WINSTON-SALEM, N.C. -- Dietary soy may reduce the risk of cancer by counteracting the cell-proliferating effect of estrogen-replacement therapy, a Wake Forest University researcher reported today (Jan. 25, 1999) at the meeting of the American Association for the Advancement of Science.
J. Mark Cline, D.V.M., Ph.D., assistant professor of comparative medicine, said that studies he conducted in postmenopausal monkeys showed that, as expected, estrogen replacement therapy induced cell proliferation -- a cancer precursor -- in both the mammary glands of the breast and in endometrial tissue.
But when monkeys got dietary soy such as found in tofu -- or soy supplements -- the cell proliferation was counteracted. "These data indicate that soy supplements may decrease breast and endometrial cell proliferation and therefore could decrease cancer risk in these tissues," Cline said.
In other words, the soy protein played much the same role as progestin does in common estrogen/progestin formulations by reducing the risk of cancer.
However, there may be some risks involved. He said the relative dose of soy protein -- which contains plant estrogens (called phytoestrogens) -- and estrogen replacement therapy may be critical.
Cline's caution came from studies he conducted in rats whose ovaries had been removed. He tested combinations of estrogen and soy proteins. The soy proteins did not cause an increase in cell proliferation either in the uterus or mammary gland. But at low doses of estrogen replacement therapy, the dose of soy estrogen found in natural soy caused an increase in cell proliferation un the breast.
At a higher dose of estrogen replacement therapy, however, the soy diminished the cell proliferation in both the uterus and breast. "At some dose combinations, the two are additive, and at other dose combinations, they are antagonistic, Cline said.