Adapted from the NCI Cancer Bulletin.
Firm recommendations on the use of green tea in liquid or extract form to prevent cancer cannot be made based on the available evidence from published studies, according to the authors of a new review published last month in the Cochrane Database of Systematic Reviews. The conflicting evidence from the available studies, lead author Dr. Katja Boehm and colleagues concluded, means “that drinking green tea remains unproven in cancer prevention, but appears to be safe at moderate, regular, and habitual use.”
Fifty-one studies involving approximately 1.6 million people were included in the review, and 45 of those studies were conducted in Japan and China. Approximately half of the studies focused on gastrointestinal (GI) cancers, mostly those in the upper GI tract (e.g., pancreas, esophagus, liver). While a single case-control study showed a benefit of green tea consumption for the reduction of liver cancer risk, the studies looking at other GI cancer sites had “highly contradictory” results, the authors explained.
Other cancers covered by the review included lung, breast, ovarian, bladder, prostate, and oral cancer.
The review itself has limitations, the authors acknowledged, including the fact that the studies included were of varying methodological quality and were largely confined to Asian populations. Only one randomized clinical trial, for prostate cancer prevention, was available for the review (the trial found a decreased risk of prostate cancer among high-risk men who received a green tea extract compared with those who received placebo).
“In our view, the possibility of measuring the therapeutic effect of green tea based on isolated case-control or cohort studies is not very likely as other confounding variables come into place,” the authors wrote. If the potential preventive effects of green tea are to be established, they continued, large, well-designed randomized clinical trials will be needed.
Despite the findings, said Dr. John Milner, chief of the Nutritional Science Research Group in NCI’s Division of Cancer Prevention, green tea is a food item worth investigating further. It’s likely, he noted, that “exposures and interactions with a host of environmental and genetic factors influence individuals’ response to green tea.”