Building on earlier research a major new study has found that girls are starting puberty at even younger ages. The most significant changes were seen in Caucasian girls and in girls who are overweight or obese. Still, girls who were not overweight were also entering puberty younger, the study found.
Researchers at three sites around the country -- including the San Francisco Bay Area -- followed 1,239 ethnically diverse girls from 2004 to 2011. They looked at breast development, a key marker for the start of puberty.
Earlier studies had shown that African-American girls had reached this milestone at younger ages. "Now it looks like it's happening earlier for Caucasian girls," said Dr. Louise Greenspan, a pediatric endocrinologist with Kaiser San Francisco and one of the authors of the study. "Particularly, the overweight Caucasian girls are developing earlier than they have in the past."
Researchers looked at a number of factors, but the "obesity epidemic appears to be a prime driver in the decrease in age at onset of breast development," the authors wrote.
Researchers cannot say that being overweight or obese actually causes the earlier development, but there are biological reasons to suggest how body fat could contribute to the change. "Fat makes hormones," Greenspan noted, "and the hormones that are being made by the fat might also be generating high enough levels to make true breast development."
Greenspan specified "true" breast development, because their research was based on examining girls at least annually during the 7-year study period as opposed to other studies which had relied on observation. "That's been a major criticism of prior studies," Greenspan said. "Because when you look at a girl's chest, you can't tell whether there's fat tissue or true breast development, and our study was different because we examined the girls physically and were able to feel the difference between fat and real breast tissue."
The study, Onset of Breast Development in a Longitudinal Cohort, was published online Monday in the journal Pediatrics. It was funded by the National Cancer Institute and the National Institute of Environmental Health Sciences.
Differences noted by ethnicity and race
Common sources for health information often list puberty as starting around age 11 for girls. But in 1997, a landmark study concluded that puberty was starting earlier, and African-American girls in particular were entering puberty younger than their peers. This study found average ages for the even-earlier breast development as follows:
- 8.76 years for African-American girls
- 9.23 years for Hispanic girls
- 9.62 years for Caucasian
- 9.92 years for Asian-American girls
This study is essentially the first to determine onset of puberty for Asian-American girls, although their study sample was still small. "Previous studies have largely focused on the white/black difference," said Lawrence Kushi, Directory of Scientific Policy at Kaiser's Division of Research and the principal investigator for the San Francisco site in this report. Hispanic girls have been somewhat more represented in prior studies than Asian-American girls, and Hispanic girls have "shifted to somewhat younger ages," Kushi said.
The big difference is in Caucasian girls. The average age of breast development for them was four months earlier than seen in the 1997 study, and that's "a pretty significant reduction" on a percentage basis, says Greenspan, since puberty only lasts two to three years. But that's the average. The more obese girls had a "greater shift," Kushi said. Some of these heavier girls were entering puberty close to a year earlier than previously reported.
African-American girls appeared to be "holding steady" compared to earlier studies, said Greenspan.
What's not clear from this study is if girls are also having their first period at a younger age. The research team has been following this group of girls since they were ages 6 to 8. Now they are 14 to 16. The researchers plan to analyze ages of the girls' first period once all the girls in the study have reached that milestone. "One question we have," Greenspan said, "is puberty starting earlier and ending earlier so there's an entire shift to an earlier age; or is puberty starting earlier and ending at the same time?"
Psychological and physical health risks
This earlier onset of puberty puts girls at risk for certain mental and physical health issues. From the study:
Girls with earlier maturation are at risk for lower self-esteem and higher rates of depression. They are more likely to be influenced by older peers and more deviant peers, and initiate intercourse, substance use and other norm-breaking behaviors at younger ages.
Many schools include classes about puberty and sexuality in 5th or 6th grade, Greenspan noted. But according to this new data, many girls are entering puberty in 3rd or 4th grade. This creates a "real disconnect between the timing of girls' puberty and the timing of the education they receive about the puberty and their bodies in general." She argued for splitting the subject matter and teaching girls about puberty in the earlier grades and leaving sexuality for the later grades.
Girls who enter puberty younger are at higher risk for developing certain cancers as adults, including breast cancer -- possibly because earlier puberty increases exposure to estrogen, a known breast cancer risk. Greenspan hopes that the researchers will continue to receive funding to follow the girls into adulthood, which could yield better understanding of breast cancer risk.
The researchers also collected blood and urine samples from the girls at the beginning of the study and are analyzing certain chemicals to see what associations there may be between chemicals and breast development. They are looking at Bisphenol A (BPA), flame retardants, phthalates, heavy metals and chemicals used in nonstick cookware.
Karuna Jagger, executive director of Breast Cancer Action, a San Francisco-based advocacy group, called the study "very important" especially since causes of breast cancer are not fully understood, she said. She wants to know if chemical exposure means "creating breasts that are susceptible to future insult and higher risk of breast cancer."
Living with uncertainty
In an accompanying commentary in Pediatrics, Marcia Herman-Giddens, lead researcher in the 1997 study, said that we may have to "live with uncertainty" for a long time, since the "exact trigger for pubertal initiation is still unknown."
Herman-Giddens agreed that "considerable research" implicates obesity as a factor in the declining age of onset of puberty, but also points to other complex changes:
Extensive interacting variables are known to be associated with earlier development in addition to weight and genetics: 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.
This study did not look at boys. Greenspan noted there's "mixed data" for them, some suggesting boys are going through puberty earlier, some not. Further investigation needs to be done on boys, she said. "The issue is that the chemicals or even fat may have a different effect on boys' puberty because [boys] have a different set of hormones."
This post has been updated with additional information about the range in onset of puberty for Caucasian girls.