Evidence about green tea and cancer

Some laboratory studies have shown that extracts from green tea can stop cancer cells from growing. Green tea contains substances called polyphenols, which scientists think give it antioxidantproperties. But while these lab results are encouraging, we need evidence from human studies to prove them.

The evidence we have at the moment from human studies is mixed. A meta-analysis published in 2006 suggested that green tea may lower the risk of breast cancer. But an earlier meta-analysis could not find any significant evidence to support a reduction in risk of breast cancer.

Other studies are also conflicting. A meta analysis in 2006 concluded there was not
enough evidence to show that green or black tea can help prevent bowel cancer, in spite of some promising results in the lab.

A Chinese study of more than 18,000 men, published in 2002, found that green tea drinkers were about half as likely to develop cancer of the stomach or food pipe as men who drank little tea . But a systematic review in 2004 was unable to show that green tea could prevent stomach or intestinal cancer.

It has also been suggested that green tea may help prevent prostate cancer. However, a study of almost 20,000 Japanese men, published in the British Journal of Cancer in 2006, found no relationship between green tea and prostate cancer.

In July 2009, Cochrane published a review of 51 studies which included over 1.5 million people. This review was looking for a link between green tea consumption and cancer. The cancers included in the review were digestive tract, breast, bladder, prostate and lung cancer as well as cancer of the oral cavity. They conclude that the research so far is conflicting which means that green tea remains unproven in preventing cancer. They say it is safe to drink in moderate amounts, 3 to 5 cups a day.

So we really need more evidence from randomised controlled clinical trials to help us know whether green tea has a role in preventing cancer. At the moment it is impossible to draw any firm conclusions because

• Only a small number of studies have been done
• There is a lack of randomised controlled trials
• There are differences in lifestyle factors within studies (most studies have been done in East Asia)
• Different amounts of tea have been taken within the studies
• There is possible interaction with other treatments or diet supplements used by people in the studies