Monday, August 20, 2012
Gender part eight: Anthropogenic Chemicals and Human Sexuality
Note: This show first aired on August 18, 2012.
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.
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.