Antioxidants have been suggested to reduce the risk of developing dementia, although evidence to support this hypothesis is under review. Free-radical reactions, which are reported to initiate cell injury, have been implicated in the pathology of various diseases including ageing processes, atherosclerosis, ischemic heart disease and neurodegenerative diseases which involve cognitive impairment. Antioxidants have therefore been considered as a means to modify and minimise neuronal degeneration in cognitive disorders.
A wide variety of plants have been associated with antioxidant effects. It is therefore not surprising that many different and structurally diverse phytochemicals have also shown antioxidant activity, including some cinnamic acids, coumarins, diterpenoids, flavonoids, monoterpenoids, phenylpropanoids and tannins. The antioxidant properties of Camellia sinensis Kuntze (Theaceae), commonly known as green tea, are well documented, and some studies suggest that Camellia sinensis extracts and some of the catechin components have protective mechanisms in neurodegenerative disorders. For example, (-)-epigallocatechin gallate had protective effects against beta-amyloid-induced neurotoxicity in vitro, an effect associated with its scavenging reactive oxygen species. However, the ability of the catechins to cross the blood-brain barrier may be restricted due to their polarity, thus limiting any therapeutic effect in practice.
Although many plants and their compounds have shown antioxidant effects in vitro, relatively few have been explored for their therapeutic and clinical relevance, particularly in relation to cognitive disorders. One plant which has shown favourable effects in the CNS is Thymus vulgaris L. (Lamiaceae). A study investigating Thymus vulgaris essential oil showed that it maintained higher polyunsaturated fatty acid (PUFA) levels in various tissues, including the brain in rats, indicating protective antioxidant effects. Other examples of plants which show antioxidant activity, particularly with reference to CNS pathologies, are described later in this site.