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Grape seed extract (GSE) is just what it sounds like ? an extract from grape seeds. The seeds are typically from red grapes (instead of white), which have a high content of compounds known as oligomeric proanthocyanidins (OPCs). The OPCs are also present in a wide variety of fruits and vegetables, including the pine bark (Pycnogenol) and green tea, where, like grape seed extract, they possess potent antioxidant properties.
The OPCs are chemically known as flavonoids or polyphenols, which can differ substantially based on their polymer arrangement. For example, polyphenols can exist in single (monomers), double (dimers), triple (trimers), quadruple (tetramers) and even longer "cyanidin" chains (tannins). Any chain length from 2-7 or so is termed and "oligomer" and longer chains are generally just called "polymers." It is generally assumed that the longer the cyanidin chain length, the less bioavailable and less active the molecule becomes. It may even be possible for the longer chain length compounds (tannins) to interfere with the absorption of other nutrients consumed at the same time. Many commercial grape seed extracts are standardized to a total OPC content, which may or may not take into account the assortment of dimers/trimers, etc. present in the final product.