Here on the show, we’ve spent the last several weeks talking
about gender. As humans, our tendency is to think of gender as a hard and fixed
quality with few exceptions. That is because we are mammals, and mammals have
the hardest and most fixed sexual differentiation system on the planet. I hope
that I’ve been able to convince you that in most other groups of organisms,
gender is a variable and plastic endeavor; that is the norm, we are the
outliers (our transgendered brethren not withstanding).
We humans may be in for a nasty surprise in the coming
decades. It turns out that our genders are getting a tiny bit more plastic as a
result of well, plastic, and a multitude of other substances that we have
created in the lab in the past 50 years or so. It turns out that many of these
chemicals (you know all the names: phthalates, bisphenol A, PCB’s, atrazine,
flame retardants, DDT etc) are what we call endocrine disruptors. They have
estrogen like molecular structures, and thus bind with estrogen receptors on
our cells and turn on estrogen sensitive genes. The few that aren’t direct
estrogen mimics still have a net estrogen like effect on the body. We talked
about hormones and their effect on the sexual development of the human body in
our discussion of why
men have nipples; it will be useful to revisit it here.
So, the human form has a default mode, and that is
essentially female. In the absence of other directions, our genetics describe a
female body whose development is governed primarily by estrogen. Males develop
because genes on the Y chromosome tell certain cells to make androgens like testosterone.
You can have a Y chromosome, but if there is damage on it, and it doesn’t do
its job, you won’t develop as fully male, or have a host of other reproductive
problems. In humans, as in most organisms, expressed gender is a result of the
balance between estrogens and androgens, regardless of the genetic make up of
the individual. Which brings us to our endocrine disruptors.
Lets start with a chemical so common scientists refer to it
as “ubiquitous” in terms of human exposure, phthalates. There are two main
catagories (and many many individual chemicals), high density and low density
phthalates. High density phthalates are
used as plasticizers in plastics and polyvinylchloride (PVC). They make
plastics softer. Chemicals in this class were banned for use in children’s toys
nationwide in 2008. Low density phthalates are used in cosmetics and other
smelly things, they make fragrances last longer (this is why the first hint
that a woman is pregnant is that she starts purchasing fragrance free personal
care products!—more on why in a moment). They are quite volatile and readily
leach out of what they are in, so, phthalates are everywhere.
Phthalates function as an antiandrogen endocrine disruptor,
by at least in part, inhibiting the synthesis of testosterone. No testosterone?
No male development. What we actually see is a group of alterations to male
sexual organs that has a name; “phthalate syndrome”; it includes reduced penis
size and impaired testicular descent among other things. Studies have also
concluded that phthalates interfere with male brain development, and result in
“reduced masculine play in boys”. Many other anthropogenic endocrine disruptors
work in similar ways, disrupting the testosterone pathways in the developing male.
Its important to note that for many of these effects, the window for problems
to occur is very early in embryonic development when those bipotential gonads
are first differentiating, and requires very low exposure levels. Hence those
pregnant ladies and their fragrance free lotions.
Endocrine disruptors affect females as well, to be sure, but
they generally do not masculate them, in a mirror image of the male effect.
Early onset of puberty and multiple reproductive abnormalities are among the
more common effects noted for females.
So its not just feminized
frogs and hermaphroditic
polar bears anymore. The bird of our toxic legacy has come home to roost. Because
many of the chemicals are relatively new (as in the past 30 to 40 years), and
their impact is often in utero, but we
don’t become reproductively active for a couple of decades after that on
average, the negative effects on our reproduction can be delayed and difficult
to connect back to our inutero exposure to an endocrine disruptor. So what does
this mean for gender in humans? We might not be able to fill our biological
roles quite as well as we once did, which, at the risk of sounding
melodramatic, may have significant long term impacts on the future of the human
species on this planet.
References:
Swan, S. H. et al, “Prenatal phthalate exposure and reduced masculine
play in boys” http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2874619/?tool=pubmed
Swan, Shana S. “Environmental phthalate exposure in relation to
reproductive outcomes and other health endpoints in humans” http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2775531/?tool=pubmed
McLachlan, John et al “Endocrine disruptors and female
reproductive health” http://www.ncbi.nlm.nih.gov/pubmed/16522520
Info about the national phthalate ban http://www.cpsc.gov/info/toysafety/phthalates.html
Nicolopoulou-Stamati, P. and M. A. Pitsos “The impact of
endocrine disrupters on the female reproductive system” http://humupd.oxfordjournals.org/content/7/3/323.short
Post Script: So, how do I avoid phthalates anyway? First, the good news is that phthalates have a very short residence time in the body, as short as a day or two. So if you are successful in lowering your exposure, your body burden will decrease rapidly. Due to the ubiquitous nature of phthalate use in consumer products, it is difficult, (but not impossible) to lower your phthalate exposure. Below are a few links with some good ways to start you on your way.
http://www.rodale.com/phthalates
http://www.oeconline.org/our-work/healthier-lives/pollutioninpeople/solutions/phthalates
http://health.usnews.com/health-news/articles/2008/05/07/how-to-limit-your-exposure-to-phthalates
