It is easier than once thought to get too little Vitamin B12. Studies have found that 20% of Americans ages 65 and over have low levels of the vitamin in their blood. Sometimes the only symptoms are subtle cognitive and neurological changes. More serious shortages can result in dementia or anemia, because B12 is essential for the production of red blood cells in the bone marrow. Breast-fed infants of mothers with a B12 deficiency are at risk for severe developmental abnormalities and irreversible neurological damage. Some experts say that it's the most common nutritional deficiency in the developing world and possibly in the United States as well.
A multivitamin or a vitamin-fortified breakfast cereal (B12 is usually included) will solve the problem. In fact, the crystalline form of the vitamin contained in pills and breakfast cereals is more readily absorbed than the "natural" form found in food.
The only dietary sources of vitamin B12 are animal-derived: meat, fish, shellfish, poultry, eggs, milk, and milk products. Beef liver and several varieties of fish contain high amounts, dairy products and eggs not so much. Vegetarians can eat tempeh, which is made from fermented soybeans (the bacteria produce B12).
Four ways deficiencies develop
In normal digestion, stomach juices pry B12 loose from animal protein. But the small intestine can't absorb the vitamin in this liberated, solo state. To be absorbed, it must be combined with a protein called intrinsic factor that is produced by cells in the lining of the stomach.
It may take quite a while for a deficiency to develop. A healthy person's liver can store up to a five-year supply of B12. And the small intestine does a marvelous job of B12 recycling, reabsorbing it from bile made by the liver.
Diets without food derived from animals. Vegans, the strict vegetarians who avoid all animal products, are most at risk for developing a full-fledged B12 deficiency. But even lacto-ovo vegetarians, who eat eggs and dairy products, may have low levels.
Lack of stomach acid. Up to 30% of people ages 50 and over suffer from atrophic gastritis, a thinning of the stomach lining. This condition reduces the acid secretions that free B12 from animal protein, so much less is absorbed by the small intestine.
Lack of stomach juices may also allow the overgrowth of bacteria in the small intestine. The bacteria grab B12 for their own purposes, leaving less for the intestine to absorb.
Lack of intrinsic factor. Some people's stomachs don't make enough intrinsic factor, so they can't absorb enough B12. The cause is an autoimmune disorder: The body's immune system gets confused and produces antibodies, which, instead of chasing down germs as they should, pounce on the stomach cells that produce intrinsic factor. The result can be anemia.
Gastrointestinal disorders and surgeries. Crohn's disease can cause a B12 shortage because it interferes with the functioning of the small intestine. Some people develop a B12 shortage after a surgical procedure that shrinks the stomach, such as gastric bypass, because a smaller stomach means far fewer of the cells secreting stomach acid and intrinsic factor.
Vegetarians and older people with atrophic gastritis can get all the B12 they need from a multivitamin pill, fortified breakfast cereal, or both. Most multivitamins contain 6 mcg, which is 100% of the Daily Value set by the FDA
People with pernicious anemia have a more difficult problem. Because they're short on intrinsic factor, they don't benefit from the extra B12 in vitamin pills and fortified cereal. At least initially, most need B12 injections. After that, some do fine with pills alone, but others need additional injections every few months.
People with Crohn's disease and other malabsorption problems may also need B12 injections depending on the severity of their condition. Gastric bypass patients may need them, too.
Should you be tested?
Those at risk for B12 deficits include older people and strict vegetarians. Research also suggests that long-term use of metformin, the diabetes drug, may lower B12 levels. Although a severe deficiency is fairly unlikely, lesser shortfalls may affect balance, memory, or perhaps mood. If you have these problems and you're in an at-risk category, ask your doctor about ordering a B12 test.
April 2006 update