Americans are likely to be exposed at higher levels than previously thought to bisphenol A, a compound that mimics hormones important to human development and is found in more than 90 percent of people in the United States, according to new research.
U.S. EPA says it is OK for humans to take in up to 50 micrograms of BPA per kilogram of body weight each day. The new study, published in the journal Environmental Health Perspectives, suggests that we are exposed to at least eight times that amount every day.
"Our data raise grave concern that regulatory agencies have grossly underestimated current human exposure levels," states the study.
The study also gives the first experimental support that some BPA is likely cleared at similar rates in mice, monkeys and humans, making it possible to extrapolate health studies in mice to humans.
Despite decades of research, questions about BPA have lingered and recently become politicized. Sen. Dianne Feinstein (D-Calif.) hopes to add an amendment to the "FDA Food Safety Modernization Act," currently under consideration in the Senate, banning the chemical from children's food and drink packaging. Republicans and industry representatives have been averse, saying that research has not shown conclusively that the chemical is harmful.
Hormones are essential during development and can determine, among other things, a child's gender. BPA, since it mimics estrogen, is an "endocrine disrupter," according to Thomas Zoeller, a biology professor at the University of Massachusetts, Amherst. And amazingly, BPA has the ability to bind to not one, but three receptors -- the estrogen, the male hormone and the thyroid hormone receptors, Zoeller said.
Controversy over method
Some scientists question whether the ability of BPA to bind receptors translates to a health effect. Detractors say that most of the chemical does not circulate in blood long enough to have health effects. All scientists agree that BPA resembles estrogen, and indeed, it was first synthesized as a man-made estrogen substitute before being used widely in the linings of canned goods and polycarbonate plastics.
Within the scientific world, the controversy hinges on the seemingly obscure question: Does the liver detox the chemical completely enough to secrete most of it out in urine, or does BPA get into human blood where it can mimic important hormones?
Feeding human volunteers a fixed dose of BPA and sampling their blood to check for the chemical would answer some of these questions, according to Zoeller. But such an experiment throws up ethical issues. The only human study of this nature was conducted in 2002 by the German researcher Wolfgang Völkel at the University of Würzburg.
Völkel found the liver removes more than 99 percent of BPA from the blood, and humans excrete it within six hours. He did find some BPA in the blood of his volunteers but found this level to be insignificant.
It is at this point that science breaks down into controversy. Some researchers say the method Völkel used to measure BPA in the blood was not sensitive enough and that he likely overestimated the ability of the chemical to pass through without causing harm.
The new study, led by Julia Taylor, a biologist at the University of Missouri, uses a more sensitive test for measuring the compound. She fed mice and monkeys a fixed amount of BPA daily. She took blood samples and found that the animals had "biologically active" amounts of the estrogen-like chemical, according to the study.
The study suggests that BPA is not completely removed by the liver and does circulate in the blood and in amounts that are cause for concern, according to Taylor.
"For those of us who work with BPA, no one has actually directly compared mice and monkeys before, and monkeys and humans before," Taylor said. "For those of us who work with it in an academic sense at least, this is confirmation of what we believe."
This study suggests that all the possible ways in which humans are exposed to BPA are not yet known, Taylor said. It also makes it possible to compare studies of BPA in mice and extrapolate it to monkeys and humans since they all clear BPA at similar rates, she said.
"These data should make us reconsider some previously held hypotheses about BPA, such as how quickly it is cleared from the body and the differences in metabolism between species," said Linda Birnbaum, director of the National Institute of Environmental Health Sciences. "The paper emphasizes the need to better understand all the potential sources of human exposure."
While some scientists found Taylor's method elegant, others such as industry-associated scientists Julie Goodman and Lorenz Rhomberg, both principals at Gradient, were not convinced. Rhomberg pointed out that Taylor had not measured blood samples in humans herself.
He also said the human blood values that she used come from studies where the samples were contaminated.
Taylor countered that she did not feel it ethically correct to conduct experiments on humans, and Völkel had in fact removed any background noise due to contamination from his results.
Gary Ginsberg, professor at the University of Connecticut and the Yale School of Medicine, said that the study was a good first step that addressed some of the controversy surrounding BPA degradation.
"It is a good exposition of data in primates that shows pharmacokinetics in administered and controlled situations," Ginsberg said. He said that more studies and an even more refined method of measuring BPA in blood would be better.
Zoeller at the University of Massachusetts, Amherst, said that this study provides some evidence that the liver allows some BPA to get into blood and that our exposure to the chemical is greater than previously thought.
"The body evolved to handle stuff that gets into our system -- the liver is designed to detoxify," he said. "There are a range of molecules that are natural, and some are incredible toxins. But when we start to make molecules that are not known to nature, we need to think a little more carefully about how they are going to interact with biological systems."