A water-soluble antioxidant compound (a peptide) made in the body and provided by the diet. The reduced form, (L-Glutathione) functions as an important antioxidant and as a co-substrate for antioxidant enzymes, such as Glutathione . Grape seed extract - Grape Seed Extract is an excellent source of biologically active flavonoids (polyphenols), which are natural compounds present in plant foods that may contribute significantly to the health and well-being of humans.