The findings, based on the self-reported nutrient intake of almost 600 men and women aged at least 60 years old, back earlier evidence of a link between folate or folic acid and risk of Alzheimer's.
Researchers from the University of California Irvine followed the individuals who were participating in the Baltimore Longitudinal Study of Aging for an average of nine years.
Those people who consumed at least 400 micrograms of folate each day (the RDA) had a 55 per cent reduction in risk of the disease, they report in the inaugural issue of Alzheimer's & Dementia, a publication of the US-based Alzheimer's Association,
Levels much above the RDA did not provide additional protection.
Folates are found in foods such as bananas and oranges, leafy green vegetables, liver, and many types of beans and peas.
But most people in the study who reached the RDA did so by taking folic acid supplements, said the researchers.
Author Maria Corrada, assistant professor of neurology at the university, cautioned however that because the study was observational, it is possible that other factors may be responsible for the reduction in risk.
"People with a high intake of one nutrient are likely to have a high intake of several nutrients, and may generally have a healthy lifestyle. Additional studies are necessary to determine whether folate has a direct, causal role in risk reduction for Alzheimer's and to determine appropriate recommendations," she said.
The study also evaluated whether the vitamins E and C, as well as the B group - folate, B6, and B12 - were related to a reduced risk of Alzheimer's. But only folates were associated with a significantly decreased risk of Alzheimer's.
There are several ways in which folate is thought to impact the risk of developing of Alzheimer's, mostly through its impact on lowering levels of the amino acid homocysteine.
Recent animal studies suggest that high homocysteine (and low folic acid levels) may make brain cells more vulnerable to damage from beta-amyloid.There is also some evidence that homocysteine may cause direct toxicity to neuronal cells.
The Alzheimer's Association said the study findings support the message in its diet-related recommendations given through the Maintain Your Brain campaign.
William Thies, vice president of medical and scientific affairs at the group, said: "A brain-healthy diet is lower in fat and cholesterol and higher in antioxidants, such as those contained in dark coloured fruits and vegetables."