Vitamin B-12 is a water soluble vitamin necessary for the maintenance of a healthy nervous system and for the metabolic utilization of fats and proteins. Vitamin B-12 is also essential for the synthesis of DNA during cell division and therefore is especially important for rapidly multiplying cells, such as blood cells. In addition, adequate intake of Vitamin B-12, along with Folic Acid and Vitamin B-6, encourages healthy serum homocysteine levels, thereby supporting cardiovascular health. Because Vitamin B-12 is found predominantly in animal products, strict vegetarians are at risk of developing Vitamin B-12 deficiency and may benefit from supplementation.
From the FDA's website: "As part of a well-balanced diet that is low in saturated fat and cholesterol, Vitamin B12 may reduce the risk of vascular disease. FDA evaluated the above claim and found that, while it is known that diets low in saturated fat and cholesterol reduce the risk of heart disease and other vascular diseases, the evidence in support of the above claim is inconclusive."