"This is convincing evidence that at dietary levels, the estrogens found in soy do not stimulate cell growth and other markers for cancer risk," said Charles E. Wood, D.V.M., lead researcher, from Wake Forest University Baptist Medical Center. "The findings should be especially interesting to women at high risk for breast cancer who take soy products."
The research is reported in the current issue of The Journal of Clinical Endocrinology & Metabolism.
Wood said there has been much debate about whether high levels of dietary soy are safe for postmenopausal women. Soy products are sold as a natural alternative to traditional hormone therapy. The most common form of hormone therapy, estrogen plus a progestin, has been shown to increase risk of breast cancer. Soy and some other plants contain estrogen-like compounds called isoflavones or phytoestrogens.
These plant estrogens are thousands of times weaker than the estrogen produced by the body, but may be present in much higher concentrations in the blood. Evidence about their safety has been mixed. It is known that populations that typically consume diets high in soy have lower rates of breast cancer. On the other hand, some studies have shown that soy isoflavones can stimulate breast cancer cells grown in the laboratory.
"Evidence from observational studies in women indicates that soy intake may help prevent breast cancer," said Wood. "But there has still been reluctance to conduct research studies in women because of concerns that isoflavones may stimulate breast cell growth and increase the risk of breast cancer."
Wood and colleagues measured how a diet high in soy isofllavones affected markers for breast and uterine cancer risk in postmenopausal monkeys. The monkeys ate one of three diets for three years: soy that didn't contain isoflavones, soy with the isoflavones intact, or soy without isoflavones, but with Premarin, or estrogen therapy, added.
The isoflavone group consumed the human equivalent of about 129 milligrams a day, more than most people would get in a soy-rich diet.
The researchers measured breast density, numbers of dividing breast and uterine cells, and levels of the estrogen produced by the body - all markers for cancer risk. Monkeys on the soy plus estrogen diet had increased levels of all markers, while monkeys that ate soy with isoflavones did not.
In fact, the monkeys eating soy with isoflavones had lower levels of the estrogen produced by the body. High levels of this estrogen are considered an important predictor of breast cancer risk in postmenopausal women.
"These findings suggest that high dietary levels of soy isoflavones do not increase markers for breast and uterine cancer risk in postmenopausal monkeys and may contribute to an estrogen profile associated with reduced breast cancer risk," said the researchers.
Wood said it is important to note that the research addressed the effects of plant estrogens on normal breast tissue, and not in breast cancer.
"A big unanswered question is whether it is safe for breast cancer survivors to turn to soy," he said.
Researchers are not certain how plant estrogens and the estrogen produced by the body, or given in pills, act together. One theory is that the plant estrogens bind to cells that have estrogen receptors, such as breast tissue, and block the effects of the other types of estrogen. Isoflavones may also help reduce the amount of active estrogen in the body.
To investigate these ideas, Wood and colleagues are currently looking at whether soy may block breast cell proliferation induced by estrogen therapy.
Other researchers involved the study included J. Mark Cline, Ph.D., D.V.M., Mary S. Anthony, Ph.D., Thomas C. Register, Ph.D., and Nancy D. Kock, Ph.D., D.V.M., all from Wake Forest Baptist.
The research was funded by grants from the National Institutes of Health.
About Wake Forest University Baptist Medical Center: Wake Forest Baptist is an academic health system comprised of North Carolina Baptist Hospital and Wake Forest University Health Sciences, which operates the university's School of Medicine. The system comprises 1,282 acute care, psychiatric, rehabilitation and long-term care beds and is consistently ranked as one of "America's Best Hospitals" by U.S. News & World Report.