Many plants produce estrogen-like substances called phytoestrogens. These plant estrogens can interfere with the interactions between mammalian estrogens and their estrogen receptors (see Our Stolen Future, Chapter 5). Industry flacks have claimed that the volume of phytoestrogens in the human diet is so large compared to synthetic estrogens that any worry about contaminants is misguided.

Their argument fails on several grounds. First, it fails because much, but not all, of the exposure to synthetic hormone disruptors is not dietary. Their argument is irrelevant to non-dietary exposures. Second, it fails because it completely ignores the fact that many hormone disruptors interfere with hormones other than estrogen.

And finally, it fails even within the realm of dietary exposure to estrogen-like compounds. It turns out that the body is quite capable of avoiding the impact of many (but not all) phytoestrogens because we have evolved defenses to cope with them.

In fact, the crucial distinction is not whether a compound is a plant vs. a synthetic estrogen. Instead the focus should be on two issues:

• Does a specific estrogen like substance overcomes the body's physiological defenses?
• Does it persist and bioaccumulate?

These two questions together determine whether the levels of a given chemical, plant-based or synthetic, will reach levels within the fetus that are physiologically relevant, i.e., will they interfere with natural hormone signalling crucial for the process of development.

About the body's physiological defenses:
People possess chemical defenses which protect against at least some endocrine disruptors, natural or synthetic. But of the endocrine disruptors that have been studied carefully, the natural compounds are more likely to be taken out of circulation by the body's defenses. Some plant compounds are not eliminated or detoxified (see Our Stolen Future, chapter 5). Some synthetic are neutralized. But the synthetic compounds are more likely to make it through the bodies defenses. This is likely due to the fact that humans have co-evolved with many phytoestrogens, and in the process developed defenses.

About bioaccumulation and persistence:
Persistent synthetic endocrine disruptors accumulate in body tissue to levels that are orders of magnitude of times higher than observed levels for phytoestrogens, and some are then transferred to the developing fetus. The half-life (an estimator of how long a molecule of a compound will persist) of DDT or DDE in the body, or of PCBs, is measured in years and decades. The half-life of plant phytoestrogens is measured in minutes and hours. Under normal circumstances, most plant phytoestrogens don't stick around long enough or accumulate to high enough levels within the fetus to cause problems. There are exceptions, and these can be as important as the impact of synthetic compounds. But their impact doesn't diminish the attention that should be paid to what synthetic compounds can do.

Continuing research into this issue will no doubt reveal a far more complex picture concerning natural defenses vs. synthetic and natural hazards. Indeed, a key part of the ongoing research will explore why the body's defense mechanisms are effective against some but not all compounds. While far from the final word, these new results do, however, counter the claim that the presence of plant estrogens in the human diet is sufficient reason to dismiss concern about synthetic compounds. It is clearly not.

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