Bisphenol A Low-Dose Endocrine Hypothesis Not Confirmed
Introduction
Low-Dose Effects Not Confirmed
Weight of Scientific Evidence
Conclusion

Introduction
The potential toxicity of BPA has been examined extensively
with numerous safety studies conducted over more than 40 years.
These studies uniformly have shown that the toxicity of BPA is
low, as assessed by standard test protocols. Exposure evaluations
have demonstrated a large margin of safety between the exposure
levels of BPA and any level of concern. Taken together, these
tests and evaluations demonstrate that consumer exposure to BPA
does not pose any risk to human health.
In recent years, a hypothesis has been advanced claiming that
exposure to extremely low doses of certain substances could
cause adverse health effects in humans, including disruption
of normal hormonal functions. In science, a hypothesis is a
limited statement regarding cause and effect that has not been
confirmed through repeated experimental tests. According to
this "low-dose hypothesis", health effects occur at doses far
below levels previously determined to be safe using well-established
toxicological procedures and principles. The hypothesis further
asserts that the dose-response relationship for these substances
is "non-monotonic", which means that health effects may only
be observed at low doses while much higher doses result in no
effects. The claimed non-monotonic dose-response relationship
of the low-dose hypothesis is contrary to a fundamental principle
of toxicology - "the dose makes the poison."
The low-dose hypothesis is largely based on several small-scale
experimental studies that report reproductive or developmental
effects in mice or rats from low doses of BPA. Several attempts
to confirm the hypothesis by repeating these initial experiments
have shown that the results cannot be replicated, which indicates
that the hypothesis is not valid. More importantly, definitive
large-scale experiments using accepted protocols have also found
no evidence for reproductive or developmental effects from low
doses of BPA. Consequently, a number of independent scientific
bodies, after reviewing all available evidence, have concluded
that the low-dose hypothesis is unproven. The following sections
provide a brief summary of key experimental findings and the
conclusions of the scientific bodies that have comprehensively
reviewed the scientific evidence.
Low-Dose
Effects Not Confirmed
The low-dose hypothesis
for BPA originated with several small-scale studies first reported
in the 1990's. These studies triggered concerns over possible health
effects from low-dose exposure to BPA and, as a result, data from
the initial studies has been re-evaluated and additional studies have
been conducted to determine if the low-dose hypothesis is valid. In
addition to showing that the low-dose hypothesis is not valid, the
additional re-evaluations and research have also highlighted shortcomings
of the initial studies that may account for the reported low-dose
effects. Several of the key studies are described in this section.
Definitive Large-Scale Studies
The most definitive tests
of the validity of the low-dose hypothesis for BPA are two large-scale
reproductive and developmental toxicity studies using accepted protocols.
Both of these studies clearly demonstrated the absence of a low-dose
effect of BPA.
The most comprehensive
of these is a three-generation study conducted at the Research Triangle
Institute (now RTI International) under the direction of Dr. Rochelle
Tyl (Tyl et al, 2002). In this study, Sprague-Dawley rats (30 males
and 30 females per dose group) were fed a diet containing BPA at levels
from 0 to 7500 parts per million, yielding approximate intakes of
0, 0.001, 0.02, 0.3, 5, 50 or 500 mg/kg body weight/day. Exposures
were continued until adulthood of the third-generation offspring.
The endpoints evaluated included parental growth rate, food intake,
reproductive performance, sperm production and motility, gross and
histopathology, organ weights, litter size, pup survival and growth,
and anogenital distance. In addition, the day of vaginal opening,
preputial separation and (in males) the presence or absence of retained
nipples were recorded. Analysis of the data for all of these endpoints
for the parental and three offspring generations revealed no evidence
of a low-dose effect of BPA. This exceptionally powerful study, which
complied with Good Laboratory Practice (GLP) standards, clearly demonstrated
the absence of low-dose effects for BPA.
The Tyl study was thoroughly
re-evaluated as part of an independent, scientific peer review of
the evidence for and against "low-dose endocrine disruptor"
effects organized by the U.S. National Toxicology Program (NTP
Report, 2001) . The peer review included a Statistics Subpanel
that re-evaluated the experimental design, data analysis and interpretation
of results of studies claiming low-dose effects as well as studies
that found no evidence of low-dose effects. In recognition of its
significance, the NTP Statistics Subpanel stated that the Tyl study
is "arguably the most comprehensive of the studies we evaluated."
