Many scientists think that man-made chemicals with an estrogenic effect (xenoestrogens) increase breast cancer incidence. As well, many argue that estrogenic plant compounds (phytoestrogens) also cause breast cancers. So the recent study comparing the effects of the xenoestrogen bisphenol-A (BPA) and the phytoestrogen genistein from soy is of great interest. This research focused on the effect of these chemicals in pre-pubertal rats because breast tissue develops extensively in prepuberty and early exposure to estrogenic compounds may change the way the breast develops and consequently how it responds to carcinogens later in life.
In every day life, we are exposed to BPA primarily from the plastic linings in food cans, from polycarbonate-softened plastic bottles, and from teeth sealants. Significant amounts of BPA are also present in our heat-treated cash register receipts. BPA is ubiquitous and some 95% of Americans tested have BPA in their urine, including young, prepubescent girls. Our exposure to BPA is chronic, with an estimated daily exposure of about 0.05 to 10 micrograms per day.
Genistein is a plant estrogen absorbed primarily from soy foods. In Asia, the daily intake of genistein ranges from 1 to 30 milligrams per day; the amount in the average Western diet is much lower. There is wide spread concern that early exposure to genistein also alters the way the breast develops later in life.
The study exposed prepubertal female rats to a daily low or high dose of either BPA or genistein. The doses were calculated to reflect the amounts of BPA or genistein that girls in the U.S. would be exposed to in daily life. Control rats were raised without these chemicals. At day 50, some of the rats were exposed to DMBA, a chemical known to cause a high incidence of breast cancer. The date of exposure was based on the fact that the female breast develops a considerable number of “terminal end buds” in the pre-pubertal period, structures that then are targets for carcinogenic chemicals such as the DMBA used in this study.
The results were fascinating. The rats exposed to BPA had a significant increase in the numbers of cancers triggered by the chemical compared to controls. In contrast, rats fed genistein had significantly fewer breast cancers compared to the control animals. In other words, the xenoestrogen increased the susceptibility to breast cancer while the phytoestrogen protected against the effect of the cancer-causing chemical.
Looking closely at the breast tissue of these rats, researchers found that genistein reduced the number of terminal end buds that are targeted by carcinogens. They also looked at the effects of BPA and genistein on cell growth, a key element in cancer. Specifically, they looked at their effect on annexin 2, a protein that enhances tumor growth and metastasis (cancer spread). BPA increased annexin 2 levels by 73% while genistein reduced annexin 2 levels by 67%. BPA and genistein had similar, opposing effects, on other proteins involved in breast cancer.
The internet is rife with postings urging us to avoid all soy foods – even traditional soy foods – because of their estrogenic effect. These postings often demonize phytoestrogens and, in my opinion, often fail to understand the complexity of phytoestrogens. While much of our exposure to phytoestrogens does come from soy, we also get phytoestrogens from flax, berries, grains, coffee, and dairy. As well, the anti-soy posts tend to ignore that that Asian women consuming a traditional diet fairly rich in soy had lower breast cancer rates than immigrant Asian women who ate a more Western diet. This most recent study tends to undermine the anti-soy position. Instead, it strongly suggests that phytoestrogens are not carcinogenic even though they are estrogenic.
I am of the opinion that phytoestrogens have a beneficial effect on our bodies because of their ability to protect us from toxins in our environment. Phytoestrogens are very different from endocrine disruptors such as the xenoestrogen BPA that make us more susceptible to the carcinogenic effect of other chemicals. Non-GM soy eaten as a traditional food in reasonable amounts – as opposed to drinking soy milk by the glass and eating processed foods filled with altered soy in the form of say soy protein isolate – can be a very healthy choice rather than something to be advocated against. And, given that we continue to increase our daily exposure to carcinogens, we likely need more phytoestrogens in our diet today than our hunter/gatherer ancestors did in the past.
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