Eating soy foods does not increase risk of breast or endometrial cancer in postmenopausal women, and may even be protective, suggests a study published in the Journal of Clinical Endocrinology and Metabolism.
Despite the fact that the isoflavones found in soy have only 1/1,000th the potency of human estrogens, and epidemiological studies indicate that populations the consume diets high in soy have lower rates of breast cancer, the safety of consuming soy has been questioned because hormone replacement therapy has been found to increase breast cancer risk.
(We'd like to point out that the debate has not taken into consideration the fact that conventional hormone replacement therapy has meant the use of estrogens derived from pregnant mares, which are not only different from, but much more potent than human estrogens, and progestin, drug compounds similar but not identical to human progesterone. So, in our opinion, comparing the potency and possible side effects of conventional HRT to that of soy foods is like contrasting Godzilla with Bambi.)
Now, a animal study conducted at Wake Forest University Baptist Medical Center, has found that consuming the amount of soy phytoestrogens that would be ingested when soyfoods are included in the diet (in women, about 129 mg/day of isoflavones) does not increase risk of breast or uterine cancer, and appears to be protective.
In this study, laboratory animals with healthy breast tissue whose ovaries had been removed (to resemble postmenopausal women), were randomly assigned to receive one of three diets for 3 years:

• Group One was given soy protein isolate from which the isoflavones had been removed.
• Group Two received soy protein isolate containing soy isoflavones in a dose comparable to 129 mg/day, more than most people would get on a soyfoods-rich diet.
• Group Three was given soy protein concentrate from which the isoflavones had been removed and replaced with conjugated equine estrogens (Premarin, the equine estrogen commonly used in HRT) in an amount comparable to the usual dose given women, 0.625 mg/day.

The researchers measured a number of markers of cancer risk: breast density, numbers of dividing breast and uterine cells, sex steroid receptor expression, and blood levels of estrogen. In the monkeys receiving Premarin, levels of all cancer markers increased significantly. In contrast, the monkeys given soy with or without its isoflavones had no increase in any of the cancer markers. And animals receiving soy with isoflavones actually had significantly lower levels of estrogen than the animals given soy from which the isoflavones had been removed.
The researchers concluded that the high dietary levels of soy isoflavones did not increase markers related to uterine and breast cancer risk in the laboratory animals studied.
Lead researcher Charles Wood noted that the research addressed the effects of plant estrogens on normal breast tissue, and not in breast cancer. Whether soy is safe for breast cancer survivors remains "a big unanswered question," according to Wood.
Wood's research team has now begun investigating whether soy isoflavones can be used to block breast cell proliferation triggered by estrogen replacement therapy. The theory is that since soy isoflavones, but not estrogens, are similar enough in structure that they can bind to estrogen receptors, they can prevent the much more powerful human estrogen from doing so, thus reducing its effects in the body.
Until this research provides more answers, it looks like soyfoods may, at the very least, reduce cancer risk in postmenopausal women with healthy breast tissue.