As a woman of a certain age who consumes a well-balanced diet of all the usual food groups, including reasonable amounts of animal protein, I tend to dismiss advice to take a multivitamin supplement. I’ve been told repeatedly by nutrition experts that the overuse of dietary supplements for “nutritional insurance” has given Americans the most expensive urine in the world.

I do take a daily supplement of vitamin D, based on considerable evidence of its multiple health benefits, especially for older people. However, based on advice from the National Academy of Medicine and an examination of accumulating research, I’m prompted to consider also taking a vitamin B12 supplement in hopes of protecting my aging brain.

Animal protein foods — meat, fish, milk, cheese and eggs — are the only reliable natural dietary sources of B12, and I do get ample amounts of several in my regular diet. But now at age 75, I wonder whether I’m still able to reap the full benefit of what I ingest.

You see, the ability to absorb B12 naturally present in foods depends on the presence of adequate stomach acid, the enzyme pepsin and a gastric protein called intrinsic factor to release the vitamin from the food protein it is attached to. Only then can the vitamin be absorbed by the small intestine. As people age, acid-producing cells in the stomach may gradually cease to function, a condition called atrophic gastritis.

A century ago, researchers discovered that some people — most likely including Mary Todd Lincoln — had a condition called pernicious anemia, a deficiency of red blood cells ultimately identified as an autoimmune disease that causes a loss of stomach cells needed for B12 absorption. Mrs. Lincoln was known to behave erratically and was ultimately committed to a mental hospital.

“Depression, dementia and mental impairment are often associated with” a deficiency of B12 and its companion B vitamin folate, “especially in the elderly,” Dr. Rajaprabhakaran Rajarethinam, a psychiatrist at Wayne State University School of Medicine, has written.

He described a 66-year-old woman hospitalized with severe depression, psychosis and a loss of energy and interest in life who had extremely low blood levels of B12 and whose symptoms were almost entirely reversed by injections of the vitamin.

European researchers have also shown that giving B12 to people deficient in the vitamin helped protect many of the areas of the brain damaged by Alzheimer’s disease. In a two-year study at the University of Oxford of 270 people older than 70 with mild cognitive impairment and low B12 levels, Dr. Helga Refsum, a professor of nutrition at the University of Oslo, found reduced cerebral atrophy in those treated with high doses of the vitamin.

“A B12 vitamin deficiency as a cause of cognitive issues is more common than we think, especially among the elderly who live alone and don’t eat properly,” Dr. Rajarethinam said.

The academy estimates that between 10 percent and 30 percent of people older than 50 produce too little stomach acid to release B12 from its carrier protein in foods, and as the years advance, the percentage of low-acid producers rises.

But many people do not know they produce inadequate amounts of stomach acid. In fact, evidence from a study of young adults called the Framingham Offspring Study suggests that insufficient absorption of B12 from foods may even be common among adults aged 26 to 49, so the following advice may pertain to them as well.

The academy recommends that adults older than 50 get most of their daily requirement of B12 — 2.4 micrograms for people 14 and older, slightly more for women who are pregnant or nursing — from a synthetic form of the vitamin found in foods fortified with B12 or in a multivitamin supplement. Synthetic B12 is not attached to protein and thus bypasses the need for stomach acid. Given that I eat very few fortified foods, a supplement with B12 is likely to be my best option.

Certain groups besides older people are also at risk of a B12 deficiency. They include vegetarians and vegans who consume little or no animal foods; people with disorders of the stomach and small intestine like celiac disease and Crohn’s disease; chronic users of proton-pump inhibitors to control acid reflux; and people whose digestive systems were surgically reduced for weight-loss or treatment for cancer or ulcerative colitis.

Among those most likely to be B12 deficient are the older patients in nursing homes whose diets are limited, and this deficiency may account in part for the symptoms of cognitive dysfunction so common among nursing home residents.

While a B12 deficiency can take years to develop, encroaching symptoms can be distressing and eventually devastating. It can also be challenging to link such symptoms to a nutrient deficiency.

In an online posting in July, David G. Schardt, the senior nutritionist for the Center for Science in the Public Interest, noted that symptoms of B12 deficiency include fatigue, tingling and numbness in the hands and feet, muscle weakness and loss of reflexes, which may progress to confusion, depression, memory loss and dementia as the deficiency grows more severe.

Early symptoms can be reversed by treatment with high doses of B12, usually given by injection. But symptoms related to nerve damage and dementia are more likely to be permanent. Thus, it is especially important for people at risk of a B12 deficiency to have their blood tested for it periodically. For example, experts at Kaiser Permanente in Oakland, Calif., suggest that chronic users of proton-pump inhibitors should have their B12 level tested every two years.

Vegetarians and vegans need not despair. In addition to B12 supplements, various commercially prepared plant-based foods, like some breakfast cereals, nondairy milks and soy products and one type of nutritional yeast, are fortified with synthetic B12. The Vegan Society recommends eating two to three servings a day of fortified foods to get at least three micrograms of B12.

However, Dr. Ralph Carmel, a retired hematologist now affiliated with New York University who studied the effects of B12 for decades, cautions against taking megadoses of the vitamin. He said in an interview that too often, “People who really need B12 don’t get it, and those who don’t need it, like athletes, often take huge doses — 2,000 or 5,000 micrograms a day. We don’t know what such doses can do in the long run. If an older person has low-ish B12 levels, I don’t object to taking 500 or 1,000 micrograms a day, but 5,000 is ridiculous.”

Jane E. Brody/The New York Times