COLUMBIA, Mo. - Over 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 to be published in Nature this week, 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 BPA is a compound that was originally made as a chemical estrogen, but that is now used as a "building block" for the production of polycarbonate plastic products such as baby bottles, tin can linings,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 post-natal 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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