BACKGROUND:More tea is consumed than any other beverage apart from water. Most teas come from the same plant, Camellia sinensis.
The intense green colour of fresh leaves is progressively lost as the leaves are processed to give green, oolong or, ultimately, black tea. Green tea contains compounds called catechins, which are believed to be the active ingredients. Further processing changes the catechins into other compounds, some of which are thought to have other effects in the body.
Green tea is the most commonly consumed tea in Asian cultures, and has long been associated with health benefits. As these have been documented more widely, green tea consumption has been increasing in western countries. Apart from green tea itself, extracts are available as food supplements, which contain higher concentrations of catechins.
EVIDENCE FROM STUDIES
Green tea catechins are good antioxidants, suggesting they may be helpful in preventing some chronic illnesses. Much interest has focused on the possible role of green tea in preventing cancer. Population studies have found that in Asian countries where green tea consumption is higher, the risk of certain cancers is lower. In addition, laboratory studies have found that individual catechins can prevent the growth of cancer cells.
This research has led to many studies examining the impact of green tea on the risk of various cancers. Three different groups published systematic reviews on this topic in 2009. The reviewers searched widely for studies, finding about 50 in total. All but two of these were observational studies. These sorts of studies can place people in different groups and observe their health and various lifestyle factors over years. Another approach is to take those who have cancer and look back at their health and lifestyles compared to others who don’t have cancer.
While these studies provide important information, they cannot determine which factors cause higher or lower risks of cancer. In addition, they have had conflicting results. Those examining cancers of the digestive tract were particularly contradictory. With liver and prostate cancer, the results were more suggestive that green tea might have some benefit. Randomised controlled trials are necessary to determine if an intervention causes benefit. Of the two located by these reviews, one found men at high risk of prostate cancer, who consumed green tea catechin supplements, developed prostate cancer less frequently than those taking a placebo. However, this study had only 60 participants. The other trial found green tea consumption did not change the risk of oesophageal cancer among 400 people.
PROBLEMATIC ASPECTS
Green tea itself is safe to consume, although excessive amounts can lead to overconsumption of caffeine. Adverse effects include irritability, restlessness and stomach upset.
Concerns were raised in 2009 about an interaction between green tea and a particular anti-cancer drug called bortezomid (Velcade). Researchers carried out laboratory and animal tests to determine whether green tea extract might boost the effectiveness of bortezomid. They found that the extract almost completely eliminated the drug’s anti-cancer activity. They established that the green tea catechins chemically combine with the drug making it inactive. Ongoing research is examining how much green tea is necessary to interfere with the drug in patients.
RECOMMENDATIONS
No clear recommendation can be given on whether green tea reduces the risk of cancer. Most studies could only suggest a potential link, but their results are highly variable. Randomised controlled trials are needed to establish whether green tea reduces the risk of cancer, but few have been conducted. In general, green tea is high in antioxidants which can supplement those in fruits and vegetables.
Dónal OMathúna has a PhD in pharmacy, researching herbal remedies, and an MA in bioethics, and is a senior lecturer in the School of Nursing, Dublin City University
