Green tea has real but often overstated health benefits: the antioxidant activity and potentially cardioprotective associations are the most robustly supported, but for cancer prevention and other claims the evidence in humans is still too limited. Supplements containing high doses of EGCG carry risks that do not apply to ordinary tea consumption.
Green tea contains high concentrations of catechins, most notably EGCG, which display potent antioxidant activity in cell and animal studies. In the human body this effect is less certain: catechins are only partially absorbed in the gut, which means the translation from laboratory to human cannot be taken for granted. The antioxidant activity is therefore real, but its precise magnitude in humans has not yet been sufficiently established.
Several review studies report an association between regular green tea consumption and a lower risk of cardiovascular disease. However, this research is almost exclusively observational: people who drink a lot of green tea may also lead healthier lives in other respects. Large randomised trials that demonstrate cause and effect are largely absent. The association is promising but does not yet constitute a proven causal effect.
The hope that green tea might help prevent cancer is largely based on cell and animal studies. In those settings, green tea polyphenols do indeed inhibit cancer cells. In humans, the evidence remains limited and insufficient to conclude that green tea prevents cancer. The same applies to skin protection and possible protection against kidney stones, bacterial infections and tooth decay: the indications exist, but the strength of the evidence is weak.
The calming amino acid L-theanine found in green tea has shown a genuine effect on stress and anxiety in a systematic review of 9 randomised trials, at a dosage of 200 to 400 mg per day. However, a single cup of green tea contains only a fraction of that amount. Whether simply drinking tea provides enough L-theanine to produce a noticeable calming effect therefore remains unclear.
Finally, an important caveat regarding safety: serious side effects are rare with normal tea consumption, but adverse effects can occur with high doses of EGCG supplements. One review explicitly warns that alongside the benefits, the risks of tea constituents must also be taken seriously. Specific safe upper limits are not precisely defined in the available research. Green tea as a beverage is safe for most people; concentrated supplements are a different matter.
The claims are based on several review studies and a systematic review of RCTs (L-theanine). Most research into cardiovascular disease and cancer is observational; large randomised trials in humans are largely absent. Cell and animal studies are numerous but cannot be directly translated to humans.