As a drink, several cups a day; or extract (watch the EGCG dose).
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Green tea is the infusion obtained from the unfermented leaves of Camellia sinensis, the same plant from which black tea (oxidized), oolong tea (partially oxidized), white tea (from very young buds), and special Japanese teas such as matcha and sencha are obtained; what distinguishes green tea from the others is that the leaves are processed quickly after harvest with steam or heat to inactivate the oxidative enzymes, thus preserving a profile of polyphenols (especially catechins) that in oxidized teas is transformed into other compounds (theaflavins, thearubigins) with different pharmacological properties.
The active components identified are the catechins (epigallocatechin gallate, or EGCG, is the most studied and accounts for up to 50-80 percent of total catechins), epigallocatechin (EGC), epicatechin gallate (ECG), and epicatechin (EC), along with caffeine, L-theanine (an amino acid unique to Camellia with an anxiolytic effect that modulates alertness), minor flavonoids, and minerals; this combined profile explains the characteristic "calm alertness" effect of green tea (thanks to the combination of caffeine + L-theanine, which is qualitatively different from the purely stimulating effect of caffeine alone in coffee) and the broad clinical profile of metabolic, cardiovascular, oncological, and cognitive modulation attributed to regular consumption.
The most robust epidemiological evidence associates regular green tea consumption (3-5 cups daily or equivalent) with modest but significant reductions in cardiovascular mortality, all-cause mortality, and cardiovascular events in prospective Japanese and Chinese cohorts; these effects are attributed to multiple synergistic mechanisms: improvement of the lipid profile (modest reduction of LDL and triglycerides), reduction of blood pressure, improvement of endothelial function, modulation of chronic inflammation, and possibly effects on body composition.
In terms of body composition, green tea catechins combined with caffeine produce modest increases in thermogenesis and fat oxidation: several meta-analyses have documented body weight reductions of 1-2 kg over 12 weeks with green tea extracts standardized for EGCG compared with placebo, clinically small magnitudes that do not justify green tea as a primary weight-loss strategy but that may have a place as a very modest complement.
In cancer prevention, the epidemiological data are interesting and consistently favorable (inverse associations with several specific cancers in populations with high consumption), but specific intervention trials are less conclusive; the role of green tea in chemoprevention remains the subject of active research, with promising but not definitive data.
In cognitive function, several studies have documented modest improvements in attention, memory, and processing speed with acute green tea consumption, attributed to the characteristic combination of caffeine (stimulant) and L-theanine (modulator), which produces a more balanced cognitive effect than caffeine alone; with chronic use, regular consumption is associated with a lower incidence of mild cognitive impairment and dementia in Asian cohorts.
An important clinical concern with concentrated commercial green tea extracts (not with drinking green tea as a beverage) is idiosyncratic hepatotoxicity: dozens of clinical cases of acute hepatitis associated with the use of green tea extract supplements have been documented, some serious, including cases that required liver transplantation; the risk appears particularly related to high doses (more than 800 mg/day of EGCG), prolonged use, consumption on an empty stomach, and individual susceptibility; the FDA and other regulatory agencies have issued specific warnings about this toxicity.
Drinking green tea as a beverage (3-5 cups daily) is the most studied form and has the best safety profile; typical commercial extracts contain 100-500 mg of EGCG per capsule and should be used with care at moderate doses (no more than 300-400 mg/day of total EGCG) and always with food to minimize the risk of hepatotoxicity; matcha and other special green teas provide particularly concentrated amounts of catechins and other components.
The most important precautions are: the hepatotoxicity with concentrated extracts discussed above; caffeine (variable depending on preparation, typically 25-50 mg per cup of green tea, less than coffee) can cause insomnia if taken late or nervousness in sensitive people; people who take anticoagulants (especially warfarin) should keep their consumption stable because the high doses of vitamin K in green tea can interfere with the INR; those who take drugs absorbed in the small intestine may experience interference with absorption because of the high tannin content (separate doses by at least 1-2 hours); people with iron-deficiency anemia should avoid taking green tea with meals because tannins significantly inhibit the absorption of non-heme iron (separate by at least 1-2 hours); in pregnancy it is recommended not to exceed 200 mg/day of total caffeine (approximately 2-3 cups of green tea); in breastfeeding, moderate consumption is acceptable; high-concentration commercial extracts should be used with caution and avoided completely in people with preexisting liver disease or who take other hepatotoxic drugs; drinking green tea as a beverage in moderate amounts (3-5 cups/day) is probably the option with the best risk-benefit balance for most people who want to incorporate the benefits of green tea into their diet.
Information, not medical advice. Scientific evidence reflects the quality of the studies.