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Assessing Hormone-Mimicking Chemicals
Assessing Hormone-Mimicking Chemicals
Some Scientists Are Alarmed About Pesticides, Contaminants
and Natural Plant Estrogens
By Susan Okie
Washington Post Staff Writer
Tuesday, August 10, 1999; Page Z08
Should you worry about being exposed to environmental
chemicals that
can act like human hormones--from natural plant estrogens
found in many
foods to pesticides and industrial contaminants such as
DDT and PCBs?
So far, there is no solid evidence from human studies
that low-level
exposure to hormone-mimicking chemicals in the
environment causes
disorders such as cancer or infertility, according to a
new report issued last
week.
But that doesn't mean there's no problem. Chemicals such
as PCBs, if
ingested by a woman before or during pregnancy, can be
stored in her
tissues and can affect her infant's brain development,
leading to a lower IQ
and a poorer short-term memory, according to the report
by an expert
committee of the National Research Council.
"I would say those are effects that . . . should be taken
seriously," said Ana
M. Soto, an associate professor of cellular biology at
Tufts University
School of Medicine and a member of the committee.
The report also found that hormonally active chemicals
have been clearly
linked with a wide range of adverse health effects in
wildlife and in
laboratory animals, including reproductive problems,
abnormal
development of the nervous system, weakening of the
immune system and
tumors of certain glands.
Because it is possible that these chemicals--which are
ubiquitous in the
environment--may cause similar effects in people, the
committee called for
extensive research, including studies that would monitor
some human
populations from conception through adulthood to find out
whether
exposure--particularly during fetal development or
infancy--could cause
health problems years later.
"As we really got into this thing, this subject became
bigger and bigger,"
said Ernst Knobil, a professor at the University of
Texas-Houston Medical
School, who chaired the expert committee. "It really
addresses just about
all of biology when you come right down to it."
Many chemicals have hormone-like activity, and everyone
is exposed to
some degree. Natural hormone-like substances include
phytoestrogens
found in many food plants, including soybeans, nuts,
plant oils, grains,
berries, vegetables and tea. Synthetic chemicals with
hormonal activity are,
of course, components of certain drugs (such as birth
control pills), but are
also present in some herbal supplements as well as in
many consumer
products, including cosmetics, plastics, dental sealants
and household
cleaners.
In addition, various pesticides and industrial chemicals
that persist for years
in soil, water or the atmosphere can have hormone-like
actions. Those
include the banned pesticides DDT, chlordecone (kepone)
and dieldrin
and the currently marketed pesticide methoxychlor, as
well as PCBs and
dioxin. Manufacture of PCBs ceased in the United States
in 1977 but the
chemicals still contaminate many rivers and lakes. Dioxin
is a byproduct of
wood-burning and of many industries, including pulp and
paper mills.
The amounts of such chemicals to which people are exposed
in daily life,
as well as their hormonal potency and how long they
remain in the body,
vary from one substance to another. For instance, the
typical diet contains
a gram of plant estrogens a day but only two
one-thousandths of a
microgram of PCBs. On the other hand, genistein, the
major phytoestrogen
in soy, is cleared by the body within a couple of days,
while studies in
monkeys show PCBs can remain in body tissues for more
than seven
years.
Environmentalists, as well as some wildlife biologists
and other scientists,
have expressed alarm in recent years about the potential
health effects of
hormone-like environmental chemicals. A 1996 book by
zoologist Theo
Colborn of the World Wildlife Fund and two coauthors
popularized the
term "endocrine disruptors" and suggested that exposure
to such
substances might cause breast cancer in women and low
sperm counts in
men.
As early as the 1960s, biologists wondered whether
hormone-like actions
were the cause of some of the reproductive
disorders--such as thinning of
eggshells and abnormal sexual behavior--seen in birds
exposed to DDT
and other pesticides, Knobil said.
Then in 1971, eight cases of a rare cancer of the vagina
were reported in
young women whose mothers had taken the synthetic
estrogen DES during
pregnancy. Subsequent studies in animals confirmed that
estrogen
exposure before birth could lead to cancer and reduced
fertility in later life,
fueling the theory that environmental chemicals acting
like estrogen or other
hormones could have long-term adverse health effects.
But the committee found that in many cases where a
chemical has been
clearly linked to an effect--such as DDT and
eggshell-thinning--it's unclear
whether a hormone-like action of the chemical is the
cause, Knobil said.
For that reason, he said, the panel rejected the term
"endocrine disruptors"
and recommended further studies to determine precisely
how pesticides,
PCBs and other chemicals act in the body.
Among the report's major conclusions:
* CANCER. Although hormone-mimicking chemicals have been
associated with tumors of the thyroid, pituitary and
adrenal glands in
animals, the evidence to date does not support a link
between adult
exposure and an increased risk of cancer (including
tumors of
hormone-sensitive organs such as the breast, prostate,
testicles and
uterus). One study did find an association between
dieldrin exposure and
breast cancer. Further research is needed.
* REPRODUCTION AND DEVELOPMENT. Consumption of
PCB-contaminated freshwater fish by pregnant women has been
associated with lower birth weights and premature births
as well as IQ and
memory deficits and delayed neuromuscular development in
infants. Many
animal studies have found that hormonally active
chemicals can produce
abnormalities of reproductive organs, and field studies
document abnormal
sexual development and behavior in exposed fish,
alligators and other
wildlife.
However, the committee found no sign of an overall
downward trend in
human sperm counts or of a link between sperm count and
exposure to
hormone-like chemicals. Knobil said studies on sperm
counts have shown
large regional differences. "These things are yo-yoing up
and down," he
said. "There's no evidence that these hormonally active
agents are
involved."
* NERVOUS SYSTEM. In American, European and Asian studies,
children exposed to PCBs before birth have shown
persistent problems
with memory and intellectual function. Monkeys, rats and
mice exposed to
these chemicals before birth also have impaired learning
and motor ability.
How the chemicals produce these effects is unknown.
* IMMUNE SYSTEM. Studies of birds, seals and other
animals show
that DDT and other pesticides suppress the immune system,
but it is not
known whether this occurs through a hormone-like
mechanism. Few
studies have been done on such chemicals' effect on the
human immune
system.
The NRC report was commissioned and funded by the
Environmental
Protection Agency, the Department of the Interior and the
Centers for
Disease Control and Prevention. The EPA is screening an
array of
chemicals for hormonal effects, and research in its
laboratories has
produced new information on how some substances affect
the body, said
Frederick S. vom Saal, a biology professor at the
University of Missouri
and a panel member.
For example, he said, DDT was long believed to act as an
estrogen-like
hormone. But a 1995 study by EPA scientists showed that
the chemical
into which DDT is converted by the body acts as an
anti-androgen,
inhibiting the action of the male sex hormone
testosterone. That probably
explains why male birds, alligators and other animals
exposed to DDT fail
to develop normal male sex organs and often behave like
females.
"It is almost as potent [an inhibitor of testosterone] as
flutamide, the drug
used to treat prostate cancer," vom Saal said.
Although the new report offers no recommendations for
consumers, vom
Saal said he thought the evidence on PCBs was strong
enough to justify
warning women of childbearing age, including teenagers,
against eating
freshwater fish, because so many bodies of water in the
United States are
contaminated.
"You can have these chemicals in your body as a result of
eating fish years
before your pregnancy, because you don't clear them," he
said.
The report, "Hormonally Active Agents in the
Environment," is available
online at http://www.nap.edu. Copies (at $64.95 each) can
also be
ordered from the National Academy Press at 202-334-3313 or
1-800-624-6242.
© Copyright 1999 The Washington Post Company
--
Neil TANGRI