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Source: Global Science
Author: Wang Yibo
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Can commonly used chemicals affect the reproductive ability of male offspring and even make it easier for people to have girls?
I still remember my structural chemistry teacher saying in class that most of the teachers in our department were girls. This is not the teacher's opinion, and there seems to be a saying in the chemistry department that chemists produce more girls than boys.
According to science writer Ivan Amato in the journal Science in 1992, many chemists believed this claim and even vied for the title for their sub-specialty. For example, several theoretical chemists told Science that the tendency to have daughters only occurs among theoretical chemists, then NMR spectroscopists and X-ray crystallographers express similar views, and then organic chemists say that organic chemists have more daughters. Regardless of what biologists and others think of these chemists' claims, they seem to like to talk about related "gossip" after dinner as a form of entertainment, although there is not enough evidence to support such a view.
In the eyes of the general public, the influence of chemicals is just as important, but the impression does not seem to be very good. If you want to advertise the adverse health effects of a certain chemical, the effect can be imagined. When I saw scientific reports that there was a class of chemicals that could affect the development of male babies and even reduce the proportion of male offspring, I didn't doubt it at first.
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Reducing penis size in men?
Speaking of these reports, we have to mention Shanna Swan, an environmental epidemiologist at the Icahn School of Medicine at Mount Sinai, who focuses on endocrine disruptors, a chemical that can interfere with the endocrine system. In 2021, Swan's book, Count Down, claimed that an environmental pollutant could affect the size of a boy's penis at birth. This statement then went viral.
In fact, Swan is focusing on a class of chemicals called phthalates, which are commonly used in a variety of products such as water bottles and food packaging, electronic devices, and personal care products. What's more, they are endocrine disruptors – broadly speaking, they can mimic or affect androgens or estrogens in the body. Through research in rats and humans, Swan found that phthalates affect fertility and cause genital malformations.
Swan's research on humans dates back to 2005, when she published a study investigating the association between prenatal phthalate exposure and human genital development. She and her colleagues collected physical data from 134 boys aged 2 to 36 months, examining the position and size of their testicles, penis size, and anogenital distance (AGD). At the same time, the researchers also collected urine samples from the mothers of 85 of the boys during pregnancy in order to analyze prenatal exposure to phthalates.
Previous rodent studies have shown that prenatal exposure of females to certain phthalates can affect the normal development of the reproductive tract of male offspring, resulting in shortening of the anogenital (anus-to-penis) distance, undescended testicles (referring to testicles not lowering to their normal position), and testicular abnormalities. Among them, anogenital distance is used as a marker of reproductive toxicity in rodents, as shorter distances are associated with lower testosterone levels, reduced sperm count, and reduced penile size.
The results showed that Swan's conclusion was consistent with animal studies: the mother's phthalate exposure was related to the length of the anogenital distance in the male infant, and the greater the exposure, the shorter the anogenital distance and the smaller the penis was likely to be.
Swan's 2015 study further showed a strong association between exposure to phthalates and fertility in human males. Specifically, when expectant mothers are exposed to a phthalate in the first trimester of pregnancy, bis(2-ethylhexyl) phthalate (DEHP), their male offspring may be at greater risk of infertility later in life.
But other scientists have questioned the experimental methods used by Swann and the conclusions reached, arguing that there is insufficient and strong evidence for the effects of phthalates on the reproductive function of male infants. But phthalates are almost ubiquitous in our daily lives, so those studies must be a cause for concern.
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In May 2022, FDA received two food additive petitions and one citizen petition asking agencies to take action on phthalates in food contact applications. In response, in September 2022, the FDA revoked the authorization of 23 phthalates for food contact use, as well as two other phthalates for use as plasticizers, adhesives, and surface lubricants. These 25 diethyl phthalates have actually been abandoned by industry. As a result of this initiative, only nine phthalates can be used in food contact applications, of which eight are authorized for use as plasticizers and one as monomers. However, this was met with some objections, and the FDA received a request to reconsider the ban on the use of some phthalates, but the FDA again denied those requests in July 2023 on the grounds that there was no evidence of their safety concerns in food contact applications.
