Green tea is brewed from the unfermented dried leaves of the plant Camellia sinensis. The predominant constituents of green tea are polyphenols belonging to the family of catechins, mainly (‒)-epigalocatechin gallate (EGCG), with lesser amounts of catechin (C), epicatechin (EC), epigalocatechin (EGC) and epicatechin gallate (ECG). In addition, caffeine, theanine, theaflavins and phenolic acids such as gallic acid are present in smaller quantities (Cooper et al., 2005). A typical brewed green tea beverage (250 mL) contains 50–100 mg of catechins and 30–40 mg of caffeine. However, the concentration of bioactive compounds of green tea can vary widely according to preparation methods, that is, brewing time or water temperature (Rains et al., 2011). Therefore, standardised green tea extract (GTE) has been developed for research to provide uniform levels of green tea catechins (GTCs).
In recent years, many health benefits of consuming green tea have been reported, including the prevention of diseases associated with free radicals and reactive oxygen species, such as cancer, or cardiovascular and neurodegenerative diseases. In addition to the antioxidant properties of the catechins, their anti-diabetic, anti-bacterial, anti-inflammatory and anti-obesity activities also have been reported (Zaveri, 2006). The health benefits of green tea are mainly attributed to its anti-oxidant properties, including the ability of catechins to scavenge reactive oxygen species or chelate with metal ions (Kashima, 1999). In addition to antioxidant effects, GTCs have been purported to influence several molecular targets in signal transduction pathways associated with cell death and survival (Murase et al., 2002). However, it is not known so far whether these effects on molecular endpoints in signal transduction pathways are downstream events of the modulation of pro-oxidant/antioxidant balance in cells or if they result from direct action of the catechins on molecular targets, independent of antioxidant properties (Zaveri, 2006).
This chapter highlights the recent research on the efficacy and mechanisms of action of GTCs on body weight, fat metabolism and oxidative stress parameters, with particular interest in their application in healthy, physically active and trained individuals.
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