Exposure to BPA May be Too Low to Cause Problems
Tue, 02/19/2013
Pacific Northwest National Laboratory
A
controversial component of plastic bottles and canned food linings that have
helped make the world’s food supply safer has recently come under attack: bisphenol
A. Widely known as BPA, it has the potential to mimic the sex hormone estrogen
if blood and tissue levels are high enough. Now, an analysis of almost 150 BPA
exposure studies shows that in the general population, people's exposure may be
many times too low for BPA to effectively mimic estrogen in the human body.
The analysis, presented at the American Association for the Advancement of Science's
annual meeting by toxicologist Justin Teeguarden of the Department of Energy's Pacific Northwest National Laboratory, shows
that BPA in the blood of the general population is many times lower than blood
levels that consistently cause toxicity in animals. The result suggests that
animal studies might not reflect the human BPA experience appropriately.
"Looking
at all the studies together reveals a remarkably consistent picture of human
exposure to BPA with implications for how the risk of human exposure is
interpreted," says Teeguarden. "At these exposure levels, exposure to
BPA can’t be compared to giving a baby the massive dose of estrogens found in a
birth control pill, a comparison made by others.”
In
addition to evaluating the likelihood of BPA mimicking estrogen in humans,
Teeguarden also analyzed another set of BPA studies that looked at the
chemical’s toxicity in animals and cells in the lab. These 130 studies are
significant as a group because they refer to the exposures as "low
dose," implying they are very relevant to human exposures.
According
to his analysis, however, the "low doses" actually span an immense
range of concentrations, a billion-fold. In addition, only a small fraction of
the exposures in these self-described “low dose” studies are in the range of
human exposures, from 0.8 percent to 7 percent depending on the study.
"The
term low-dose cannot be understood to mean either relevant to human exposures
or in the range of human exposures. However, this is in fact what it has come
to mean to the public, as well as many in the media," says Teeguarden.
Analysis
of 150 Exposure Studies
The
first analysis covered 30,000 individuals, including women and infants, in 19
countries. Human blood concentrations were calculated multiple ways using many
kinds of exposure data.
Teeguarden
looked to see if BPA concentrations were sufficiently high to be a significant
source of estrogen-like activity in the blood. Researchers have long known that
BPA can bind to the same proteins that estrogen does – called estrogen
receptors – when estrogen is doing its job in the body. However, in most cases,
BPA does so much more weakly than estrogen. To trigger biological effects
through receptors, BPA concentrations have to be high enough in the blood to
overcome that weakness.
"Systematically
testing the estrogenicity, or the bioactivity of BPA at the part per trillion
concentrations we expect in human blood would seem the most scientific way to
substantiate or refute this conclusion," says Teeguarden.
Teeguarden
analyzed the data in these studies using multiple independent approaches
applied systematically to the data from thousands of individuals. The results
showed that human blood levels of BPA are expected to be too far below levels
required for significant binding to four of the five key estrogen receptors to
cause biological effects.
Teeguarden's
analysis also confirmed the findings of many academic and government scientists
that biologically active BPA is at such low concentrations in the blood that it
is beneath toxicologists' current ability to detect it, raising questions about
the role of sample contamination in studies reporting high levels of BPA.
Analysis
of 130 Toxicity Studies
In
this analysis, Teeguarden compiled all the BPA studies that included the term
"low dose" as it referred to human exposure by using such terms as
"low-concentration," "environmentally relevant," or
"human exposure." From the 130 studies found, he and his PNNL
biologist Sesha Hanson-Drury compiled all the doses that were actually used in
the studies.
The
results showed that a small fraction of the "low doses” used in these
studies are within the range of human exposures, with the vast majority being
at least 10 to thousands of times higher than what humans are exposed to daily.
In addition, the range of concentrations spans from upwards of 10 grams per
kilogram of weight per day down to 100 picograms per kilogram of weight per day
(a picogram is one millionth of a gram).
"Unfortunately,
the low dose moniker has been used by some to promote the importance of
selected toxicity studies, for example, in arguments to ban BPA," says
Teeguarden. "For BPA and all chemicals, we need more accurate language to
present these findings so the public and scientists in other disciplines can
understand how human exposures compare to exposures in laboratory studies
reporting toxicity.”
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