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Risk of Cancer by Consuming Soy Isoflavones

written by: Jillita Horton • edited by: Rhonda Callow • updated: 8/17/2010

You've perhaps heard that the isoflavones in soy "replace" the body's natural estrogen, thus lowering breast cancer risk. Being that lifetime exposure to estrogen influences breast cancer risk, doesn't it stand to reason that a woman does not need to be eating foods with estrogen-like compounds?

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    The promotion is that a “soy-ful” diet helps lower breast cancer risk. After all, many breast cancers are triggered by excess circulating estrogen. Soy, with its estrogenic compounds (isoflavones), supposedly cuts cancer risk because the isoflavones bind to estrogen receptor sites in breast tissue. This creates the net effect of less estrogen in a woman’s body, thereby lowering cancer risk.

    But watch out, because isoflavones have been shown to stimulate breast tissue growth. This is a precursor to cancer development, since malignancies are out-of-control cell division. We’ve been taught to believe that the phytoestrogens (“phyto” means plant) in soy, when binding to those estrogen receptor sites, block the more potent natural estrogens from attaching to those sites.

    Phytoestrogens have about 1/1,000th the biological activity of human estrogens. A 2005 issue of Breast Cancer Research and Treatment published results of a study: Isoflavones produce an antioxidant effect on breast cancer survivors. Phytoestrogens tie up the parking spots for natural estrogens. This all sounds sweet on paper.

    There are two kinds of soy: fermented and unfermented. While Asians typically eat only the fermented type, Americans gobble down in much larger quantities, the unfermented version, and therein lies a real problem. When soy is processed, its isoflavone composition is altered in a way that nature did not intend.

    Isoflavones are not antioxidants. They are estrogenic compounds (“estrogenic” means estrogen-like). A woman does not need more estrogen-like substances in her body, being that lifetime exposure to estrogen plays a role in breast cancer risk! When a woman eats estrogenic compounds, this gets tacked onto her lifetime exposure to estrogen!

    Alarm was created by a study that was reported in Cancer Epidemiology and Biomarkers Prevention (October 1996), in which women took 38 grams of genistein, a soy isoflavone, daily for one year. This resulted in higher blood levels of the most potent human estrogen, estradiol. However, this doesn’t seem to happen when whole soy is consumed. It depends on how you ingest soy.

    Breast tumor growth was stimulated when genistein was eaten, but not when whole soy was eaten, according to a study published in the May 2004 Carcinogenesis. And consider this statement from the May 2002 Proceedings of the National Academy of Sciences: “The greater an animal’s exposure to genistein, the smaller its thymus…Genistein also reduces the number of immune cells…”

    An impaired immune system is a risk factor for any cancer. Anything that creates fertile conditions for stimulated breast tissue growth is cause for caution and further investigation. In the meantime, it’d be smart to avoid soy isoflavone supplements that come in a bottle, as well as processed soy, in which the isoflavone composition has been altered from nature’s intent. Though studies have been focused more on isolated phytoestrogens, versus whole soy, why take a chance?

    Nevertheless, avoid processed soy (soy powders, soy bars, soy sausage, soy yogurt, etc.), because in nature, genistein occurs in a certain ratio to soy’s many other constituents. The ratio jumps up when whole soy sustains processing methods. You do not want disproportionately high-genistein foods.

    If you truly enjoy soy for its flavor, stick to tempeh, miso, natto and other whole-soy forms; unfermented soy. And at that, aim for no more than several 6-8 gram servings per week.