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NEW YORK (Reuters Health) - Vitamin B12 may help protect against Alzheimer's disease, according to a study out Monday.The study suggests that seniors with more of the active part of the vitamin in their blood have a lower risk of developing the disease, which eats away at the minds of one in eight Americans aged 65 and older, according to the Alzheimer's Association.However, the findings don't necessarily mean that taking B vitamin supplements will stave off mental decline. Just last summer, for instance, a pair of studies deflated long-held hopes that B vitamins -- like B12 and folic acid -- would help patients who had suffered strokes or heart attack (see Reuters Health stories of June 22 and August 4, 2010)."More research is needed before we can get a conclusion on the role of vitamin B12 supplements on neurodegenerative diseases such as Alzheimer's disease," said Dr. Babak Hooshmand from Karolinska Institutet in Stockholm, Sweden, whose findings appear in the journal Neurology.But he added that many elderly people suffer from B12 deficiency, so the results could turn out to be important."Our findings indicate that vitamin B12 and related metabolites may have an important role in Alzheimer's disease," Hooshmand told Reuters Health by e-mail.The researchers took blood samples from 271 Finnish seniors without dementia. At a second examination about seven years later, they found 17 (six percent) had developed Alzheimer's.Those who did had higher levels of holotranscobalamin -- the active portion of vitamin B12 -- and lower levels of homocysteine, an amino acid tied to mental decline, stroke and heart disease. Folic acid was not linked to Alzheimer's.B vitamins decrease homocysteine levels, and so have attracted a lot of attention as a potentially cheap and safe treatment. But it is unclear if they are just a sign of disease or have a causal role.Neurologist Dr. Sudha Seshadri, of Boston University, said he wouldn't advise taking extra B vitamins unless a doctor had diagnosed signs of deficiency."Too much folate in the presence of B12 deficiency can be harmful," he told Reuters Health by e-mail.However, he added, "A healthy diet with adequate B12 may still be useful in reducing risk despite the failure of initial clinical trials to show a benefit on cognition."Vitamin B12 is found in a variety of foods, including dairy, eggs, fish and meat.