The skins of red grapes contain resveratrol, an organic chemical which contains polyphenols. Similar to many fruits and vegetables, these polyphenols act as antioxidants in the body and can help scavenge free radicals. These unstable molecules have damaging effects throughout the body and to many bodily systems.
Resveratrol has important regulatory function in grapes; it helps protect them from invasion by bacteria and fungi.
Resveratrol is commonly found in red wine—the skin is taken off earlier in the fermentation process of white wines, so they have less resveratrol—and also and can be purchased as supplements.
The resveratrol in grapes is also a proven aromatase inhibitor. Aromatase is an enzyme which converts testosterone and other androgens into estrogen.
Inhibiting its action is an important factor for people wanting to reduce estrogen in favor of maintaining and increasing lean mass, helping to promote testosterone.
Of note: The concentration of resveratrol is much higher in the skin of red grapes compared to red wine. Bottom line: Eat the fruit and steer clear of the drink.