Younger Girls, Bigger Breasts: Are Chemicals to Blame?

We’ve all heard the stories and seen the evidence: Girls are getting breasts a lot earlier than we remember from our own elementary school days. Nobody really knows why. But because girls are also getting chubbier (along with the rest of the population), that’s been the favored theory of causation. Here’s the general evolutionary scenario: If you can eat more, you can reproduce earlier. And in fact, thanks to better nutrition and less disease, the average age of sexual maturity in girls has dropped slowly but steadily, about three months per decade, since 1850.

In 1997, though, Marcia Herman-Giddens, a scientist at the University of North Carolina, noticed that the pace of advancing puberty had greatly, unexpectedly, accelerated. And that’s no longer a good or neutral thing, because girls who enter puberty on the early end of the spectrum are at greater risk for breast cancer later on. They also tend to suffer more psychological and behavioral problems such as depression and substance abuse.

In a bombshell of a paper, Herman-Giddens wrote that girls were developing breasts, sprouting pubic hair, and getting their periods one to two years younger than previously thought, with white girls getting breasts at a mean age of 9.8 and black girls at 8.8 (menstruation typically starts two to three years later). The data were controversial: European scientists weren’t finding similar changes, and the breast observations were considered subjective—how did researchers know what was a budding breast and what was a little extra fat? To the extent the paper was credited as telling a true story, the cause was again chalked up to body weight: During the last 30 years, the percentage of American girls between the ages of six and 11 who are obese has more than tripled, to 16 percent. Fat stores estrogen and triggers leptin, another hormone involved in the puberty sweepstakes. So far so clear.

But a new study out of Denmark, published last month in Pediatrics, shows that the link between early puberty and fat [2] isn’t the whole story. And, note from Hamlet: We ignore strange things in the state of Denmark at our peril.

For starters, the new paper confirms the American findings almost exactly. Girls are getting breasts earlier, and it’s happening fast. Researchers used the exact same protocols to measure almost 1,000 girls in Copenhagen in 1991 and a second group of the same age in 2006. The girls—all white and middle class—started budding breasts a full year earlier than their counterparts just 15 years ago (the age of menstruation had advanced about four months). While that’s a stunner in itself, the real head-scratcher was that the change in girls’ body weight was minimal and couldn’t account for the difference. Nearly all the girls in both groups were relatively thin, says Dr. Lise Askglaede, the study’s principal author at the University of Copenhagen.

So if fat isn’t resetting the puberty clock, what is?

Two other leading theories have been divorce and the media. It’s been documented that girls not living with a biological father tend to mature earlier, just like elephants, believe it or not. But if anything, families are slightly more stable now than they were 20 years ago. And if girls are becoming more sexualized from the constant stream of media images, their hormone levels would also be rising, and, notably, that’s not happening. While the Copenhagen girls are growing breasts earlier (which requires estrogen), their bodies are not making any more estrogen than the first group of girls were in 1991. To Asklaede, this indicates that the estrogen must be coming from somewhere else.

“Our best suggestion is that it is something from outside,” Aksglaede says. “The main discussion is environmental factors.” Specifically, chemicals that mimic hormones, many of which girls are exposed to every day. Called endocrine disrupting compounds, or EDCs, these chemicals include the much publicized baby-bottle ingredient, BPA, as well as pesticides, compounds in cigarettes, and phthalates (a family of molecules used as aroma stabilizers in lotions and shampoos as well as a common additive in household plastics), among many others. The U.S. government does not currently test chemicals for their effects on human hormonal systems, and the European Union is only just starting to do so.

Many of these compounds actually resemble the molecular structure of estrogen. And our cells’ estrogen receptors turn out not to be very picky. It’s possible, says Aksglaede, that the chemicals may be doing an end-run around the body’s normal hormone-making process, faking out the estrogen receptors in girls’ breast tissues and switching them on before their time. Or they might somehow be changing the gene expression that influences fat-storage (making girls fatter, for example) or speeds up sexual development, as other scientists have found in lab animals. But nobody knows for sure exactly how these external molecules might be causing miscues in the body. “What is scary is that we don’t have any idea what the mechanism is. It’s a big black box,” Askglaede says.

The next step that she’s taking, along with some well-funded scientists in the U.S., is to measure girls’ blood and urine for a variety of chemicals, including the ones mentioned above. It’s the first time many of these industrial chemicals are being measured in girls. Preliminary findings show that nearly all the girls have nearly all of the substances in their bodies, sometimes in larger concentrations than found in adults. There are also some geographical ticks: Girls in New York City girls carry more cotinine, a molecule found in cigarettes and second-hand smoke, while girls in Cincinnati carry more PFOA, used in making products with Teflon.

The study those findings come from, funded by the National Cancer Institute and the National Institutes of Environmental Health Sciences, is following 1,200 American girls through the onset of puberty while measuring their chemical exposures. Already, says Dr. Frank Biro at the University of Cincinnati, he’s seeing an even earlier age of breasts than ever recorded before. Biro believes body weight is still the main driver in the puberty clock, but not the only one. “Heavier girls are more likely to enter puberty first,” he says, “but something above and beyond that is going on, and that’s where it gets really interesting. There are lots of others who believe that chemicals are the major cause. I clearly believe that they are contributing.”

Not all experts are convinced. “If external estrogens were really affecting girls, you’d think you’d see more breast development in boys as well. We do see some of that, but I haven’t seen a big increase,” says Dr. Paul Kaplowitz, chief of Endocrinology and Diabetes at Children’s National Medical Center in Washington, D.C. Biro counters with studies documenting that boys are indeed showing signs of unusual estrogen exposure, such as a rising rate of undescended testicles in newborns and reduced sperm count in adult males.

How can parents influence the puberty clock? Larry Kushi, associate director of research at Kaiser Permanente, points to data suggesting that girls with relatively low-fat, high-fiber diets tend to develop later. Girls should also play sports and get regular exercise, he says. It’s difficult to control chemical exposures in the U.S., because the government (unlike Denmark’s) does not require chemical ingredients to be labeled in personal care products as it does in food. Askglaede recommends, if possible, avoiding products containing parabens (a preservative), phthalates (most common in scented and vinyl products), and Bisphenol A (known as BPA and sometimes found in the lining of canned food, dental sealants, and hard plastic bottles. See Slate’s take on this chemical [3]. The non-profit Environmental Working Group has tested some cosmetics, sunscreens, and other products for hazardous chemicals [4]. And here are the Danish government’s recommendations for pregnant and nursing women [5] to reduce their exposures.

Aksglaede is currently nursing a 16-month-old. After finding high levels of parabens in her body one day after wearing sunscreen, she started avoiding products with that chemical. Overly cautious, based on what we know now? Maybe. But she believes her environmental puberty hypothesis has legs. Or, should we say, breasts.

This is the first of several pieces looking at the emerging science of the relationship of our hormones to the environment.