Jan. 27, 1999 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.
Cline said further work is needed to explore the basic mechanisms by which soy estrogens decrease the incidence of breast cancer.
"Widespread consumption of phytochemicals in the human diet and as supplements make it imperative that we understand their effects, particularly as they related to differing developmental and functional stages of the breast," Cline said.
Cline also talked about the differences between people with apparently similar genetic heritage, particularly Asians. "Asian diets typically contain less fat and a higher proportion of vegetables, including soy protein, when compared to Western diets," he said. Striking differences in cancer rates between Asians living in Asia and their cousins living in America has long been known.
Perhaps more important, recent studies have shown that American-born children of Asian immigrants have a 60 percent higher risk of developing breast cancer than do people born in Asia who immigrate to America and now are eating Western diets. "This implies that the breast-cancer protective effect may occur early in life," he said.
But the mechanism is still a mystery.
"The contribution of vegetable and fiber intake to lower cancer risk is clear. However the degree to which one can attribute the anticancer effect to plant-derived estrogenic compounds is still uncertain."
Cline said not all women would benefit equally from consuming estrogens in the soy, because of individual differences both in how the soy estrogen is metabolized and in the woman's ability to respond to the estrogen.
"The benefit of soy phytoestrogens is clearer for dietary exposures early in life and for premenopausal women," Cline said. "The benefit for older women and breast cancer survivors remains to be determined."
Cline's study was supported both by the Department of the Army and the National Institutes of Health.
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