Is green tea really as healthy as they say?
Green tea as a daily beverage is safe for most people and there are promising indications of cardiovascular benefits, but the evidence still rests largely on observational and laboratory research rather than clinically proven effects.
Green tea contains high concentrations of catechins, most notably EGCG, which demonstrate powerful antioxidant activity in cell and animal studies. In the human body that effect is less certain: catechins are only partially absorbed in the gut, which means the leap from laboratory to human is by no means straightforward. The antioxidant activity has therefore been convincingly demonstrated in the lab, but its precise magnitude in people 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 a proven causal effect it is not.
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: there are indications, but the strength of 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 delivers enough L-theanine for a noticeable calming effect therefore remains unclear.
At normal levels of tea consumption serious side effects are rare, but they can occur at high doses of EGCG supplements, up to and including liver damage. This is not only a theoretical risk: since the end of 2022 the EU has a legal limit. Green tea extract with 800 mg EGCG or more per daily portion is prohibited, and products containing EGCG must carry mandatory warnings about possible liver damage and are not intended for pregnant or breastfeeding women or children. None of this applies to drinking ordinary tea: that 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.