To Your Health
Are tannins responsible for the heart-healthy benefits of drinking wine?
A. Tannins are among the large number of substances found naturally in plants, collectively known as phytochemicals. Tannins can either be helpful or harmful to humans (as can be said for all phytochemicals). They are present in plants to protect against overconsumption by animals because they taste bitter. You can find more information about tannins in the "To Your Health" column published on April 30, 2004.
The word "tannin" is very old, and the topic of tannins is difficult to discuss because tannins are not in a single chemical group. What most tannins have in common is that they bind proteins and make them inactivate. We can use this property of tannins to our advantage while at the same time keeping in mind that it can be hazardous. In animal experiments, while small amounts of tannins generally improve animal health, when the amount exceeds about 5% of the diet, growth rate is slowed down. It takes a lot of high-tannin food to exceed that 5% number, but grazing animals can do it. We humans seldom come anywhere near this amount.
Cosmetically, we can use the fact that tannins constrict blood vessels to remove the dark circles from under our eyes with the application of used tea bags. Tannins will indeed constrict blood vessels, but our 5,000-year history of drinking wine and tea, both of which are high in tannins, is testimony that ill effects are outweighed by benefits. "Proanthocyanidins" are among the substances labeled as "tannins." Proanthocyanidins are simply several molecules of the phytochemicals that we call flavonoids joined together. These tannins, when broken down into units, yield bioflavonoids, valuable antioxidants that are well documented to protect the heart.
The tannins in the skin and seeds of grapes are particularly harsh, so winemakers extract the juice very gently in order to minimize tannins and thus improve the taste of wine. Wine aged in oak barrels will also pick up additional tannins. In addition, tannins are changed during the process of fermentation and aging, so the tannins found in grape juice are not quite the same as those found in wine. Until recently it was thought that resveratrol, a tannin in wine credited with extending the life of those who eat a "Mediterranean diet," was found exclusively in red wine. It is clear now that purple grape juice, as well as purple grapes, contains resveratrol and that the alcohol in wine is not a major factor in life extension.
We should be cautious about interpreting the research on the apparent health benefits of wine since people who drink wine regularly have other lifestyle differences, such as being less likely to smoke.