Green tea is safe and may taste delicious, but if you're counting on it to prevent cancer, you may want to reconsider: A new review of studies including more than 1.6 million people has found "limited" evidence that green tea might help prevent some types of cancer.

"We can say for certain that green tea consumption can never account for cancer prevention alone," Dr. Katja Boehm of the Unconventional and Complementary Methods in Oncology Study Group in Nuremberg, Germany, who led the review, told Reuters Health via E-mail. "The lack of interventional studies in this field, where many health claims have been made in the past, is one of the most salient points of the review," she added.

Boehm and her colleagues looked at the existing scientific literature on green tea consumption-either by drinking the tea or taking green tea extracts-and found 51 studies suitable for inclusion in their review. Just one of the studies, of prostate cancer risk, was a randomized controlled trial, the highest standard of clinical evidence.

The investigations of green tea consumption and digestive cancers had "highly contradictory" results, Boehm and her team found, with "limited evidence" that green tea could reduce liver cancer risk, but conflicting evidence on a number of other cancers involving the digestive system.

For prostate cancer, the higher quality studies did indeed suggest a link between higher consumption of green tea in beverage or extract form and lower risk, the researchers found. Evidence on bladder and lung cancer was "limited to moderate," while there was some evidence that green tea could actually boost bladder cancer risk. Evidence that green tea was not protective against death from cancer of the stomach was "moderate to strong."

Forty-seven of the studies were done in Asia, Boehm and her colleagues note, where green tea consumption is much more common than elsewhere. "It is impossible to generalize the findings to the world more widely since the main bulk of evidence derives mainly from Asian cultures," the researcher said.

To get better answers on green tea and cancer prevention, she added, it will be necessary to perform a "large, well-designed" study with adequate green tea consumption," even though resources for doing these kinds of studies are limited. "We need to aim at creating high-level evidence in this much talked about but little researched topic area. Funding and infrastructure for clinical trials remain major challenges for the future."

Nevertheless, she and her colleagues conclude, people who enjoy drinking green tea should continue to do so. "Drinking green tea appears to be safe at regular, habitual and moderate use, within its recommended dose of 1200 milliliters per day, which is about 5-6 average cups per day," Boehm said.

SOURCE: Cochrane Database of Systematic Reviews Issue 3, 2009.