In a similar study commissioned
by the Japanese National Institute of Health Sciences and carried
out by the Chemical Compound Safety Research Institute (Ema et al,
2001), Crj:CD (SD) IGS rats were dosed orally by stomach tube over
two generations. The doses applied were 0, 0.2, 2.0, 20 or 200 microgram/kg
body weight/day of BPA. Endpoints assessed included parental growth,
food intake, reproductive performance, sperm production and motility,
gross and histopathology, organ weights, litter size, pup survival
and growth, and anogenital distance. In addition, the study measured
reflex development, maze performance, and several hormones related
to reproduction. Analysis of the data for all of these endpoints for
the parental and two offspring generations revealed no evidence of
a low-dose effect of BPA, which is fully consistent with the results
of the Tyl study. The Ema study was also re-evaluated by the NTP peer
review panel and the results were fully confirmed (NTP
Report, 2001).
Small-Scale Studies
In contrast to these two
definitive studies, reports of low-dose effects for BPA have generally
been based on small-scale experiments that included a small number
of doses (i.e. sometimes as few as one or two), tested many fewer
animals per dose group (i.e. as low as five to seven), did not comply
with GLP standards, and frequently used a route of exposure that is
not relevant to humans (i.e. subcutaneous or intraperitoneal injection).
When other scientists tried to repeat the small-scale studies on a
larger scale with more animals and doses, the results of the small-scale
studies could not be reproduced. In some cases, even the re-evaluation
of the original data from the initial study has revealed that the
reported effects cannot be substantiated.
For example, researchers
from the University of Missouri (Nagel et al, 1997) reported an increase
in prostate weight in male offspring of CF1 mice that were exposed
to BPA during pregnancy at 2 or 20 microgram/kg body weight/day. This
study included only 7 females per treatment group and 11 unexposed
controls, and only one male from each litter was examined. In a subsequent
publication (vom Saal et al, 1998) derived from the same study, the
authors reported that the 2 microgram/kg body weight/day dose decreased
body weight and epididymis weight. Preputial gland weight was reported
to be increased although seminal vesicle and testis weights were unaffected.
The 20 microgram/kg body weight/day dose was associated with a 20%
reduction in daily sperm production per gram of testis weight, but
that dose had no statistically significant effect on body, preputial,
epididymal, seminal vesicle or testis weight. These parameters were
apparently all assessed in the same individual males per litter as
were used in the first report (Nagel et al, 1997), but only 5 males
per BPA-treated group and 8 control males were used for the sperm
production test.
Two independent larger-scale
studies were subsequently conducted using the University of Missouri
test conditions to determine if the reported results could be replicated.
In both cases, the reported low-dose effects could not be replicated.
A study sponsored by the Society of the Plastics Industry,
Inc. (SPI) and the European Chemical Industry Council
(CEFIC) attempted to duplicate the original experimental
conditions as closely as possible (Cagen
et al, 1999, ,
512kb). Although the SPI/CEFIC study employed more doses
(0, 0.2, 2, 20 and 200 microgram/kg body weight/day),
more mice per group (28 females per treatment group
and controls, all male offspring examined) and a greater
number of endpoints than the original studies, no low-dose
effects were observed.
Similarly, John Ashby and
colleagues at the AstraZeneca Central Toxicology Laboratory in the
United Kingdom (Ashby et al, 1999) also used the University of Missouri
test methods to the extent possible, with the same BPA doses in CF1
mice. The findings reported by the vom Saal laboratory could not be
replicated. Ashby and his coworkers found no statistically significant
effects on prostate weight or efficiency of sperm production.
Re-evaluation of the raw
data from the SPI/CEFIC and Ashby studies by the NTP peer review Statistics
Subpanel confirmed the results of both studies. In contrast, the effects
reported by vom Saal (vom Saal et al, 1998) were not statistically
significant and were not confirmed (NTP
Report, 2001).
In a similar sequence of
events, a group of researchers at the Freie Universität Berlin
reported that exposure of pregnant Sprague-Dawley rats to 20, 100
or 50,000 microgram/kg body weight/day of BPA for 16 days (gestation
6 through 21) led to a range of effects on the sexual development
of the male and female pups at all doses. These results were presented
in a series of abstracts (Chahoud et al, 2001; Fialkowski et al, 2000;
Schönfelder et al, 2001; Talsness et al, 2000b, 2001) and one
paper (Talsness et al, 2000a), which are not entirely consistent.
However, none of the results could be replicated in either Sprague-Dawley
rats or in another rat strain (Tinwell et al, 2002). In addition,
re-evaluation by the NTP Statistics Subpanel revealed that concurrent
controls were not used in the Chahoud study, which lead the NTP panel
to conclude, "The lack of concurrent controls in this study was
a serious design deficiency. This confounding of possible treatment
effects with time-related changes precludes any reliable assessment."
(NTP
Report, 2001).
