The Minnesota Green Tea Trial (MGTT), a randomized controlled trial of the efficacy of green tea extract on biomarkers of breast cancer risk: study rationale, design, methods, and participant characteristics

Purpose: The Minnesota Green Tea Trial (MGTT) was a randomized, placebo-controlled, double-blinded trial investigating the effect of daily green tea extract consumption for 12 months on biomarkers of breast cancer risk. Methods: Participants were healthy postmenopausal women at high risk of breast cancer due to dense breast tissue with differing catechol-O-methyltransferase (COMT) genotypes. The intervention was a green tea catechin extract containing 843.0 ± 44.0 mg/day epigallocatechin gallate or placebo capsules for 1 year. Annual digital screening mammograms were obtained at baseline and month 12, and fasting blood and 24-h urine samples were provided at baseline and at months 6 and 12. Primary endpoints included changes in percent mammographic density, circulating endogenous sex hormones, and insulin-like growth factor axis proteins; secondary endpoints were changes in urinary estrogens and estrogen metabolites and circulating F2-isoprostanes, a biomarker of oxidative stress. Results: The MGTT screened more than 100,000 mammograms and randomized 1,075 participants based on treatment (green tea extract vs. placebo), stratified by COMT genotype activity (high COMT vs. low/intermediate COMT genotype activity). A total of 937 women successfully completed the study and 138 dropped out (overall dropout rate = 12.8 %). Conclusions: In this paper we report the rationale, design, recruitment, participant characteristics, and methods for biomarker and statistical analyses.