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

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. 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 +/- A 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. 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 %). In this paper we report the rationale, design, recruitment, participant characteristics, and methods for biomarker and statistical analyses.