A variety of antioxidant compounds derived from natural products\r\n(nutraceuticals) have demonstrated neuroprotective activity in either in vitro or in vivo\r\nmodels of neuronal cell death or neurodegeneration, respectively. These natural\r\nantioxidants fall into several distinct groups based on their chemical structures:\r\n(1) flavonoid polyphenols like epigallocatechin 3-gallate (EGCG) from green tea and\r\nquercetin from apples; (2) non-flavonoid polyphenols such as curcumin from tumeric and\r\nresveratrol from grapes; (3) phenolic acids or phenolic diterpenes such as rosmarinic acid\r\nor carnosic acid, respectively, both from rosemary; and (4) organosulfur compounds\r\nincluding the isothiocyanate, L-sulforaphane, from broccoli and the thiosulfonate allicin,\r\nfrom garlic. All of these compounds are generally considered to be antioxidants. They may\r\nbe classified this way either because they directly scavenge free radicals or they indirectly\r\nincrease endogenous cellular antioxidant defenses, for example, via activation of the\r\nnuclear factor erythroid-derived 2-related factor 2 (Nrf2) transcription factor pathway.\r\nAlternative mechanisms of action have also been suggested for the neuroprotective effects\r\nof these compounds such as modulation of signal transduction cascades or effects on\r\ngene expression. Here, we review the literature pertaining to these various classes\r\nof nutraceutical antioxidants and discuss their potential therapeutic value in\r\nneurodegenerative diseases.
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