Vitamin B12 refers to a group of cobalt-containing corrinoids known as cobalamins. It slowly loses its activity when exposed to light, oxygen, and acid or alkali environments. The main cobalamins in humans and animals are hydroxocobalamins, adenosylcobalamin, and methylcobalamin, the last two being the active coenzyme forms. Cyanocobalamin is widely used clinically, because of its availability and stability, and is transformed into the active factors in the body.
Reactions involving methylcobalamin include biosynthesis of methionine, methane, and acetate.
Vitamin B12 is better absorbed when combined with calcium, taken with several meals during the day, and during pregnancy. Absorption decreases with age and with deficiencies of vitamin B6, calcium, and iron. It is the key component in synthesizing hemoglobin, increases energy, increases resistance to infection, important in reproduction and lactation, helps reduce the effects of bruising and black eyes, and promotes growth. Folic acid should be taken with Vitamin B12. Vegetarians are usually deficient in vitamin B12.