Cochrane reviews – in their own words

Abstract

“Tea brewed from the leaves of the plant Camellia sinensis is the second most common drink in the world. Only plain water is more common. Globally, 20% of the tea that is drunk is green tea, with it being especially popular in Asia. Many claims have been made for its ability to protect against illness and we investigated one of these claims in our Cochrane review (1). We analyzed a total of 51 studies, including over 1.6 million participants, to assess possible associations between green tea and cancer. This is the first investigation on such a large scale. But, despite this large amount of data, the jury still seems to be out on whether green tea can prevent the development of various cancer types. We searched through decades of literature and contacted makers of green tea. The earliest study was published in 1985 and the most recent one in 2008. Studies investigated several different types of cancer, including gastro-intestinal, uro-genital tract, breast, oral and lung cancer. However, very few randomised trials have been done and we had to rely almost completely on observational studies. Such studies do not allow us to decide whether there is a casual relationship between green tea and cancer, as things other than tea might have influenced the development of cancer. The studies are also at risk of recall bias, which means that participants may not remember the exact amount or duration of their green tea consumption when asked retrospectively. When investigating the effects of green tea, we knew that it would be very unlikely that its effects would be “good” in all circumstances. Cancerous diseases and their mechanisms of progression are too varied. Therefore, if green tea has an effect, it is possible that there will be groups of people or diseases for which it plays a protective role, others for which it has no effect and others for which it is harmful. We also needed to distinguish between a behavior that is culturally based, such as drinking tea, and what might be perceived as a medical measure, such as taking an extract or a tablet. For some people, the tea may be part of their everyday behavior and for others it might be seen as something more medical. In summary, we found that the evidence for green tea preventing cancer is sparse and very inconsistent. The research that we reviewed does not allow us to draw any valid conclusions about the associations of green tea consumption on cancer incidence and mortality. It is also impossible to generalize the findings to the world more widely. The evidence comes mainly from Asian cultures, since this is where most of the included studies were done. Research from Europe and North America is lacking and such studies are needed before making generalizations across different cultures. One thing is certain: green tea consumption can never account for cancer prevention alone. But, more research is still justified. Evidence-based medicine has changed preventative practice and has produced a lasting need for more randomized trials, systematic reviews and meta-analyses. We need to aim at the creation of high quality evidence for this much talked about, but little researched, topic. One should always keep in mind that damaging effects can also be possible, since “natural” does not automatically equal “safe”. As well as trials, we strongly recommend the conduct of large, well-designed, prospective cohort studies, investigating adequate levels of green tea consumption. In conclusion, people who enjoy a cup of green tea should continue to drink it. Drinking green tea appears to be safe at regular, habitual and moderate use, within its recommended dose of 1200 ml per day. However, getting an answer to whether or not it prevents cancer needs to wait for new, and better, research.”