OPC (oligomeric proanthocyanidins), also known as vitamin P, is a group of flavonols consisting of polymers of catechins and epicatechins. Proanthocyanidins are found in a whole range of fruits and plants, including the seeds and skins of grapes, in berries and citrus fruits, but also in tree bark.
The various proanthocyanidins are grouped together, depending on the number of units, as monomers (one unit) and dimers (two units). The exact definition of OPC is somewhat blurred - depending on the interpretation, either only the di- to tetramers, di- to pentamers, di- to hexamers or all proanthocyanidins except the monomers are referred to as OPC.
For a long time, only the dimers, i.e. consisting of two units, and the trimers, i.e. consisting of three units, were considered bioactive proanthocyanidins. However, this has been challenged with increasing regularity, and it is now believed that all components (including the polymeric proanthocyanidins such as hexamers and heptamers) of the grape seed extract interact in a complex way. OPC compounds are contained in prominent quantities in pine bark and grape seeds, with the individual proanthocyanidins contained therein differing from one another both in form and in their effect. The large amount of the important proanthocyanidin-B3-3'-O-gallate in grape seed extract is particularly noteworthy.