Nearly all the women in the study had already gone through menopause, and most took the estrogen-blocking drug tamoxifen. So where did their estrogen come from? Fat tissue, says study investigator Cheryl Rock, PhD, RD, a professor in the cancer prevention program at the University of California, San Diego.
"Fat tissue is the primary non-ovarian site for estrogen production in the body," Rock tells WebMD. "And what makes this worse is that the elevated insulin levels associated with obesity are suppressing sex-hormone binding globulin [which removes free estrogen from the circulation]."
"Anti-estrogen drugs are not going to solve this problem for everybody, so let's look for the lifestyle issues that can help," she says. "Exercise and losing weight gives breast cancer survivors two bangs for the buck. You not only reduce estrogen from fat -- the main organ making estrogen in postmenopausal women -- but you improve your heart health, too."
Rock's advice makes a lot of sense to Charles Cox, MD, a surgical oncologist in the breast program at Tampa's Moffitt Cancer Center.
"Exercise and reducing weight can be an effective therapy for breast cancer patients because this lowers their overall stores of estrogen," Cox tells WebMD. "This puts patients in charge of their own life expectancy. What they do doesn't just reduce their odds of cancer recurrence: It improves their overall outcome."
The study by Rock and colleagues looked at baseline estrogen levels in 153 breast cancer survivors whose breast cancer returned and in 153 age- and cancer-stage-matched women whose breast cancer did not come back. All of the women were followed for more than seven years as part of a study of how diet affects breast cancer recurrence.
Rock and colleagues report the findings in the March issue of the journal Cancer Epidemiology, Biomarkers, and Prevention.