Because black, green, and red grapes contain high levels of naturally occurring antioxidants, the fruits may reduce hypertension that can lead to heart failure, shows the study, published in the October issue of the Journal of Gerontology: Biological Sciences.
Scientists including Mitchell Seymour, MS, report that flavonoids -- found in abundance in the skin, flesh, and seeds of grapes -- may be the substances that provide the beneficial effects they found in their study of laboratory rats.
Grapes and High Blood Pressure
The researchers studied the effects of regular table grapes -- a blend of green, red, and black fruits -- that were fed to rats in powdered form.
After 18 weeks, rats that ate the grape-enriched powder had lower blood pressure, better heart function, and reduced inflammation throughout their bodies than comparable rodents that didn't receive the mixture. Rats on salty diets plus hydralazine, a blood pressure medicine, had lower blood pressure, but their hearts weren't as protected from damage as the animals fed grapes.
"These findings support our theory that something within the grapes themselves has a direct impact on cardiovascular risk, beyond the simple blood-pressure-lowering impact that we already know can come from a diet rich in fruits and vegetables," says Seymour, who manages the University of Michigan Cardioprotection Research Laboratory.
Steven Bolling, MD, who heads the program, says the rats in the study were in a similar situation as millions of Americans who have high blood pressure related to their diets and who develop heart failure because of prolonged hypertension.
"The inevitable downhill sequence to hypertension and heart failure was changed by the addition of grape powder to a high-salt diet," Bolling says. "Although there are many natural compounds in the grape powder itself that may have an effect, the things that we think are having an effect against the hypertension may be the flavonoids, either by direct antioxidant effects, by indirect effects on cell function, or both."
Such naturally occurring substances already have been shown to reduce other potentially harmful molecular and cellular activity, the researchers say in a news release.
The study notes that grapes and other fruits high in antioxidant phytochemials show promise, as does research on the impact of red wine on heart health.
Still, the best advice for people with blood pressure problems is to cut down on the amount of salt they get, Bolling says.
"There is, as we know, a great variability, perhaps genetic even, in sensitivity to salt causing hypertension," he says. "Some people are very sensitive to salt intake, some are only moderately so, and there are perhaps some people who are salt resistant. But in general we say, 'Stay away from excess salt.'"
The researchers studied several groups of rats and assigned each group of 12 to various combinations of salty foods, grape powder, and hydralazine. All the rodents were fed the same weight of food daily, with powdered grapes making up 3% of the diet for the animals that received grapes as part of either a low-salt or high-salt diet. Rats receiving hydralazine lapped it up in their water supply.
After looking at various factors, including molecular indicators of cardiac stress, the researchers still found that the rats in the high-salt grape and high-salt hydralazine groups developed high blood pressure over time but had lower systolic blood pressures than the high-salt rats deprived of grape powder.
"Though it's true that your mom told you to eat all your fruits and your vegetables, and that we are learning a lot about what fruits, including grapes, can do ... we would not directly tell patients to throw all their pills away and just eat grapes," Bolling says.
The researchers say the study suggests that a grape-enriched diet can have broad effects on hypertension, but that more work is needed to see if the beneficial effects will apply to humans.
They write that the findings "may have particular importance to our aging population, which has reduced intake of both fruit and vegetables."
According to recent data, only 35% of women and 39% of men over age 60 consumed two servings of fruits per day, and only 6% of both women and men met the goal of three servings daily of vegetables.
The study was funded in part by the California Table Grape Commission, but the authors note that the commission had no involvement in study design, data analysis, or manuscript preparation.