The human body stores several years' worth of vitamin B12, so nutritional deficiency of this vitamin is extremely rare.

However, deficiency can result from being unable to use vitamin B12. Inability to absorb vitamin B12 from the intestinal tract can be caused by a disease known as pernicious anemia. Additionally, strict vegetarians or vegans who are not taking in proper amounts of B12 by way of supplements are also prone to a deficiency state.

Low levels of B12 can cause anemia as well as numbness or tingling in the extremities and other neurologic symptoms such as weakness and loss of balance.

Recommended dietary 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.

Because vitamin B12 comes primarily from animal products, people who follow a strict vegetarian diet and do not consume 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 offers a PDF file that lists these recommendations.