Vitamin B-12: Also called cobalamin, is one of eight water-soluble B vitamins. All B vitamins help the body to convert carbohydrates into glucose (sugar), which is "burned" to produce energy. These B vitamins, often referred to as B complex vitamins, are essential in the breakdown of fats and protein. B complex vitamins also play an important role in maintaining muscle tone lining the digestive tract and promoting the health of the nervous system, skin, hair, eyes, mouth, and liver.

Vitamin B12 is an especially important vitamin for maintaining healthy nerve cells and it aids in the production of DNA and RNA, the body's genetic material. Vitamin B12 also works closely together with vitamin B9 (folate) to regulate the formation of red blood cells and to help iron function better in the body. The synthesis of S-adenosylmethionine (SAMe), a compound involved in immune function and mood, depends on the participation of folate and vitamin B12.

Similar to other B complex vitamins, cobalamine is considered an "anti-stress" vitamin because it is believed to enhance the activity of the immune system and improve the body's ability to withstand stressful conditions.

Vitamins B12, B6, and B9 (folate) work closely together to control blood levels of the amino acid homocysteine. Elevated levels of this substance appear to be linked with heart disease and, possibly, depression and Alzheimer's disease.

Deficiencies of vitamin B12 are usually caused by a lack of intrinsic factor, a substance that allows the body to absorb vitamin B12 from the digestive system. Such a deficiency can cause a range of symptoms including fatigue, shortness of breath, diarrhea, nervousness, numbness or tingling sensation in the fingers and toes., People with the blood disorder pernicious anemia do not produce sufficient intrinsic factor and must take high doses of vitamin B12 to maintain their health. Similarly, people who have had stomach surgery (for example, for a severe ulcer) are at risk for vitamin B12 deficiency and pernicious anemia. They require lifetime B12 injections after the surgery.

Others at risk for B12 deficiency include vegetarians who follow a strict vegan or macrobiotic diet; those with certain intestinal infections such as tapeworm and, possibly, Helicobacter pylori (an organism in the intestines that can cause an ulcer); and those with an eating disorder.

Good dietary sources of vitamin B12 include fish, dairy products, organ meats (particularly liver and kidney), eggs, beef, and pork.