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msnbc.com
updated 11/18/2005 7:51:51 AM ET 2005-11-18T12:51:51

Most Americans don't worry about vitamin B-12. Health experts once thought that only strict vegetarians were likely to have a shortage. As long as basic blood counts didn't show any anemia, even these people were considered safe. Now research shows that vitamin B-12 may be a concern for many more people.

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Vitamin B-12 is the shorthand term for a group of substances called cobalamins. Cyanocobalamin is the major form in supplements. We need this vitamin for healthy nerve and blood cells and the production of DNA. Researchers are also studying its role in brain function. It is possible that it could help prevent Alzheimer's disease and other forms of dementia.

Strict vegetarians - those who avoid meat, poultry, fish and dairy products - are among those most likely to lack vitamin B-12, because animal foods are the primary sources. Plant foods like cereal, soy products, nutritional yeast and meat substitutes (for example, veggie burgers) only provide the vitamin if they are fortified with it.

According to surveys, most Americans do meet the recommended dietary allowance (RDA) of 2.4 micrograms (mcg) of vitamin B-12 per day. The average consumption ranges from 2.9 to 5.1 mcg. However, if people cut back on animal foods, as they should, to eat a mostly plant-based diet that will lower their cancer risk, careless choices could leave them short.

Two modest servings of poultry, lean meat, or seafood plus two cups of lowfat, skim, or nonfat milk or yogurt allow a person to meet this RDA. But if you skip dairy products and have only a moderate serving of meat at one meal, you could fall short without including fortified cereal or soy products in your daily choices.

Falling below the RDA occasionally isn't a problem for most adults, because the body can store enough to cover days with a low intake for several years. Children need less vitamin B-12, too, but their ability to store less means that they can develop health problems more quickly with an inadequate diet.

Supplements encouraged
Now, a new concern about vitamin B-12 has arisen. It seems that adults over the age of 50 may have a reduced ability to absorb it. Eating sufficient amounts of animal foods daily won't help these people. Since acids in our stomachs' digestive juices release the bonds that bind vitamin B-12 to protein in food, we need enough of these acids for proper absorption.

As we get older, however, we secrete less digestive acids. Studies suggest that 10 to 43 percent of people over the age of 50 may lack the acids to release vitamin B-12 from protein so it can be absorbed. To meet the RDA of 2.4 mcg, people over 50 are encouraged to take a multivitamin supplement, or eat fortified foods, where vitamin B-12 is not bound to protein.

Others who may have trouble absorbing this vitamin are people with digestive disorders such as Crohn's and celiac disease, those who have had substantial portions of their stomach or lower intestine removed, and those who take certain medications for gastroesophageal reflux (GERD), ulcers, or diabetes. Individuals who have had gastric surgery for weight control may also be at risk.

Although it was thought that a simple test for anemia would show a lack of vitamin B-12 before any other damage could occur, health experts no longer agree. A high intake of folate from fortified grains can hide the changes in red blood cells that show a lack of vitamin B-12. Studies also indicate that nerve and brain changes can occur without blood cell variations.

If you are at risk of not getting enough vitamin B-12 - either because of your food choices or your inability to absorb it - you should discuss the matter with your doctor. To be on the safe side, you could undergo tests that measure the function of vitamin B-12 through blood levels of homocysteine or methylmalonic acid (MMA). Or you could have a test that measures blood levels of vitamin B-12 using updated standards.

Nutrition Notes is provided by the American Institute for Cancer Research in Washington, D.C.

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