Imagine having a disease that will very likely kill you—unless you can somehow choke down 2/3 pound of raw liver a day. Awful? Yes. But that was once the cure offered to people with pernicious.

The researchers who discovered this cure won the Nobel Prize for medicine in 1934. It wasn’t until 1948 that researchers isolated from raw liver the small red crystals of the nutrient that we now call vitamin B₁₂.

Unless you’re a vegan (strict vegetarian), pernicious is not often due to a lack of this essential vitamin. It’s actually a problem with absorption. B₁₂ differs from other vitamins in that before it can be absorbed into the cells lining the intestines, it has to hook up with a protein called intrinsic factor that is secreted as part of the stomach’s digestive juices.

Some people, especially when they’re older and on low-protein diets, don’t make enough intrinsic factor to absorb all the B₁₂ they need. Since intrinsic factor is provided by the body, not by food, when the body doesn’t make enough, we can be at risk for B₁₂.-related health risks.

Only about 1 percent of people develop the classic symptoms of B₁₂ deficiency— and nerve damage. About 30 percent of older people show signs of deficiency when their blood is tested, says Robert Allen, M.D., professor of medicine and biochemistry and director of hematology at the University of Colorado Health Sciences Center in Denver.

Even a low-level deficiency can have negative health effects. People may have depression, forgetfulness, trouble walking, and even incontinence. Sometimes, though, these are problems of old age rather than an outcome of vitamin deficiency. That’s why proper testing is so important.

This vitamin is essential to every cell in your body. It must be on hand for DNA synthesis, the process by which your body makes the genetic material that comprises the cell nucleus. Vitamin B₁₂ is far more than a cheerleader in this process. It rolls up its sleeves and goes to work, helping to make the nucleic acids that are strung together like pearls to form DNA, the genetic e-mail system. It also helps make RNA, the copy of DNA that’s sent along to each cell.

When cells are rapidly dividing, as they do during growth and development, more B₁₂ is needed for this process, according to John Pinto, Ph.D., associate professor of biochemistry at Weill Medical College of Cornell University and director of the nutrition research laboratory at Memorial Sloan-Kettering Cancer Center, both in New York City. Extra B₁₂ is also necessary in areas where cells normally have rapid turnover, such as the intestines and blood, he says.

In some places in the body, such as the cervix and intestines, a shortage of  vitamin B₁₂ can begin to interfere with cell growth; the resulting cell abnormalities can lead to cancer. Your ability to fight infection may slow down because your body can’t crank out enough infection-fighting white blood cells. In fact, one study showed that people with HIV, the virus that causes AIDS, were able to hold off full-blown AIDS longer if they had high levels of B₁₂. "The bottom line is, how fast can your system adapt to an adverse situation, such as infection, by making more DNA?" Dr. Pinto says. "When B₁₂ is in short supply, your body slows down and can’t defend itself as well."

Vitamin B₁₂ works hand in hand with folic acid, which also helps to make DNA. One form of folic acid circulates throughout the body. When B₁₂ removes a piece of the molecule from this circulating form, the folic acid becomes available for other reactions. As it goes about its business, B₁₂ uses the stolen piece of molecule for some of its own important functions.

Since B₁₂ and folic acid are so closely linked, a deficiency of one can leave the other in cold storage. You may be getting plenty of folate (the food form of folic acid) in your diet, for instance, but if you’re coming up short on B₁₂, the folate stays inside cells, trapped in its inactive form.

Vitamin B₁₂ and folic acid are also necessary for amino acid synthesis, meaning that they need to be there when your body sends the wrecking ball up against the proteins in food and then reassembles those proteins to create new structures.

This demolition and reconstruction work is going on all the time. If your body wants to take those scrambled eggs you had this morning and turn them into useful energy and hearty body cells, B₁₂ and folic acid have to be on hand to do their part. They help to ensure that the protein parts—amino acids—are in the proper amounts to make new proteins.

As the redevelopment authority inside your body, B₁₂ can make trouble for your heart if it goes on strike. Along with folic acid and B₆, it is needed to break down an amino acid by-product called homocysteine. When you don’t have enough, blood levels of homocysteine rise, "and high levels of homocysteine seem to increase your risk for heart disease and stroke even more than high cholesterol," Dr. Pinto says.

Homocysteine damages the cells lining the blood vessels, creating rough spots that attract cholesterol deposits and issue invitations for blood to start clotting. Exactly how much of the three B vitamins you need to prevent this problem isn’t known. In one study where people received daily doses of 400 micrograms of B₁₂, 1 milligram of folic acid, and 10 milligrams of B₆, homocysteine levels dropped significantly.

So far, researchers have found that high levels of homocysteine are associated with heart disease and that B vitamins can help lower homocysteine levels. They are trying to complete the circle by showing that lowering homocysteine levels with B vitamins can also lower heart disease risk, says Dr. Allen.

12 also maintains the fatty sheath, called myelin, that surrounds and protects nerve fibers and promotes their normal growth, Dr. Pinto says. Like insulation around copper wires, this sheath allows your radiating network of nerves to send their electrical messages without short-circuiting. When B₁₂ is missing, the myelin sheath breaks down, which eventually leads to nerve damage.

Researchers used to think that since B₁₂ was needed to put together the fatty acids that make up the myelin sheath, a fatty acid problem caused nerve damage. "Now it looks more like this, too, is a problem with high homocysteine levels," Dr. Pinto says. "Homocysteine seems to be directly toxic to nerve cells."

This may partly explain the symptoms of B₁₂ deficiency—numbness, tingling feet or hands, trouble walking, memory loss, and personality and mood changes. It doesn’t explain everything, though, since only about one-third of B₁₂ deficient people develop these symptoms, Dr. Allen says. "My feeling is that these symptoms are caused by a combination of deficiency and some environmental or genetic factor."

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