The study included more than 16,000 women who are taking part in the Nurses' Health Study.
Researchers have been keeping tabs on the women's diets since 1980. Between 1995 and 2001, researchers also measured the mental function of women who were over 70 and had not had a stroke.
Mental functioning was measured during three telephone interviews that were spaced about two years apart. In the interviews, researchers asked the women to recall details from a paragraph they'd just heard, for example, or to remember the order of words or numbers in a list.
When researchers compared women who ate the most blueberries and strawberries to those who ate the fewest, they found that those who ate the most had a slower rate of developing memory problems. The difference was equal to about two-and-a-half years of aging.
"This is pretty compelling evidence to suggest that berries do appear to have memory benefits," says researcher Elizabeth E. Devore, ScD, instructor in medicine at Brigham and Women's Hospital in Boston.
What may be even better news is that the biggest berry eaters in the study weren't eating mounds of them every day. On average, they were eating a single half-cup serving of blueberries or two half-cup servings of strawberries each week.
"These are simple interventions that appear to have pretty healthful effects," Devore says.
The study can't prove that berries protected the women's brains directly.
In fact, women in the study who ate berries regularly also got more exercise and had higher incomes -- two factors that are also linked to having better health.
But researchers say that even after they adjusted their results to account for differences like that, having a diet high in fruits and vegetables, particularly berries, still appeared to be linked to having a sharper memory.
How Berries May Be Good for the Brain
The study, which is published in the Annals of Neurology, builds on smaller studies in mice and humans that have suggested that berries may benefit the brain.
Berries, particularly blueberries, are rich in a particular kind of antioxidant compound called anthocyanidins. Anthocyanidins have the ability to move from the blood into the brain. And studies in animals have shown that these compounds concentrate in brain centers responsible for memory and learning.
Further studies in mice that are bred to develop brain changes similar to those seen in Alzheimer's patients show that blueberries protect these mice from memory declines. And small studies in humans with early memory loss have shown that adding 1/2 to 1 cup of blueberry juice to the diet each day for three months improved some measures of memory.
"I think it's very exciting," says Brent Small, PhD, professor of aging studies at the University of South Florida in Tampa. "Because there's not a lot that's been reported in the human literature that's focused on these types of compounds."
Small is testing a blueberry-based supplement on memory in humans, but he was not involved in the new study.
"The fact that they were able to see an effect for people who are generally healthy and who aren't experiencing significant drops in performance is interesting," Small says.
Berries on a Budget
Other experts agree that more studies need to be conducted to prove the berries alone are behind the benefit.
But they say there's no real harm in adding more berries to your diet, even before all the evidence is in.
"We know that flavonoids in fruits and vegetables act as really good protection for a range of chronic diseases," including cancer and diabetes, says Nancy Copperman, MS, RD, director of public health initiatives at the North Shore-LIJ Health System in Great Neck, N.Y.
"And now, with this study, they've actually looked at how these flavonoids, especially the types of flavonoids found in blueberries and strawberries, really might protect cognitive function in women," she says.
Copperman acknowledges that at $3 to $6 a carton, fresh berries can be expensive. To cut costs, she recommends buying frozen berries, or waiting until the fresh berries go on sale and freezing what you don't need right away.
"It's pretty easy," Copperman says, "Wash them. Put them on a baking sheet in the freezer for a few hours. Then transfer them to a freezer-safe bag."
She recommends using frozen berries in smoothies and in baked goods like pancakes and muffins.