The research adds to mounting evidence that a diet higher in folate - a B vitamin found in grains and certain dark-colored fruits and vegetables - is important for a variety of diseases. It's proven to lower women's risk of devastating birth defects of the brain and spinal cord, and research suggests it helps ward off heart disease and strokes, too.
As people age, some decline in brain function is inevitable. The Dutch study tested whether otherwise healthy people could slow that brain drain by taking double the recommended daily U.S. dose of folic acid, the amount in 2.5 pounds of strawberries.
The study divided 818 people ages 50 to 75 to take either a vitamin containing 800 micrograms of folic acid a day, or a dummy pill, for three years.
The folic acid protected users' brains, lead researcher Jane Durga of Wageningen University reported Monday at a meeting of the Alzheimer's Association.
On memory tests, the supplement users had scores comparable to people 5.5 years younger, Durga said. On tests of cognitive speed, the folic acid helped users perform as well as people 1.9 years younger.
The study involved healthy older people, not those with Alzheimer's symptoms, so it doesn't show if folic acid might ward off that disease.
"That's the key question," Durga said.
Still, folic acid offered significant brain protection, said Johns Hopkins University neuroscientist Marilyn Albert, who chairs the Alzheimer's Association's science advisory council.
Scientists have long thought that folic acid might play a role in dementia. Previous studies have shown people with low folate levels are more at risk for both heart disease and diminished cognitive function; clogged arteries slow blood flow in the brain.
"I think I would take folic acid, assuming my doctor said it was OK," Albert said, noting that long study of folic acid shows these levels are safe.
"We know Alzheimer's disease, the pathology, begins many, many years before the symptoms. We ought to be thinking about the health of our brain the same way we think about the health of our heart," she added.
Folate is found in such foods as oranges and strawberries, dark green leafy vegetables and beans. In the United States, it also is added to cereal and flour products. The recommended daily dose is 400 micrograms; doctors advise women of childbearing age to take a supplement to ensure they get that much.
Durga said it's not clear how folic acid might work to protect the brain. Some studies suggest folate lowers inflammation; others suggest it may play a role in expression of dementia-related genes.