You can meet your RDA by eating foods that contain vitamin B12, including canned and fresh fish, beef, eggs and dairy products, or by taking supplements if your doctor recommends them for you.
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Vitamin B12 (cobalamin) is necessary in only very small quantities. The RDA is 2.0 micrograms. Vitamin B12 is found in significant quantities only in animal foods. The richest sources are liver and kidney, followed by fish, eggs, meat, and cheese. Strict vegetarians (vegans) are often told that fermented foods, such as tempeh are excellent sources of vitamin B12, and, depending upon the medium on which it is grown, nutritional brewer's yeast may also provide B12. However, in addition to the tremendous variation in B12 content in fermented foods and brewer's yeast, there is some evidence that the form of B12 in these foods is not exactly the form that meets our bodily requirements. Although the vitamin B12 content of certain cooked sea vegetables is in the same range as beef, it is not known if this form is utilized in the same manner either. Therefore, at this time it appears to be an extremely good idea for vegetarians to supplement their diets with vitamin B12.