Vitamin B12

Daily Value: 6 micrograms

Good Food Sources: Clams, ham, cooked oysters, king crab, herring, salmon, tuna

Short of an accident, there are few faster ways to short-circuit your body than bypassing vitamin B₁₂. That's because B₁₂--also called cobalamin--is vital to the production of myelin, the fatty sheath that insulates nerve fibers, keeping electrical impulses moving through your body.

Because of the nutrient's important nerve-protecting function, a whole host of problems have been linked to low levels of vitamin B₁₂, including 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. The stuff begins to literally erode," says John Pinto, Ph.D., director of the nutrition research laboratory at Memorial Sloan-Kettering Cancer Center and associate professor of biochemistry in medicine at Cornell University Medical College, both in New York City.

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 Dr. Pinto.

There's evidence that in some, the accumulation of homocysteine may be caused by a genetic defect, while in others, it's simply the result of a vitamin B₁₂ deficiency. Such clotting and homocysteine buildup also seem to occur during folate and B₆ deficiencies.

Because vitamin B₁₂ is also important for the production of red blood cells, a severe deficiency, called pernicious anemia, can lower energy levels. "When you take B₁₂, if you do have so-called tired blood (a decreased number of red blood cells), almost immediately you will see a burst of activity in the bone marrow--more cells--and that will mean more oxygen-carrying capacity to tissues," says Dr. Pinto.

Because vitamin B₁₂ is found in animal products, vegans--strict vegetarians who avoid dairy products and eggs as well as meats--are at risk for becoming B₁₂-deficient. In fact, one study documented cases of children of vegans whose growth was stunted because they did not get adequate B₁₂.

And even when they do eat meats and dairy products, nearly one-third of people over age 60 can't extract the vitamin B₁₂ they need from what they eat. That's because their stomachs no longer secrete enough gastric acid, the stuff that breaks down food so that B₁₂ can be stored in the liver and muscles until it's needed. Without gastric acid, even someone who gets adequate amounts of B₁₂ in his diet may become deficient.

Using Vitamin B₁₂ Safely

It's easy to get adequate amounts of vitamin B₁₂ from food sources, because you need so little of it. So there's really no need to take a supplement unless your doctor tells you to do so.

Doctors often prescribe shots for those who have trouble absorbing vitamin B₁₂, however. A typical regimen for someone who has been low for a while might include a daily shot of 100 to 1,000 micrograms for one to two weeks, then a weekly shot that provides between 100 and 1,000 micrograms to normalize B₁₂ stores. This is followed by monthly injections of 250 to 1,000 micrograms.

If you have absorption problems, doctors recommend using sublingual B₁₂ tablets--normally placed under the tongue--or a B₁₂ nasal gel. Both are available in health food stores. For those without absorption problems, oral tablets are available, but doctors recommend both sublingual tablets and nasal gel as alternatives.

Vitamin B₁₂ supplements are considered extremely safe; even huge excess doses are harmlessly excreted in your urine. If you get injections, there may be some discomfort at the injection site, and in rare cases, sensitive people could have allergic reactions to B₁₂. If you have any of the following conditions, you should check with your doctor before using B₁₂: folate deficiency, iron deficiency, any kind of infection, Leber's disease, polycythemia vera or uremia.