An important new study of young girls has determined that childhood obesity is most likely the key driver behind the disturbing, and now widely acknowledged, phenomenon of girls starting puberty at a younger age.
The study, published in the journal Pediatrics, tracked the breast development (thelarche) of girls ages 6-8 in three geographic regions of the U.S. — San Francisco, Cincinnati and New York City.
According to the report, “girls with greater BMI reached breast stage 2 [development] at younger ages.” Ethnicity, too, was an important factor: The “median age at onset of breast stage 2 was 8.8, 9.3, 9.7, and 9.7 years for African American, Hispanic, white non-Hispanic, and Asian participants, respectively,” researchers report. The bottom line:
We observed the onset of thelarche at younger ages than previously documented, with important differences associated with race/ethnicity and BMI, confirming and extending patterns seen previously. These findings are consistent with temporal changes in BMI.
In an accompanying editorial, “The Enigmatic Pursuit of Puberty in Girls,” Marcia. E. Herman-Giddens, of the Gillings School of Global Public Health at the University of North Carolina, Chapel Hill, raises an important question, that is, what else, beyond obesity, might be to blame? “Extensive interacting variables are known to be associated with earlier development in addition to weight and genetics,” she writes, noting a long list of possibilites, including “certain intrauterine conditions and exposures, preschool high-meat diets, dairy products, low ﬁber intake, isoﬂavones, high-stress families, absent fathers, certain endocrine disruptors, the microbiome as it inﬂuences weight, epigenetics, light exposure, hormone-laced hair products, insulin resistance, activity level, geographical location, and others.”
Indeed, the study’s conclusions, as in so much of research, aren’t completely satisfying, particularly for any mother who has surreptitiously surveyed her daughter’s elementary school and seen breasts clearly budding on second, third and fourth grade girls, who happen not to be obese.
I know, I know, my little scan around the playground means pretty much nothing when it comes to figuring out what’s going on in the population. But it certainly seems like there may be more to this trend than simply the spike in obesity — though that clearly plays a role. (Also, is it still merely a “trend” when it’s on the cover of The New York Times Magazine?)
I asked Diane E. J. Stafford, MD, with the Division of Endocrinology at Children’s Hospital Boston for her initial take on the Pediatrics study and editorial. Here’s are seven (lightly edited) points she made, via email:
1. This supports the previous studies that imply that the “normal” age for puberty may be younger than had been previously documented. It is more helpful in that it was a prospective longitudinal study so it followed the same children over time rather than extrapolating from a cross-sectional study.
2. It is very interesting and, in my opinion, not surprising that those with an increased BMI may be more likely to progress to puberty earlier. I think we have all felt that this was likely be associated. However, “the data support, but do not establish, causality.” This is an important caveat and one that will not likely be brought out in the general conversation about this study.
3. There are very likely to be many influences on the timing of puberty, but most of them cannot be easily measured on a population basis, if at all. The assumption of this and other population based studies is that the various other influences (like genetics) are spread evenly throughout the population and “come out in the wash”, making obesity the “largest driver” on a population basis. A reasonable conclusion for a large population, but not for an individual. Continue reading