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AUDIE CORNISH, HOST: When it comes to staying healthy, most of us know we're supposed to eat our five a day. But not all fruits and veggies were created nutritionally equal. New evidence today suggests some may give us more bang for our buck.

NPR's Allison Aubrey reports on a study that finds women who ate at least three servings per week of blueberries and strawberries had fewer heart attacks.

ALLISON AUBREY, BYLINE: When it comes to foods that are thought to be supernutritious foods, the blueberry has long had a health halo floating over it, going back centuries when blueberries grew wild all over American colonies. Foodways historian Kathleen Wall says they were considered an important part of the diet.

KATHLEEN WALL: Going back to colonial times, Native Americans and the English colonists were eating blueberries. They were drying them. They would pound them into a paste. They would add them to their porridge.

AUBREY: Not so different from us topping off our oatmeal with berries.

Now, Native Americans may have had some clues and beliefs about the health-promoting effects of berries. But it's only in recent years that scientists have started to really try to nail them down. For instance, research suggests that berry consumption may be good for the brain. And one large study links berries to a decreased risk of Type 2 diabetes.

Today, researchers add another finding to the growing body of evidence. A study published in the journal Circulation finds blueberries, as well as strawberries, seem to help cut the risk of heart disease in some women. Aedin Cassidy, of the University of East Anglia, is the study author.

AEDIN CASSIDY: We showed for the first time that a regular intake of substances that are naturally present in red, blue-colored fruits and vegetables can reduce the risk of a heart attack, by about 32 percent in young and middle-aged women.

AUBREY: The findings come from the huge Harvard study of some 90,000 nurses in the U.S., who for years have reported details about their diets and lifestyles to researchers. At the same time, researchers have monitored the nurses' health to see which diseases they go on to develop or not.

It's certainly not a perfect way to detect how certain foods may influence health. But Cassidy says in this instance, the association was strengthened when she ruled out other reasons that the blueberry-eating nurses didn't have heart attacks.

CASSIDY: Even when we adjusted for things like fat intake, fiber intake, or medication use, or body size and, for example, exercise, we still got the strong reductions in risk.

AUBREY: So what is it about berries that could explain such an effect? In lab experiments, researchers have pinpointed a class of plant compounds, known as anthocyanins. They give the red and blue color to everything from berries to eggplants to cherries. And Cassidy says when you eat a lot of them, they seem to work in a number ways.

CASSIDY: They have effects on blood pressure in animal models. And they do things like exert anti-inflammatory effects, for example.

AUBREY: Which may help keep arteries more elastic and flexible. Dr. Robert Eckel, a preventive cardiologist at the University of Colorado, says that's a good thing. But he says there is a caveat here. The women in the study were not prime candidates for heart attacks.

DR. ROBERT ECKEL: Having a heart attack when you're a woman between the ages of 45 and 60 is distinctly unusual.

AUBREY: So does the protective effect hold up for older women? Eckel says, it's not clear.

ECKEL: But nevertheless, for the first time, I think we have some evidence that the intake of theses anthocyanins may have a protective effect.

AUBREY: So, Eckel says, go ahead, add in more berries. It can't hurt. And if they're out of season, like now, studies show frozen berries have similar levels of beneficial compounds.

Allison Aubrey, NPR News

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