Vitamin B12 (cobalamin) was first isolated from a liver extract in 1948. It is necessary for the rapid synthesis of DNA during cell division. This is especially important in tissues where cells are dividing rapidly, such as, the bone marrow tissues responsible for red blood cell formation. If B12 deficiency occurs, DNA production is disrupted and abnormally large red blood cells are formed, and a low blood count (anemia) results. Vitamin B12 also contributes to the metabolism of fatty acids essential in myelin, the insulating sheath that surrounds nerve cells. A deficiency of Vitamin B12, folic acid, or Vitamin B6 may increase blood levels of homocysteine. An elevated blood level of homocysteine is an independent risk factor for heart disease and stroke.
Vitamin B12 is bound to the protein in food until it is released by the hydrochloric acid in the stomach. It then combines with a substance called intrinsic factor and is absorbed into the bloodstream. Vitamin B12 is naturally found in animal foods including fish, milk and milk products, eggs, meat, and poultry. Fortified breakfast cereals are another source of Vitamin B12.
Individuals with stomach and small intestinal disorders may not absorb enough Vitamin B12 from food, and some people are unable to produce an intrinsic factor to bind Vitamin B12. Ten to 30 percent of older people may be unable to absorb Vitamin B12 in food. Vegetarians who do not eat meats, fish, eggs, milk or milk products, or B12 fortified foods are at risk of developing a deficiency of Vitamin B12. No adverse affects have been associated with Vitamin B12 intake from food and supplements. (Ann Intern Med 1996; 124:469).