Daily Value: 6 mcg
Safe Upper Limit: 3,000 mcg
Good Food Sources: Clams, ham, herring, king crab, oysters (cooked), salmon, tuna
Also called cobalamin, B12 is vital to the production of myelin, the fatty sheath that insulates nerve fibers, keeping electrical impulses moving through the body as they should. Because of this important function, a whole host of problems can arise when B12 is in short supply: memory loss, confusion, delusion, fatigue, loss of balance, decreased reflexes, impaired touch or pain perception, numbness and tingling in the arms and legs, tinnitus, and noise-induced hearing loss. Deficiencies of B12 have also been linked to multiple sclerosis-like symptoms and dementia. "In a severe deficiency, there is actually a degeneration of the myelin sheath," says John Pinto, PhD, director of the nutrition research laboratory at American Health Foundation in Valhalla, NY.
But that's only the beginning of vitamin B12's importance. Researchers have discovered that a deficiency raises blood levels of a substance known as homocysteine. In addition to being toxic to brain cells in high doses--raising serious questions about its possible role in Alzheimer's disease--homocysteine may be one of the primary causes of heart disease. "It has been shown to activate a clotting system that makes blood cells become a little more adhesive, a little more sticky, making them cling to arterial walls," says Pinto. There's evidence that in some people, the accumulation of homocysteine may be caused by a genetic defect, but in others it's simply the result of a vitamin B12 deficiency (although shortages of folate and vitamin B6 can do the same).
Because vitamin B12 is also important for the production of red blood cells, a severe deficiency can lead to a condition called pernicious anemia, which can lower energy levels. "When you take B12, you will almost immediately see a burst of activity in the bone marrow--more cells--and that will mean more oxygen-carrying capacity to tissues," says Pinto.
Unfortunately, however, 10 to 30% of people over age 50 can't get enough B12 from their normal diet because their stomach doesn't secrete enough gastric acid to break down food so that B12 can be stored in the liver and muscles until it's needed. (If you have absorption problems, doctors recommend using sublingual B12 tablets--taken by placing them under the tongue--or a B12 nasal gel, both available in health food stores.)
Unless you're vegan and avoid all animal products, it's easy to get adequate amounts of vitamin B12 from food sources because you need so little of it. There's probably no need to take a supplement, therefore, unless you've been instructed by your doctor. Vegans should look for B12-fortified products or take a supplement.
Vitamin B12 supplements are considered extremely safe, even in larger doses. People suffering from any of the following conditions, however, should check first with their doctor before supplementing with this vitamin: folate deficiency, iron deficiency, any kind of infection, Leber's disease, polycythemia vera (a condition marked by an abnormal increase of red blood cells), or uremia (a toxic accumulation in the blood of substances in the urine).