Tired? Depressed? Forgetting things? Who isn't these days?

Those are also symptoms of a deficiency of B-12, a key nutrient needed to make red blood cells and DNA and keep the nervoussystemworking right.

Vitamin B-12 deficiency is officially considered rare, affecting about 1 in 1,000 Americans, according to a 2005 study. But the incidence rises with age, to about 15% of elderly people. The rate is also much higher among people who don't eat meat or dairy products, people with absorption problems, people taking acid-blocking medications and those with Type 2 diabetes who take the drug Metformin.

'B-12 deficiency is much more common than the textbooks and journal articles say it is,' says Alan Pocinki, an internist in Washington D.C., who routinely tests his patients who fall into those categories. He also notes that since the Metformin connection was discovered only recently, some physicians aren't aware of it. 'They assume that if patients complain of numbness and tingling in the feet, it's a diabetes issue and not a B-12 issue.'

Other symptoms of low B-12 include anemia, depression, dementia, confusion, loss of appetite and balance problems. Long-term deficiency can bring severe anemia, nerve damage and neurological changes that may be irreversible.

Sometimes the symptoms are subtle. Internist Linda Assatourians, one of Dr. Pocinki's partners, says that a surprising number of her young female patients also have low levels of B-12. Typically they are healthy and active, but they don't eat much meat and they have minor mood, memory or balance problems. 'When I supplement their B-12, they feel better,' Dr. Assatourians says. 'It's not a controlled study, but I see a lot of them.'

'I was sort of tired, but I thought, 'It's winter and I'm doing too much,'' says Jessica Riester, 27, editor of publications for a German-American think tank. Her B-12 level was slightly over 200 picograms per milliliter (the normal range is considered 200 to 800 pg per ml). After several weeks of B-12 injections, then 1,000 milligrams daily in pill form, her B-12 is now over 600 pg per ml and she says she feels better. 'My color is better and the shadows under my eyes are gone,' she says.

Indeed, several studies have shown that some symptoms of B-12 deficiency, particularly early problems with cognitive function, can be apparent even when patients' blood levels are still in the normal range.

Still, vitamin B-12 isn't a miracle pick-me-up, a performance enhancer, a weight-loss aid or a quick way to sober up after a long night of drinking, as some celebrities and Internet marketers claim. For people who aren't deficient, 'there's no evidence whatsoever that these injections are useful to pep you up,' says Irwin H. Rosenberg, a professor and former dean of the Friedman School of Nutrition at Tufts University. Taking too much B-12 is unlikely to be harmful, he adds; the excess is simply excreted.

B-12 is one of the newest of the human vitamins, identified in 1948. It originates in bacteria, yeast and microbes in soil. Plants can't store it, so people get their B-12 almost exclusively from meat, liver, poultry, fish and dairy products. Adults need just 2.4 micrograms a day, the amount in three ounces of beef, and can generally store it in fat tissue for several years. Dr. Pocinki speculates that some of his very lean patients are B-12 deficient because they don't have much fat to store it in.

Some people have trouble absorbing B-12 in the first place. Stomach acid is needed to release it from food particles. People who have used acid-blocking medications, particularly proton-pump inhibitors, for years may not have enough. And many elderly people naturally stop making stomach acid, so B-12 deficiencies are increasingly common as people age.

That's why the Institute of Medicine recommends that people over age 51 get most of their daily requirement from B-12 supplements or cereals fortified with the vitamin that break down more easily in the digestive tract.

Further down the digestive tract, B-12 must combine with a protein called intrinsic factor to be absorbed into the body. Without intrinsic factor, people absorb much less B-12, resulting in a condition called pernicious anemia, in which red blood cells don't develop properly. Left untreated, pernicious anemia can bring on neurological changes, delusions and hallucinations. The discovery that eating large amounts of liver could cure pernicious anemia led to the Noble Prize for medicine in 1934.

Other conditions can also interfere with B-12 absorption, including celiac disease and Crohn's disease. 'I thought I was going to die in my sleep because I had so little energy,' says Ellen Icochea, a seniorexecutive at the Federal Bureau of Investigation who had a very low B-12 level along with several major medical problems. Weekly B-12 injections made a dramatic difference. 'It was like, 'Wow, here's energy!'' she says.

David H. Johnson, a 54-year old wildlife biologist, takes B-12 shots every three days to cope with the challenges of chronicfatigue syndrome. 'Imagine you are on a small roller coaster,' he says. 'You feel up one day and the next and then you start slumping until you're really flat again.'