Recent studies have suggested  B-12 may aid  in fighting heart disease, stroke,  dementia, frailty, depression, osteoporosis and even some cancers. (Although another study concluded it had little impact on Alzheimer’s.)
But several studies do show that symptoms of some diseases were evident in people with B12 levels slightly below normal. Could the disease be related to a B12 deficiency?  It’s hard to say conclusively.
The studies suggest considerable 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.
Vitamin B12 deficiency is traditionally thought of as a condition of elderly people who have poor eating habits or inadequate digestion. Also, it may be found in anyone who chooses to avoid meat. However, individuals with stomach and small intestine disorders also may  not be unable to absorb enough vitamin B12 from food.

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, explained that every cell in the body needs this vitamin. 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 acid-producing stomach cells, and those who chronically 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.
Others who are at serious risk of a B12 deficiency are those who lose major parts of their stomachs or parts of their small intestine, through, for example, surgery for weight loss or ulcers. They must take daily B12 supplements to stay healthy.