Recent
media stories have reported on two new scientific studies involving BPA’s
effects on birth weight. One study
reported a statistical association between prenatal exposure to BPA and increased
birth weight, while the other reported an association with decreased birth
weight.
However, both of the new studies
use cross-sectional study designs that are inherently incapable of providing much
meaningful information on whether exposure to BPA has any effect on birth
weight. Furthermore, the BPA exposure
measurements reported in one of the studies are not likely to be valid at all.
In comparison, multiple comprehensive studies
on laboratory animals have found that exposure to BPA has no effect on
birth weight. The lesson to be learned
(again) is that cross-sectional epidemiology studies are unlikely to provide
useful information on any potential health effects from exposure to BPA.
Background
For
10 years or more, the popular media has repeatedly written about the chemical bisphenol A
(BPA). Particularly popular are stories about
new scientific studies, along with the suggestion that we’re all at risk of one
health effect or another due to everyday exposure to BPA. In that context, recent reports on prenatal
exposure to BPA and birth weight were just more of the same.
In
scientific terms, more of the same might be called replication, which generally
is a good thing. The results of one
study are more likely to be valid if the results are replicated in another
study, in particular a study conducted independently by other researchers. In fact, one of the hallmarks of the
scientific method is replication, or reproducibility of results.
Credit and link: Examiner.com
New Studies and Results
But
replication isn’t what happened a couple of weeks ago, although you wouldn’t
have known that from a quick review of media stories. What did happen is that two new studies were
published in the scientific literature, about a week apart, on the subject of
prenatal exposure to BPA and birth weight, and both studies were separately
covered by the media. Along with birth
weight, both studies measured BPA exposure at the time of birth and both
studies reported an association between BPA exposure and birth weight.
The
problem is that one study reported that prenatal exposure to BPA was statistically
associated with decreased
birth weight while the other study reported an association
with increased
birth weight.
So which is it, does prenatal exposure to BPA increase or decrease birth
weight? Digging a little deeper into the
study designs and some of the data reveals that neither study is capable of
answering the question.
A
fundamental limitation of both studies is that these associations with birth
weight were based on cross-sectional study designs in which health outcome and
exposure data are collected at a single point in time. With cross-sectional study designs, it
is simply not possible to know if the exposure preceded the health effect,
which is a necessary element to establish causation.
The lack of a temporal sequence is a
particular issue for BPA because it has a very
short half-life in the body and it is well established that
exposure measured at one point in time says very little about exposure at
preceding times. For this reason alone,
neither study can speak to the effect of prenatal exposure on birth weight.
Furthermore,
the BPA exposure measurements in one of the studies
are likely to be invalid. It is well
known that BPA is efficiently metabolized and quickly eliminated from the body
in urine. Accordingly, urine is the “optimal matrix for
measuring nonpersistent, semivolatile, hydrophilic agents,”
such as BPA. That’s not just an
uninformed opinion but represents the views of a group of highly qualified
experts led by a prominent scientist from the U.S. Centers for Disease Control
and Prevention (CDC), which is recognized as a worldwide authority on
biomonitoring.
In
contrast, measurement of BPA in blood is not optimal due to significant
challenges, including the high demonstrated potential for sample contamination
with BPA. The same group led by CDC previously
advised against using blood measurements, stating that “it is seldom possible
to verify that [blood] concentrations of [BPA] are valid measures of exposure.”
Recognizing
the difficulty in measuring trace levels of BPA in blood, researchers from the
Pacific Northwest National Laboratory and the U.S. Food and Drug Administration
(FDA) recently published clear criteria to identify contaminated blood samples. For oral exposure to BPA, which is the
primary way we contact BPA through our diets, blood samples containing “>1% BPA [in
unmetabolized form as a percentage of total measured BPA including its
metabolized forms] are likely contaminated.”
This makes sense since extensive research in
human volunteers has shown that very little unmetabolized BPA would ever be
present in human blood. The levels of unmetabolized
BPA reported in the recent study are frequently above 90% (!), a clear indication
of contamination rather than a measure of exposure.
Does Prenatal BPA Exposure Affect
Birth Weight?
Although
the two recent studies provide essentially no meaningful information on this
question, we have a wealth
of relevant information from comprehensive, high-quality
studies in laboratory animals. These
studies provide extensive, replicated evidence that prenatal exposure to BPA has
no effect on birth weight, in particular at any level of exposure even remotely
close to actual human exposures.
What
we should have learned from the two new studies, contrary to the headlines they
generated, is that cross-sectional epidemiology studies on BPA are generally
incapable of providing any useful information on whether exposure to BPA could
cause health effects. That lesson is not
new, but bears repeating based on recent history since misunderstanding of the
value of cross-sectional studies certainly contributes to misinformation in the
popular media.
