MILWAUKEE — A growing consensus that the B vitamin folic acid can prevent Alzheimer's disease was strengthened today by a new study showing that people who consumed sufficient amounts of the nutrient had substantially less risk of getting the disease.

The study looked at a number of nutrients, including vitamins E, C, B6, B12 and carotenoids, but it was folic acid that stood out as having the strongest association with a reduced risk of getting Alzheimer's.

The study followed 579 people aged 60 and older for more than nine years. Those who consumed at least the recommended daily allowance of 400 micrograms had 55 percent less risk of getting Alzheimer's than those who consumed less than 400 mcg.

"It looks like there's a threshold effect (needed) to get a benefit," said lead author Maria Corrada, an epidemiologist at the University of California, Irvine's Institute for Brain Aging and Dementia. "If you go above a certain level, you don't get any improvement in reduction in risk."

While the new study does not prove that folic acid prevents Alzheimer's, it adds to other research suggesting that the nutrient may be beneficial, said Dr. Piero Antuono, a professor of neurology at the Medical College of Wisconsin.

"It's a continuation of several lines of evidence that show that folic acid is good for the brain," he said.

Folic acid is the synthetic form of folate, a B vitamin found in high amounts in chicken liver, legumes, leafy green vegetables, asparagus and orange juice. Since 1998, bread, flour, cereals, pasta, rice and other grains have been fortified with folic acid in the United States as a measure to prevent neural-tube birth defects.

Among those in the study, the average daily intake of folic acid was 318 mcg, although the study was begun before folic acid was added to grains.

The study was published in Alzheimer's & Dementia, a new journal of the Alzheimer's Association. It comes on the heels of other research suggesting a link between folic acid and memory and Alzheimer's.

In June, a Dutch study found that middle-aged people who took 800 mcg of folic acid a day for three years had significantly improved scores on memory tests. Those who took the supplements essentially performed as though they were two to five years younger.

Other studies have linked high levels of homocysteine, an amino acid found in the blood that has been implicated in heart disease, stroke and vascular disease, with Alzheimer's. Folic acid lowers homocysteine, which is believed to be toxic to brain cells.

In 2002, the ongoing Framingham Heart Study found that people with high levels of homocysteine had nearly twice the risk of getting Alzheimer's as those with low levels.

Other studies that used brain imaging have showed that people who had low levels of folic acid in their blood were more likely to have brain atrophy.