The flesh of grapes is just as heart healthy as the skin, says a laboratory study by Italian and US researchers, a result that may challenge the idea that red wine offers better cardiovascular protection than white.
The researchers prepared grape skin and grape flesh extracts from four varieties of red grape and tested their cardioprotective effects in rats. They found that the flesh extract was just as protective as the skin extract.
Several studies have linked regular consumption of wine to reduced risk of heart disease. The basis for these observations is that the skin of red grapes is a rich source of anthocyanins, potent antioxidants that contribute to the red colour of the fruit. Red grapes are usually crushed whole, meaning the anthocyanins are transferred to resulting wine and juice.
To make most white wine or white grape juices however the skins are separated from the flesh. That situation led to the conventional belief that red wines and red grape juice are healthier for the heart than white.
The new study, published on-line ahead of print in the Journal of Agricultural and Food Chemistry (doi: 10.1021/jf061048k), challenges this view by reporting that both skin and flesh have cardioprotective potential despite vastly differing polyphenol content.
"The results indicate for the first time that the flesh of grapes is equally cardioprotective as skin, and the antioxidant potential of skin and flesh of grapes are comparable with each other despite of the fact that flesh does not possess any anthocyanin activities," wrote the researchers.
Dipak Das from the University Of Connecticut School Of Medicine, collaborated with researchers from the University of Milan and several other research institutes in Italy.
The researchers randomly assigned male Sprangue Dawley rats to receive dietary supplementation with one of three preparations: water only (control), grape skin extract, or grape flesh extract.
After 30 days, the hearts of the animals was subjected to injury to reduce blood flow (ischemia) and heart attack (myocardial infarction). Under such conditions, concentrations of malondialdehyde (MDA), a reactive carbonyl compound related to oxidative stress, increase.
The animals given either the grape flesh or grape skin extracts had significantly reduced heart attack size, said the researchers, compared to the control rats. No difference was observed between the flesh and skin extracts.
Levels of MDA were also about 50 per cent lower in the grape extract groups.
Quantification of the polyphenol content confirmed that, while the skins had anthocyanin concentrations of about 128 milligrams per 100 grams, the flesh contained no such compounds.
However, the radical scavenging abilities of both the flesh and skin extracts were found to be the same.
The flesh of the grapes did contain polyphenols, said the researchers, but not of the anthocyanin type. Significant concentrations of caffeic acid, caftaric acid, and coutaric acid have been reported. Such compounds are also present in white grape varieties.
"On the basis of the findings that resveratrol and proanythocyanidins are present in the skins and seeds of the grapes, much attention has been paid on these parts and not the flesh," wrote the researchers.
"The present study indicates that several organic acids and polyphenols possessing potent antioxidant activities present in the flesh of grapes are also found in white wines."
"Although further study is needed to identify the principle ingredients responsible for the cardioprotective abilities of the grape flesh, to the best of our knowledge, our study provides evidence for the first time that the flesh of grapes is equally cardioprotective with respect to the skins," concluded the researchers.
Previous research with red grape juice has shown that the polyphenol rich juice could reduce the oxidation of LDL (bad) cholesterol.
This led to Gerber Foods, distributor of Welch's purple grape juice in the UK, announcing at the end of 2005 that it had been approved by the heart health charity Heart UK, and now carries the charity's logo on its packaging.