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 ver the past decades, researchers have reported a
dramatic increase in the number of overweight Americans, reproductive
deformities and the number of youth reaching puberty at early
ages. In the past, these findings have been attributed to nutrition,
lifestyle and genetics.
 However,
in an article published in Nature, a team of researchers
from the University of Missouri-Columbia and North Carolina State
University report that a chemical estrogen that is used to make
plastics could be a contributing factor.
The researchers,
Kembra Howdeshell, an MU doctoral candidate; and Frederick vom
Saal, MU professor of biological sciences; exposed mice to bisphenol
A (BPA), similar to levels at which humans are routinely exposed.
BPA is a compound that was originally made as a chemical estrogen,
but that is now used as a "building block" for the
production of polycarbonate plastic products such as baby bottles,
tin can linings, certain toys and certain types of food storage
containers. What resulted were findings that included an earlier
onset of puberty and an increase in body weight after birth.
Howdeshell and vom Saal also found that mice with more natural
estrogen in them were much more sensitive to the chemical than
those with low levels of natural estrogen.
"People
sometimes get confused if 100 percent of the population doesn't
exhibit the exact same symptoms to a chemical, but there are
varying effects in the animal as well as human population,"
Howdeshell said. "That's to be expected, but there is no
way to predict natural levels of estrogens in humans without
doing several series of invasive tests. The whole idea of government
regulation is to protect the most sensitive subpopulation, which
our findings identify as having the highest levels of natural
estrogen."
The study
was conducted by exposing mice to BPA while still in the womb.
Exposure was done just during pregnancy, not after birth. However,
the study concluded that exposure to the chemical while still
in the womb programs postnatal growth. On average, Howdeshell
and vom Saal found that mice exposed to BPA weighed 20 percent
more than normal when examined at puberty. The research was conducted
over a period of one year and was funded by a $500,000 grant
from the National Institutes of Health.
"We found
that the largest effects happened to the babies of the pregnant
mother," Howdeshell said. "The chemical did not affect
the mother, but instead it altered the babies' growth patterns
and accelerated timing of sexual maturity. Our study shows that
this chemical may be a factor for contributing to trends seen
in human populations over the past several decades."
The researchers'
findings indicated that more work is needed to discover the exact
effects of BPA on humans. However, in previous research, when
both humans and mice were exposed to the same relative dose of
chemical estrogens, the effects were nearly identical. Both experienced
similar types of abnormalities of the reproductive system. This
points to the need for research on humans.
"We're
not offering an answer concerning effects in humans with these
findings; instead, the findings pose a question regarding human
health," vom Saal said. "This study should serve as
a guide for human research. We believe that the medical community
should take a long look at this study and consider looking at
BPA as a possible cause for the changes in growth, sexual maturation
and reproductive abnormalities that have been reported in the
humans in the past decades."
Vom Saal also
has pointed out that food containers are not the only source
of exposure to this chemical. BPA also is used to make computers,
toys and other household items. Products made with BPA have been
claimed to be nondegradable. However, at the Estrogens in the
Environment conference in New Orleans Oct. 18-21, Howdeshell
and vom Saal are presenting evidence from another study showing
that BPA leaches out into the environment from plastic products
with repeated use. 
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