk. Source: NIH Office of Dietary Supplements,
www.bragg.com
Vitamin B12 Deficiency and Toxicity
When there is a deficiency in vitamin B 12 , inactive folate (from food) is unable to be
converted to active folate and used in the body for the synthesis of DNA. However, folic
acid from supplements or fortified foods is available to be used as active folate in the body
without vitamin B 12 . Therefore, if there is a deficiency in vitamin B 12 , macrocytic anemia
may occur. Vitamin B 12 deficiency also results in nerve degeneration and abnormalities that
can often precede the development of anemia. These include a decline in mental function
and burning, tingling, and numbness of legs. These symptoms can continue to worsen, and
deficiency can be fatal. 4
The most common cause of vitamin B 12 deficiency is the condition of inadequate intrinsic
factor production that leads to poor vitamin B 12 absorption, resulting in pernicious anemia.
This condition is common in people over the age of 50, because production of intrinsic factor
decreases with age. Pernicious anemia is treated by large oral doses of vitamin B 12 or by
putting the vitamin under the tongue (sublingual), where it is absorbed into the bloodstream
without passing through the intestine. In patients that do not respond to oral or sublingual
treatment, vitamin B 12 is given by injection.
Because vitamin B 12 is found primarily in foods of animal origin, a strict vegetarian diet
can result in cases of vitamin B 12 deficiency. Therefore, careful dietary planning to include
fortified sources or supplements of vitamin B 12 is important to prevent deficiency in a vegan
diet.
No toxicity of vitamin B 12 has been reported. Because of the low risk for toxicity, a
tolerable upper intake level (UL) has not been established for vitamin B 12 . 4