By JANE E. BRODY

It has long been known that vitamins must be obtained from sources outside the body - food and drink, and for vitamin D, exposure to sunlight - and that failing to get enough of a vitamin can result in welldefined and sometimes deadly diseases.
In recent decades, epidemiological studies have linked several nutrients, especially vitamins C and E, beta carotene and folic acid, to chronic ills including heart disease and cancer. People took large doses in hopes of warding off dire consequences.
But when scientifically designed clinical trials were conducted, most early promises proved false. Now another vitamin, B12, is being discussed as a factor in several ailments that commonly afflict older people, including heart disease and stroke, Alzheimer’s disease and dementia, frailty, depression, osteoporosis and even some cancers.
In many of the studies, symptoms were seen in people with B12 levels just slightly below normal. In some cases, symptoms were seen in people with B12 levels considerably above the levels that cause the best-known disease of B12 deficiency, anemia. The findings have prompted some experts to question whether blood levels of B12 now considered normal are optimal.
The studies suggest benefits from the increasing of B12 levels, especially in adults over 50. But these types of studies cannot prove cause and effect. Until placebo-controlled clinical trials are conducted, it is not known whether artificially increasing levels of B12 among people at the low end is safe and beneficial.
Still, a growing number of experts, who cite well-established explanations for drops in B12 levels, especially in older people, are urging everyone over 50 to increase their B12 intake through supplements or fortified foods. These experts believe it cannot hurt and may help to keep people healthy.
Dr. Donald Jacobsen, a biochemist at the Cleveland Clinic who has studied B12 for 40 years and is a consultant for a company developing a new B12 supplement, said B12 is needed by every cell in the body.
Since it is water-soluble and only a small fraction of the amount consumed is absorbed by the body, taking large doses of it appears to be safe, Dr. Jacobsen said in an interview.
The only dietary sources are animal products and bacteria: meat, fish, poultry, eggs and milk and nutritional yeast.
The body has a complicated means of acquiring naturally occurring B12. In animal foods the vitamin enters the body attached to protein; to be absorbed, it must first be separated from protein by stomach acid. The vitamin then combines with a substance in the gut called intrinsic factor, which enables it to pass through the small intestine into the bloodstream.
People with low levels of stomach acid or who lack intrinsic factor are at risk of developing a B12 deficiency. Among them are many millions of older people who develop atrophic gastritis, a loss of acidproducing stomach cells, and those who take acid-lowering drugs like Prilosec, Prevacid and Zantac to control reflux. Because the body has a temporary storage system for B12 in the liver, a deficiency may not show up for several years after acid levels fall.
But more often it is the elderly - as many as 30 percent over age 65 - who have less than ideal B12 levels.
“It’s a huge problem,” said Dr. J. David Spence, a neurologist and stroke specialist at the Robarts Research Institute in London, Ontario.
“Close to 80 percent of older adults with a B12 deficiency don’t know it,” he said. “Neither do their doctors. Doctors tend to think ‘normal’ means adequate.