High doses of certain B vitamins can slow the rate of brain shrinkage that frequently leads to Alzheimer's disease and dementia, a new study finds.

Although all brains shrink some with age, the process speeds up in people with mild cognitive impairment. About one in six people over age 70 suffers from the condition, which is characterized by problems with memory, concentration and language, problems severe enough to be noticeable but not serious enough to interfere with everyday life. Mild cognitive impairment is an indication that something is amiss. Indeed, up to half the people who live with it go on to develop dementia — mainly Alzheimer's disease — within five years of diagnosis. The impairment has been linked to a number of factors, one of which is a high blood level of an amino acid called homocysteine.

Since the B vitamins folic acid (folate), vitamin B6 and vitamin B12 help break down homocysteine, David Smith, a professor at the University of Oxford in England, and his colleagues set out to learn whether reducing homocysteine levels could slow the rate of brain shrinkage.

For two years, the researchers followed 168 volunteers over age 70 with mild cognitive impairment. Half took high-dose B vitamin pills each day and the other half took a placebo pill. The pills contained mega-doses of vitamin B, more than people could get from either foods or traditional vitamin supplements. The researchers also measured brain volume with MRI scans.

By the end of the study, homocysteine levels had dropped almost 23 percent for those taking the vitamins pills and had risen nearly 8 percent in the placebo group. Further, the rate of brain shrinkage in the vitamin group was 30 percent less than in the placebo group. There were no safety issues with the high-dose vitamins. The researchers are planning a longer study to determine whether the vitamins can delay the onset of Alzheimer's.

But experts don't recommend that everyone start popping vitamin B pills based on this one study. The study size was small — fewer than 200 people. Also, the researchers didn't design the study to look at whether the vitamins help improve memory, although they did find that those whose brains shrank the least did the best on mental tests.

Accelerated brain shrinkage is not an inevitable part of aging, says Smith. "You can do something about it." Would he recommend high-dose B vitamins for everyone? "No. But if you have memory problems, especially if you're diagnosed with mild cognitive impairment, have your homocysteine level measured. If it's above 10, I would recommend starting on high-dose B vitamins." Since this vitamin combination is prescription-only, speak with your doctor.

"This is a landmark paper," says Christopher van Dyck, M.D., director of the Alzheimer's Disease Research Unit at Yale School of Medicine, who was not associated with the research. He adds that high-dose vitamins won't stamp out dementia, but if the study findings are replicated "to show a reduced risk of Alzheimer's disease in those with mild cognitive impairment, it will be a huge advance."

Nissa Simon is a freelance writer who lives in New Haven, Conn.