The skin of the grapeseed is a rich source of proanthocyanidins (also referred to as procyanidins). Mixtures of procyanidins are referred to as OPCs. Grapeseed extract contains OPCs made up of dimers or trimers of (+)-catechin and (-)-epicatechin (Fine 2000) and also trimers and polymers of proanthocyanidins. Vitis vinifera also contains stilbenes (resveratrol and viniferins) (Bavaresco et al 1999); however, it is unclear whether significant amounts are present in the seeds.
Clinical note— Proanthocyanidins
Proanthocyanidins (PCs) are a group of naturally occurring polyphenols bioflavonoids that are present in many fruits (e.g. apples, pears, grapes and peaches), vegetables, nuts, beans (e.g. cocoa), seeds, flowers and bark (e.g. pine) (Bavaresco et al 1999). Grapeseeds are a particularly rich source of PCs, containing more than any other grape products, such as red, white or rose wine or grape juice,
and more than most commonly available foods (Rasmussen et al 2005a). Proanthocyanidins are also found in many medicinal herbs such as Ginkgo biloba, Camellia sinensis, Hypericum perforatum and Crataegus monogyna; however, GSE is considered the superior source. Proanthocyanidins demonstrate a wide range of biological actions according to various in vitro, in vivo and clinical studies. However, in recent years, bioavailability studies have demonstrated that not all orally ingested PCs are absorbed. In particular, PC polymers have negligible absorption from the gastrointestinal tract whereas low-molecular-weight PCs (monomers, dimers and trimers) are absorbed (Rasmussen et al 2005b). In addition, some PCs are degraded by microflora in the caecum and large intestine into low-molecular-weight phenolic acids, chiefly hydroxyphenylpropionic acid and 4-O-methylgallic acid (Ward et al 2004), which are likely to contribute to the biological effects. These findings have implications when interpreting in vitro data because this method of testing does not take into account variations in bioavailability and metabolism in the body.
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