Plasma levels of enterolactone and percentage mammographic density among postmenopausal women.

AIMS Certain phytoestrogens, such as lignans, may protect against developing breast cancer. Enterolactone is a lignan metabolite produced by the intestinal flora from dietary precursors such as whole grains, vegetables, and fruits. Enterolactone has been shown to have weak estrogenic and antiestrogenic properties. We decided to examine the association between plasma levels of enterolactone and mammographic density, a biomarker for breast cancer risk. METHODS We included data from postmenopausal women ages 55 and older who participated in a cross-sectional mammogram study in Tromsø, Norway. Mammograms, plasma enterolactone measurements, as well as information on anthropometric and hormonal/reproduction factors were available on 616 women. We assessed mammographic density using a previously validated computer-assisted method. We estimated correlation coefficients and conducted multiple regression analyses. RESULTS Mean mammographic density increased slightly across quartiles of enterolactone; the women in the highest quartile had, on average, 3.1% (absolute difference) higher percentage mammographic density compared with the lowest quartile (P(trend) < 0.01). After adjustment for age, body mass index, number of full-term pregnancies, age at first birth, and use of postmenopausal hormone therapy, the mean difference in density was reduced to 2.0% (P(trend) = 0.05). Results were similar when restricted to the 454 current hormone nonusers. The fully adjusted statistical model explained 28.3% of the total variability in mammographic percentage density, with body mass index contributing 18.2% and enterolactone only 0.9%. CONCLUSION In our study, higher levels of enterolactone were associated with slightly higher percentage mammographic density. Our results suggest that if higher enterolactone levels reduce the risk of developing breast cancer in postmenopausal women, then this effect is not through lowering mammographic density.