According to the National Institutes of Health, vitamin B12 helps the body maintain healthy nerves and blood cells and helps in the production of DNA. It also helps prevent a type of anemia called megaloblastic anemia that makes people tired and weak.
Vitamin B12 is found naturally in a wide variety of animal foods and is added to some fortified foods, like cereal. Plant foods have no vitamin B12 unless they are fortified. Clams and beef liver are the best sources of vitamin B12. The vitamin can also be found in meat, poultry, fish and dairy products. Most multivitamins also contain vitamin B12.