Blueberries have topped the list of “superfoods” — foods packed with vitamins and nutrients — for a number of years. In the case of berries, it’s the antioxidants that are the key ingredient. These little compounds are known to help virtually every cell of the body, from heart to brain, fight off disease.

The problem with many studies linking berries and cardiovascular health is that they often rely on the relationship between people’s eating habits and their risk of having a heart attack later on. This sort of approach makes it hard to know for certain whether it’s really the berries that are doing the trick, or whether other factors — or foods — are at play.

A study from Maine shows that blueberries have definite heart-healthy effects.

To tackle this question, a team from the University of Maine studied obese rats and were able to discover that eating blueberries really did make a positive cardiovascular difference. Obese rats suffer from the same kinds of problems that obese humans do, and it's much easier to keep track of what they eat.

Overweight rats and people usually have metabolic syndrome: an unhealthy combination of high blood pressure, high cholesterol, inflammation, insulin resistance, and veins and arteries that have lost their elasticity. Metabolic syndrome affects about a third of adults in the U.S. and raises the risk of heart disease and type 2 diabetes.

The fat rats were divided into two groups. One group was fed the human equivalent of two cups of wild blueberries for eight weeks. The other group received no blueberries.

After eight weeks, rats that consumed blueberries had blood vessels that were able to constrict and dilate (expand) more easily than those rats that did not have blueberries in their diet, and this flexibility helped their blood pressure. The researchers believe these improvements are the result of antioxidants in blueberries reducing inflammation and ultimately reversing the metabolic syndrome.

The findings underscore the importance of making healthy lifestyle choices every day. Eating high-nutrient foods like blueberries can literally prevent disease — and in the case of this study, perhaps even reverse it — and may help avoid the need for medication.

The study is published in Applied Physiology, Nutrition, and Metabolism.