Does green tea protect against cancer?

Green tea does not demonstrably reduce overall cancer risk. The largest systematic analysis, covering more than 1.1 million participants, did find a slightly lower risk at high consumption levels, but that difference was not statistically significant. When looking at cancer mortality, there was no difference at all: the risk among tea drinkers and non-drinkers was virtually identical.

In the laboratory, the picture looks more promising. Green tea catechins, particularly the compound EGCG, inhibit processes that promote tumour growth, metastasis and blood vessel formation in cell and animal studies. But this type of laboratory result does not automatically translate into protection in humans. So far, that translated evidence simply does not exist.

For specific cancer types, the results are frankly contradictory. For oesophageal, prostate, urinary tract and blood cancer (leukaemia), some studies show a higher risk among tea drinkers while others show a lower risk. A reliable conclusion cannot be drawn from this evidence. The same applies to stomach cancer: of seventeen studies, just over half found possible protection, while the rest found none or even the opposite.

Concentrated green tea extracts taken as a supplement carry real risks. Several randomised trials show that gastrointestinal problems and elevated liver enzymes are the most common side effects. At high doses, liver injury can also occur. This applies to supplements, not to drinking ordinary tea. Three small studies observed a slight improvement in quality of life among cancer patients using the extract, but the supporting evidence is too thin to justify firm recommendations.