Weight
of Scientific Evidence
The lack of low-dose effects in the
definitive large-scale studies and the inability to replicate low-dose
effects reported in small-scale studies demonstrates that the low-dose
hypothesis for BPA cannot be valid. A series of independent expert
bodies have comprehensively reviewed the evidence for and against
low-dose effects and have consistently reached this same conclusion.
U.S. National Toxicology
Program
In 2000, the U.S. National
Toxicology Program (NTP) conducted an independent scientific peer
review of the evidence for and against "low-dose endocrine disruptor"
effects (NTP
Report, 2001). This review was co-sponsored by the U.S. Environmental
Protection Agency and the U.S. National Institute of Environmental
Health Sciences. The peer review included a Statistics Subpanel, which
re-evaluated the experimental design, data analysis and interpretation
of results, and a Bisphenol A Subpanel, which reviewed all available
studies on BPA. In regard to the studies that demonstrated the absence
of low-dose effects of BPA, the Bisphenol A Subpanel concluded:
"As a group these
studies are very consistent, the conclusions are supported by appropriate
statistical analyses, and the Statistics Subpanel confirmed the lack
of BPA effects for the studies
" and "Collectively,
these studies found no evidence for a low-dose effect of BPA, despite
the considerable strength and statistical power they represent, which
the subpanel considered especially noteworthy."
In their overall conclusion,
the Bisphenol A Subpanel stated:
"There is credible
evidence that low doses of BPA can cause effects on specific endpoints.
However, due to the inability of other credible studies in several
different laboratories to observe low dose effects of BPA, and the
consistency of these negative studies, the Subpanel is not persuaded
that a low dose effect of BPA has been conclusively established as
a general or reproducible finding. In addition, for those studies
in which low dose effects have been observed, the mechanism(s) is
uncertain (i.e., hormone related or otherwise) and the biological
relevance is unclear."
U. S. Environmental
Protection Agency
The conclusions of the NTP peer review panel were further
evaluated by the U.S. Environmental Protection Agency,
leading to a statement on their view of the low-dose
hypothesis (EPA,
2002 ).
Significantly, EPA refers to the claimed low-dose phenomenon
as still a "hypothesis", indicating their
view it has not been proven. Because of this, EPA further
stated, "it would be premature to require routine
testing of substances for low-dose effects."
EC Scientific Committee
for Toxicity, Ecotoxicity and the Environment
The European Commission
has completed a comprehensive risk assessment on BPA, which includes
a review of evidence for and against low-dose effects (EU, 2003). The European Commission's Scientific Committee on Toxicity,
Ecotoxicity and the Environment (CSTEE,
2002 )
independently reviewed the Risk Assessment Report (RAR) and stated:
"[A] number of studies
using non-standard protocols have reported effects of bisphenol A
administration on development using substantially lower doses than
the studies performed according to testing guidelines. The RAR critically
describes the many weaknesses (lack of repeatability, problems with
experimental design and statistical evaluation, poor reporting) of
the low dose studies. The CSTEE agrees with the conclusion of the
RAR that there is no convincing evidence that low doses of bisphenol
A have effects on developmental parameters in offspring and remarks
that effects observed are not adverse."
The CSTEE further remarked,
"a number of high quality studies on the reproductive and developmental
effects of bisphenol A are already available and do not support low-dose
effects."
Japanese Ministry of
Economy, Trade and Industry
Also in 2002, the Japanese
Ministry of Economy, Trade and Industry released a hazard assessment
of BPA (METI, 2002). In regard to the need for risk assessment or
other measures, the Ministry stated:
"Though it is necessary to collect further information
on so-called 'low dose effects' represented by BPA from
academic point of view, it seems unnecessary to take
any specific measure other than the above, considering
the view expressed by NTP Low Dose Effect Panel that
the low dose effect of BPA at present is a phenomenon
observed under considerably limited experimental conditions
and it is hardly considered to be the general phenomenon."
Japanese Ministry of
Health, Labor and Welfare
The Japanese Ministry of
Health, Labor and Welfare has also recently released a report from
an expert review committee that has been evaluating the potential
risks of endocrine disrupting substances (MHLW, 2002). After evaluating
experimental reports on low-dose endocrine disruption, the expert
committee concluded, "no reproducible experimental results have
been obtained, and at this point of time, it is doubtful whether we
can conclude that there are endocrine disrupting effects in the low
dose range."
Conclusion
The low-dose hypothesis
for BPA has been thoroughly tested with a series of comprehensive,
carefully conducted studies. Included are definitive large-scale studies
as well as studies aimed at replicating the results of studies reporting
low-dose effects. The consistent lack of low-dose effects found in
these studies demonstrates that the low-dose hypothesis is not valid.
The weight of scientific evidence provided by these
studies clearly supports the safety of BPA and provides
strong reassurance that there is no basis for human
health concerns from exposure to low doses of BPA.

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