The bulk of grapes' health benefits comes from their high content in flavonoids and phytonutrients in general: they give the color to grapes, and the more vibrant the color, the higher the content in phytonutrients.
Among these, the most important are quercitin and resveratrol (which is chemically a stilbene), which are responsible, in particular, for the heart-protecting effects: they appear to reduce blood clotting due to an antiaggregant effect on platelets, as well as protecting LDL cholesterol from oxidation (which has been linked to arterial damage).
What is currently known as the "French paradox" consists of the unusually high life expectancy of Frenchmen, if we take into account their diets, exceptionally high in fats. Many nutritionists believe this to be caused by their high consumption of red wine, and in particular the high quantities of antioxidants and flavonoids contained within.
In a study involving 20 healthy volunteers, they had their blood drawn before and after drinking grape juice. After drinking it, their blood contained higher levels of nitric oxide (a compound known to reduce the formation of clots), as well as a decrease in platelet aggregation. A third effect was t e increase of alpha-tocopherol, a member of the antioxidant vitamin E, and general antioxidant levels in blood were found to be 50% higher.
This study confirms what was discovered in a previous study, where an additional effect of drinking grape juice consisted of protecting LDL cholesterol from oxidation: normally, oxidized LDL acts on arteries damaging their walls and increasing the risk of developing atherosclerosis.
It also appears that some phenolic compounds contained in grape skins inhibit protein tyrosine kinases: this is a class of enzymes that regulates cells, inhibiting production of a vessel-constricting hormone (endothelin-1) believed to be a key component in the development of several heart conditions.
The journal Hypertension has published a study showing the way resveratrol inhibits production of endothelin-1 (ET-1): it appears to regulate ET-1 genes, preventing them from being expressed. ET-1 is normally synthesized by endothelial cells when they are damaged by free radicals: resveratrol greatly reduces the formation of free radicals, in turn reducing the expression of the ET-1 genes.
The American Chemical Society presented research, at its 226th national meeting, showing another explanation for grapes' health benefits: saponins. These phytonutrients appear to lower blood cholesterol, by binding to its molecules and preventing their absorption, as well as reducing inflammation (which may also suggest a cancer-protecting effect). Additionally, the researchers, led by Andrew Waterhouse, PhD, from the University of California, Davis, think that alcohol may improve the bioavailability of saponins, making them more easily absorbed in wine.
Chemically, saponins are glucose-based compounds, in the class of phynutrients, and they're being actively researched and found in several foods such as soybeans and olives: Waterhouse also tested six California grape cultivars (two white and four red) for their saponin content.
He proved that red wines contain 3 to 10 times the amount of saponins contained in white wines, as well as showing a positive correlation between alcoholic concentration and saponin concentration (stronger wines had more saponins), with red Zinfandel (16% alcohol) containing the highest levels. In order of saponins, Syrah was second, followed by Pinot noir and Cabernet Sauvignon, wihle the white varieties (Sauvignon blanc and Chardonnay) contained much less.
Quoting Waterhouse: Average dietary saponin intake has been estimated at 15 mg, while one glass of red wine has a total saponin concentration of about half that, making red wine a significant dietary source
Grapes also contain another antioxidant compound called Pterostilbene, known for its cancer-fighting and cholesterol-lowering properties.
In a study carried out by the USDA Agricultural Research Service on animal liver cells, pterostilbene was tested versus ciprofibrate (a drug that lowers blood lipids) and resveratrol for its cholesterol-lowering effects.
The comparison was based on these compounds' ability to activate PPAR-alpha, which is a family of cell membrane receptors involved in the transport of several energy-related compounds (including cholesterol) in and out of the cell.
As it turned out, the effects of Pterostilbene were higher than resveratrol, and on par with ciprofibrate in activating PPAR-alpha. Pterostilbene can not only be found in grapes, but also in blueberries and cranberries.
A study, published in the Journal of Nutrition, was carried out by having 24 pre and 20 post-menopausal women drink a glass of water with 36g of freeze-dried grape powder, every day, for 4 weeks.
After this period, several cardiovascular benefits could be demonstrated, including:
These protective effects can be explained by the presence of several important antioxidants in grapes, including kaempferol, resveratrol, myriceting, quercetin, flavans and anthocyanins.
It has been suggested by several studies that drinking 1 to 2 glasses of red wine per day significantly lowers your risk of hypertension. According to research published in the American Journal of Clinical Nutrition, this is caused by the presence of important antioxidant polyphenols and phytonutrients, but also by the usage of moderate quantities of ethanol (alcohol) that causes a slight vasodilation.