are demonstrable effects outside of the laboratory in wild animal
populations and in groups of people exposed by accident or through
at low exposure
levels, very low exposures, show that the classic high dose
experiments can completely miss important low dose effects. Moreover,
these results are reproducible in the lab. This science does
not rest on extrapolating high dose curves down to the low end
not just estrogens. While the first two decades of this work
focused largely on man-made chemicals capable of mimicking estrogen,
within the last five years we've seen that expand to include estrogen
blockers, androgen blockers, progesterone blockers and compounds
that interfere with thyroid. This last one is especially important
because thyroid hormone is key to proper brain development.
not just the disruption of the endocrine system. Natural chemical
signals are important at all levels of organization of life--within
cells, among cells, between organs, even between organisms, including
from one species to another. Any of these chemical signals, in
principle, are vulnerable to disruption. Scientists, for example,
have just begun to look at the chemical signals that mediate communication
between symbiotic organisms, such as nitrogen-fixing bacteria
and the roots of the plants in which they live, and are examining
how synthetic chemicals might interfere with these signals. Disrupting
these 'signals of life' could have important ecosystem impacts.
developing fetus is exquisitely sensitive to both the natural
hormone signals used to guide its development, and the unexpected
chemical signals that reach it from the environment. Both the
natural signals and the chemicals that disrupt them act as "morphogens."
They guide the fetus through forks in its developmental path and
also help set its sensitivity to subsequent hormonal signals.
This involvement of setting sensitivity can have life-long consequences.
New science, for example, on the developing prostate, shows that
natural and synthetic estrogens experienced in the womb can lead
to enlarged and hypersensitive prostate in adulthood.
with the best known of the endocrine disrupters, particularly
diethylstilbestrol, or DES, led to great scientific confidence
about the predictive value of lab animal studies for anticipating
human endocrine impacts.
contamination is very widespread and it comes from many sources,
some quite conspicuous and others completely unexpected.
of the synthetic chemical compounds are notorious because they
accumulate in our fatty tissue. These chemicals can then bioaccumulate
up the food chain, and can be passed on in the womb and through
breast milk. Because, they are not flushed through our bodies
chemical accumulation in the fat tissue and breast milk can reach
very high concentrations over time, concentrations millions of
times higher or more than plant estrogens.
estrogens are widespread in our diets and when eaten in large
volume they can cause serious problems. In fact they have been
used as antifertility agents by many cultures for a long time.
But normal diets usually don't create risks because our gut chemistry
quickly flushes them through and because there are special proteins
in our blood that are effective at ensuring many (but not all)
of these natural compounds don't reach the fetus at levels sufficient
to have an effect. For example, while the most common plant estrogen
in soy has a half life in our body of about 6-8 hours, DDT's half
life is 10-12 years.