The study was published earlier this year in the FASEB Journal, published by the American Societies for Experimental Biology.
Cardiovascular disease is the leading cause of death in the United States. Previous studies with cell cultures have shown that antioxidants such as anthocyanines contained in wild blueberries may help protect cells. Wild blueberry consumption in laboratory rats has also been linked to improvements in memory and motor skills.
At UMaine, Dorothy Klimis-Zacas, professor in the Dept. of Food Science and Human Nutrition, led a team of graduate and undergraduate students in a two-year research project that was supported by the Maine Agriculture and Forestry Experiment Station, the Maine Wild Blueberry Commission, the U.S. Dept. of Agriculture and the Wild Blueberry Association of North America. "Our experiments focused on the effect of whole wild blueberries on the biomechanical properties of arteries as related to cardiovascular disease," says Klimis-Zacas. "This is the first in-vivo study to examine this relationship."
Students working on the project included Cynthia Norton and Anastasia Kalea, master's and Ph.D. candidates respectively in the department.
Researchers found that arteries of Sprague-Dawley laboratory rats fed a diet enriched with wild blueberries generated less force in response to phenylephrine, a stress hormone, than did arteries in rats fed the same diet without blueberries. "Those arteries (in rats fed the blueberry enriched diet) were more relaxed. When they were challenged with the stress hormone, they didn't develop as much force. We know now that blueberries affect the contractile machinery of the artery," says Klimis-Zacas.
The finding is important because the force with which an artery responds to stress can directly affect blood pressure. Norton and Klimis-Zacas presented the results of the study at the 2003 annual conference of the Federation of American Societies for Experimental Biology in San Diego, California and to the Wild Blueberry Association of North America in Bar Harbor, Maine.
The apparent benefit of the blueberry enriched diet carried over to older rats which received blueberries later in the study. The implication is that the addition of wild blueberries to the diet later in life may still have a protective effect on arteries.
The study has not been replicated in humans, and the researchers did not identify the compounds in wild blueberries that affect arteries. However, it is likely that high concentrations of antioxidants and trace minerals such as manganese which are high in blueberries could explain the beneficial effect, says Klimis-Zacas.
During the project, three groups of rats containing ten animals per group were fed the same diets with the exception of ground, whole wild blueberries. One group received the blueberry addition for the entire time, while another group had the same diet without blueberries. To test the addition of blueberries to the diet later in life, the third group received a diet without blueberries for 14 weeks followed by a blueberry enriched diet for eight weeks.
The amount of blueberries given to the rats per day corresponds to between one and two cups of blueberries per day for humans.
Researchers then surgically removed the aortas from each rat. They cut four ring sections from each aorta and tested the force generated by each section in response to the presence of hormones that stimulate arteries to relax or contract. During the tests, the arterial ring sections were hung in a tissue bath under conditions that mimicked the body's internal chemical environment.
In a second round of experiments, researchers wanted to find out what layers in the artery are affected by blueberries. They focused on the inside surface of the artery, a layer of cells known as the endothelium.
"Increasing vascular resistance may lead to an elevation of blood pressure which may in turn damage the delicate endothelial layer," says Klimis-Zacas. "This layer is affected by many things in the blood. By removing the endothelium, we are left with the smooth muscle layer of the artery, and we can localize the effect of wild blueberries in response to stress hormones. "
In these tests, researchers purposely damaged a portion of the endothelium and then exposed the arteries to the hormones. "We found that when we remove the endothelium, the artery cannot relax. And the contractile force it exerts in response to the stress hormone is about three times what it was with the intact arterial rings," says Klimis-Zacas.
The endothelial layer is known to be an important source of nitric oxide that helps to relax the arteries. "You can imagine what happens with atherosclerosis. Your endothelium gets damaged. There are many different relaxation factors in the endothelium, but nitric oxide is a major one. We think that blueberries may function by preserving the bioavailability of nitric oxide," says Klimis-Zacas.
"We know that nitric oxide concentration decreases at the onset of cardiovascular disease. By preserving nitric oxide bioavailabilty, blueberries may aid in maintaining arterial relaxation and thus prevent elevation of blood pressure that damages the endothelium and contributes to cardiovascular disease," says Klimis-Zacas.
Future research is planned with rats that have high blood pressure, she says, to see if blueberries will lower blood pressure. A key will be the role of antioxidants in endothelium function.