In addition, phthalates can be used as solvents and stabilizers in perfume and other fragrance preparations, and cosmetics that may contain phthalates include nail polish, hairspray, and shampoo. To date, four phthalates have been used in cosmetics, but according to a 2010 investigation by the U.S. Food and Drug Administration (FDA), diethyl phthalate (DEP) appears to be the only phthalate ester still used in cosmetics. Consumers may not be able to determine the presence of phthalates from the ingredient claims on product labels, but the FDA says there are no safety concerns regarding the current use of DEP for cosmetics and perfumes. However, if consumers do not want to buy cosmetics that contain DEP, they may wish to choose products that do not contain fragrance on the ingredient label.
Change the sex of the fetus
Even more alarming than these studies is the debate among scientists about whether chemicals, including POPs, affect the sex ratio of future generations.
According to a 2007 report in The Gurdian, twice as many girls are born in Greenland and eastern Russia as boys, and scientists at the Arctic Monitoring and Assessment Programme (AMAP) say the fact that pregnant women have high levels of man-made chemicals in their blood could explain why there are more female babies than boys. The scientists measured man-made chemicals in women's blood that mimic human hormones, such as polychlorinated biphenyls (PCBs), and found that they were able to influence hormones to cause a sex transition — from a male embryo to a female embryo — in babies unborn before three weeks of pregnancy.
But in 2014, AMAP released the latest results of their "ArcRisk" project. Meta-analyses have shown that parental exposure to PCBs does not affect the sex ratio of offspring, but is strongly associated with low birth weight, preterm birth, stillbirth, and severe malformations.
In addition, in a 2018 study, scientists from South Korea and the United States initially found that maternal exposure to PCB 128 and paternal exposure to HCB were significantly associated with an increase in female offspring, while maternal exposure to mirex and paternal exposure to PCB 128 were significantly associated with an increase in male offspring. However, when they adjusted their comparisons, they found that only maternal exposure to mirex was associated with a second sex ratio (sex ratio of newborn babies, SSR), and the rest of the associations were not established. These studies show that there is no clear evidence that these chemicals are associated with sex ratios in offspring.
Gender bias
One of the popular hypotheses about the second sex ratio is the hormonal hypothesis proposed by W.H. James, which holds that the hormone levels of the parents at the time of conception are partly responsible for the change in the second sex ratio.
In fact, in the 70s of the 20th century, scientists accidentally discovered that estrogen can affect the differentiation of the genitals. At that time, they gave pregnant women a synthetic estrogen called diethylstilbestrol (DES) to prevent them from having a miscarriage. However, the treatment did not show the desired effect, and instead found that supraphysiological doses of estrogen could inhibit the development of the genitals in male offspring.
This may not seem surprising, but in studies on sex determination, female sexual development is often thought to occur through a "default" pathway, and the finding that estrogen has an inhibitory effect in preventing male sexual development calls into question.
Research on gender determination has developed from the very beginning in the male-dominated world of science. As such, it clearly inherits a male-centric bias. For example, testicular formation is seen as a key sex-determining event, while female development is "default". Scientists also focus on androgens and the development of the Y chromosome, as well as the male phenotype. Even in gender-related research, there is a gender bias.
Cover image source: pixabay
Reference Links:
https://web.archive.org/web/20210324211507/https:/www.foxnews.com/health/pollution-taking-a-toll-on-penis-size-scientist-claims
https://ehp.niehs.nih.gov/doi/10.1289/ehp.8100
https://academic.oup.com/humrep/article/30/4/963/613595
https://www.fda.gov/food/cfsan-constituent-updates/fda-limits-use-certain-phthalates-food-packaging-and-issues-request-information-about-current-food
https://www.fda.gov/food/cfsan-constituent-updates/fda-responds-petition-phthalates-food-packaging-and-food-contact-applications
https://www.fda.gov/cosmetics/cosmetic-ingredients/phthalates-cosmetics
https://www.theguardian.com/world/2007/sep/12/gender.sciencenews
https://www.amap.no/documents/download/1901/inline
https://www.sciencedirect.com/science/article/abs/pii/S0013935117311544?via%3Dihub
https://www.degruyter.com/document/doi/10.1515/medgen-2023-2040/html
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