Does green tea protect against cancer?

In cell and animal research the picture is fairly consistent: the active compound in green tea (EGCG) kills cancer cells and inhibits their division, while healthy cells are affected far less. In animal experiments, tumour formation was suppressed at multiple sites, from skin and lung to breast and colon. However, the quantities that worked in animal studies were often much higher than what you would normally drink.

In humans the picture is considerably more erratic. Of ten epidemiological studies on stomach cancer, six suggest a protective association, but three actually found a higher risk. Studies on colon and rectal cancer show a similarly divided picture. For oesophageal cancer the association is more favourable, but there are few studies. In short: for no single type of cancer has a clear, consistent protective association been demonstrated in humans at this time.

A large Japanese population study found that people who drank more than ten Japanese cups per day developed cancer later than people who drank less. Premenopausal women with breast cancer who drank a lot of green tea had fewer lymph node metastases. These are interesting associations, but they do not prove a causal relationship: people who drink a lot of green tea often live differently in other ways too.

Supplements containing concentrated green tea extract call for caution. A small six-month safety study (equivalent to approximately fifteen cups per day) reported no serious side effects. But at high doses, the same substances that normally act as antioxidants can actually cause cell damage. The long-term risks of concentrated supplements therefore cannot yet be ruled out.