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The explanation is found in an enzyme called aromatase, which is abundant in fatty tissues. Aromatase raises the level of estrogen in the body, in part by converting testosterone to estradiol, a close estrogen relative. In girls, this process amounts to stepping on the puberty accelerator even harder. In boys, it means slamming on the brakes. One study in the Archives of Pediatrics & Adolescent Medicine showed that among boys with the highest body mass index (BMI), 14% still showed no signs of puberty by age 11.5. Among boys with the lowest BMI, just 8% showed no signs.
While overweight boys may enter full-blown puberty later than their normal-weight peers, they may develop one sign of maturity earlier: pubic and underarm hair. That's because of the extra androgens--which aromatase doesn't convert to female hormones--that their weight causes their adrenal glands to release. "This isn't the same as true puberty," says Freemark. "It's just one piece of it."
The Chemical Soup
Much more worrisome than obesity--if only because it's much less understood--is the impact of chemical contaminants in the environment on our bodies. One recent study by the Centers for Disease Control found that the blood of the average American carries traces of 212 different chemicals, including such toxins as arsenic and cadmium. A stunning 93% of Americans have traces of BPA in their urine. It's that chemical, along with phthalates and, to a lesser extent, PBBs, polychlorinated biphenyls (PCBs) and DDE (a breakdown product of DDT), that causes the most concern among doctors studying precocious puberty.
All of these chemicals have the power to disrupt the endocrine system by either mimicking hormones, blocking them or changing the way they're metabolized and excreted. None of these are healthy for any human, but at least for adults, whose tissues have long since formed and set, the risk may be tolerable. For kids, who are still biological works in progress, it can spell big trouble. "There are key development windows during which hormones are important for organizing different parts of the body," says Heather Patisaul, an assistant professor of biology at North Carolina State University specializing in environmental estrogens. "Numerous organ systems appear to be targets, including the gonads and the brain."
Phthalates and BPA have been linked to correctable urethral deformities in baby boys who were exposed to the chemicals in utero. Earlier investigations turned up all manner of similarly alarming findings: a 2001 study of girls adopted in the U.S. from overseas showed both high levels of DDE in their tissue and a high incidence of precocious puberty; a 1999 study in the Journal of Pediatrics showed that girls exposed to PCBs and boys exposed to DDE in utero weighed more by age 14 than unexposed kids and also entered puberty earlier; an oft discussed--if very small--study of 76 Puerto Rican girls found that of those who tested high for phthalate exposure, 68% entered puberty too early. For those with low exposure, the figure was just 3%.