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 extracts.
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, the skins are separated from the flesh. This situation led to the conventional belief that red wines and red grape juice are the most heart healthy.
The research study
The new study, published online 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 are equally cardioprotective as skin, and the antioxidant potential of skin and flesh of grapes are comparable with each other despite 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.
Reduced heart attack size
After 30 days, the hearts of the animals were subjected to injury to reduce blood flow (ischaemia) 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 percent lower in the grape extract groups.
Quantification of the polyphenol content confirmed that, while the skins had anthocyanin concentrations of about 128mg per 100g, 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 to 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. - (Decision News Media, August 2006)
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