Women should not take dietary supplements containing green tea extract (GTE) in hopes of reducing the risk of breast cancer. Lab studies have found that green tea extract or certain polyphenols in green tea may have anti-cancer effects, but while some observational studies have found that tea drinkers have a lower risk of certain cancers, others have not.
It has been theorized that GTE might reduce cancer risk by altering blood levels of hormones and insulin-like growth factor (IGF). To test this notion, in an NIH-funded study in the Journal of Nutrition, researchers had 538 postmenopausal women at elevated risk for breast cancer (due to high breast density) take either a high-dose GTE supplement or a placebo for a year.
Contrary to expectations, GTE did not alter hormones or IGF in directions consistent with reduction of breast cancer risk. In fact, women taking the supplements ended up with increased blood levels of estradiol, on average, while the placebo group had a normal age-related decline in the hormone.
Previous research has found that elevated levels of estradiol are associated with increased breast cancer risk in postmenopausal women. High-dose GTE, found in many weight-loss supplements, can also cause liver damage, according to a 2017 analysis of data from the National Institute of Health’s Drug-Induced Liver Injury Network.
This article first appeared in the UC Berkeley Wellness Letter.
Also see Liver Damage from Dietary Supplements.