Consumption of
phytoestrogen (PE)-rich foods (i. e., soy and flaxseed (FS)) is increasing because of their suggested health benefits. However, recent studies raise concern over the safety of soy and its
isoflavones, particularly
genistein (GEN), for postmenopausal
breast cancer (BC), due to their potential stimulatory effects on human breast tissue and on the growth of existing
tumors in rodents. FS, rich in PE
lignans, which is metabolized to the mammalian
lignans enterolactone (ENL) and
enterodiol (END), has consistently been shown to have
tumor inhibitory effects in a human clinical trial as well as rodent BC models. Using the preclinical athymic mouse postmenopausal BC model, combining FS with
soy protein or GEN with END and ENL, was found to negate the
tumor stimulatory effects of
soy protein or GEN alone. The mechanism may be related to the modulation of
estrogen receptor and MAPK signaling pathways. If these studies can be confirmed in clinical trials, then consumption of combined soy and FS, or their PEs, may reduce the
tumor growth stimulatory effect of soy or GEN. This may indicate that if soy is consumed with
lignan-rich foods, it may continue to induce its other beneficial health effects, without inducing adverse effect on postmenopausal BC.