Dr. Katherine D. Crew of Columbia University Medical Center in New York and colleagues said they made the discovery during a secondary analysis of an earlier randomized, placebo-controlled study of Polyphenon E in a group of 40 women with hormone receptor-negative breast cancer.
"Many preclinical studies looked at epigallocatechin-3-gallate, or EGCG, which is one of the main components of green tea, and the various possible mechanisms of its action against cancer, but it is very difficult to do those same kinds of studies in humans," Crew said.
"This study was too small to say for sure if green tea will prevent breast cancer, but it may move us forward in terms of understanding anti-tumor mechanisms."
In the primary analysis, 40 women were randomly assigned to 400 milligrams, 600 mg or 800 mg of Polyphenon E or to placebo twice daily for six months. During the study, the researchers collected blood and urine samples from participants at baseline and at two, four and six months.
Women assigned to the extract had a significant reduction in hepatocyte growth factor levels -- important to breast cancer development -- at two months compared with women assigned the placebo.
However, at the four-month and six-month follow-ups, the difference was no longer statistically significant, the study said.
The findings were presented at the annual American Association for Cancer Research International Conference on Frontiers in Cancer Prevention Research in Los Angeles.