More important, when the scientists removed the isoflavones from soy and again fed it to rats, the soy no longer suppressed tumor growth, further suggesting anti-cancer properties. The mechanism by which isoflavones are thought to block tumor production works as follows. But some common types of breast cancer essentially feed off estrogen, so the goal is to keep estrogen out. That's where isoflavones of the sort found in soy products come in. These weak plant estrogens are close enough in structure to human estrogen that breast receptors mistake them as such and allow them in, which in effect blocks the entry of harmful human estrogen.
Epidemiologic evidence supports the theory.
Women throughout Asia, who for centuries have eaten much more tofu and other soy products than Americans, are much less likely to develop breast cancer than American women. Word about the cancer-fighting value of soy estrogens has certainly gotten out, no doubt contributing to the hundreds of millions of dollars in sales of soy products annually. But there's a soy snafu, with emerging science presenting a less certain picture of soy's benefits. In one study, women who were given soy supplements experienced increased proliferation of breast cells, at least initially. That's a potential problem because the more breast cells proliferate, the greater the chance of a mutation that could give rise to cancerous cells that would quickly grow into a tumor.