The cardioprotective effect of red wine has been attributed to a group of bioflavonoid compounds called proanthocyanidins. Proanthocyanidins are concentrated in the seeds of grapes. These compounds are incorporated into red wine during the fermentation process, when the must (crushed grapes, seeds, stems, skin, and juice) is held in vats for several weeks.

The bioflavonoids in red wine and grape seed extract comprise a complex mixture of two monomer (or single unit) compounds named catechin and epicatechin, and various polymeric products of these two compounds. In vitro studies on catechin, epicatechin, and the grape bioflavonoids have shown them to be free radical scavengers that may be even more potent than the antioxidant vitamins C and E.

These compounds effectively inhibit oxidation of human low-density lipoproteins (LDL). Grape seed extract also demonstrates anti-inflammatory activity, and a growing body of research suggests an additional role in enhancing capillary strength.

The principal food source for grape seed bioflavonoids is red wine. White wine and grape juice contain smaller amounts of proanthocyanidins.