The study also found that folates appear to have more impact on reducing Alzheimer's risk than vitamin E, a noted antioxidant, and other nutrients considered for their effect as a brain-aging deterrent.
Maria Corrada and Dr. Claudia Kawas of UC Irvine's Institute for Brain Aging and Dementia led the effort, which analyzed the diets of non-demented men and women age 60 and older. They compared the food nutrient and supplement intake of those who later developed Alzheimer's disease to the intake of those who did not develop the disease. It is the largest study to date to report on the association between folate intake and Alzheimer's risk and to analyze antioxidants and B vitamins simultaneously.
Results appear in the inaugural issue of the quarterly peer-reviewed research journal, Alzheimer's & Dementia: The Journal of the Alzheimer's Association.
"Although folates appear to be more beneficial than other nutrients, the primary message should be that overall healthy diets seem to have an impact on limiting Alzheimer's disease risk," said Corrada, who like Kawas started with the study while at Johns Hopkins University in Baltimore.
The researchers used data from the Baltimore Longitudinal Study of Aging to identify the relationship between dietary factors and Alzheimer's disease risk. Between 1984 and 1991, study volunteers provided detailed dietary diaries, which included supplement intake and calorie amounts, for a typical seven-day period.
Ultimately, 57 of the original 579 participants developed Alzheimer's disease. But the researchers found that those with higher intake of folates, vitamin E and vitamin B6 shared lower comparative rates of the disease. And when the three vitamins were analyzed together, only folates were associated with a significantly decreased risk.
In turn, no association was found between vitamin C, carotenoids (such as beta-carotene) or vitamin B-12 intake and decreased Alzheimer's risk.
"The participants who had intakes at or above the 400-microgram recommended dietary allowance of folates had a 55-percent reduction in risk of developing Alzheimer's," said Corrada, an assistant professor of neurology. "But most people who reached that level did so by taking folic acid supplements, which suggests that many people do not get the recommended amounts of folates in their diets."
Folates have already been proven to reduce birth defects, and research suggests that they are beneficial to warding off heart disease and strokes. Although folates are abundant in foods such as liver, kidneys, yeast, fruits (like bananas and oranges), leafy vegetables, whole-wheat bread, lima beans, eggs and milk, they are often destroyed by cooking or processing. Because of their link to reducing birth defects, folates have been added to grain products sold in the U.S. since 1998. But even with this supplement, it is thought that many Americans have folate-deficient diets.
Recent research is beginning to show relationships between folates and brain aging. Earlier this year, Dutch scientists showed that adults who took 800 micrograms of folic acid daily had significant improved memory test scores, giving evidence that folates can slow cognitive decline.
"Given the observational nature of this study, it is still possible that other unmeasured factors also may be responsible for this reduction in risk," said Kawas, the Al and Trish Nichols Chair in Clinical Neuroscience. "People with a high intake of one nutrient are likely to have a high intake of several other nutrients and may generally have a healthy lifestyle. But further research and clinical studies on this subject will be necessary."
Judith Hallfrisch of the U.S. Department of Agriculture, Denis Muller with the National Institute on Aging and Ron Brookmeyer with Johns Hopkins collaborated on the study, which was originally undertaken at the Gerontology Research Center of the NIA and the Department of Neurology at Johns Hopkins. Study funding came from the Extramural Programs of the NIA.
Begun in 1958 by the NIA, the Baltimore Longitudinal Study of Aging is America's longest-running scientific study of human aging. BLSA scientists are learning what happens as people age and how to sort out changes due to aging from those due to disease or other causes. More than 1,400 men and women are study volunteers. For more information, see: www.grc.nia.nih.gov/branches/blsa/blsa.htm.
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