Definition
A water-soluble vitamin; part of the vitamin B complex.

Function
Vitamin B12, like the other B vitamins, is important for metabolism. It helps in the formation of red blood cells and in the maintenance of the central nervous system.

Food Sources
Vitamin B12 is found in eggs, meat, poultry, shellfish, and milk and milk products.

Side Effects
Because the body stores several years' worth of vitamin B12, nutritional deficiency of vitamin B12 is extremely rare.An inability to absorb vitamin B12 from the intestinal tract can, however, occur.This can be caused by a disease known as pernicious anemia.Low levels of B-12 can cause anemia as well as numbness or tingling in the extremities or other neurologic symptoms.

Recommendations
Recommended daily allowances (RDAs) are defined as the levels of intake of essential nutrients that, on the basis of scientific knowledge, the Food and Nutrition Board judges to be adequate to meet the known nutrient needs of practically all healthy persons.

The best way to get the daily requirement of essential vitamins is to eat a balanced diet that contains a variety of foods from the food guide pyramid.

Since vitamin B12 comes primarily from animal products, people following a strict vegetarian diet ( i.e., not consuming eggs or dairy products) may require vitamin B12 supplements. Non-animal sources of vitamin B12 exist, but are highly variable in their B12 content, and are therefore unreliable sources.

Specific recommendations for each vitamin depend on age, gender, and other factors (such as pregnancy).The U.S. Department of Agriculture has a PDF file that lists these recommendations.