In Monday's issue of the journal Pediatrics, researchers found the onset of puberty in terms of breast development varied by body mass index and race among more than 1,200 girls in the study.
"The obesity epidemic appears to be a prime driver in the decrease in age of onset of breast development in contemporary girls," Dr. Frank Biro of the adolescent medicine division at Cincinnati Children's Hospital and his team concluded.
The findings suggest that the ages for both early and late maturation in girls may need to be redefined, the researchers said.
The biological impact of early maturation includes greater risk of breast, ovarian and endometrial cancer as well as obesity and hypertension, the study's authors said. The picture isn't clear though, since obese children also often become obese adults.
As part of the study, doctors observed and measured breast development of 1,239 girls in San Francisco, Cincinnati and New York City who were aged six to eight when the study began. The girls were examined at least once a year from 2004 to 2011.
Trend observed in Canada
Breast buds started at 8.8 years of age for African-American girls, 9.3 for Hispanic girls, and 9.7 for white non-Hispanic and Asian girls, the researchers found. The onset of breast development in white girls was four months earlier than a previous U.S. study from 15 years ago.
Overall, the findings likely reflect what is happening in Canada and doctors are observing the same trend at clinics in this country, said Dr. Jean-Pierre Chanoine, head of endocrinology at British Columbia's Children's Hospital.
While the results may seem scary to parents, the age of menarche or first menstrual cycle is quite stable, Chanoine noted. In the study, it went from 12.5 years to 12.3 years, which suggests earlier breast development is not associated with the full-blown changes of puberty.
Menarche at age 11 or younger is thought to increases the length of time women are exposed to estrogen and other hormones, which may increase the risk of breast cancer, cancer experts say.
While the study participants were diverse in racial/ethnic and socio-economic terms, they were not nationally representative, the study's authors said.
A journal commentary accompanying the study said the factors involved in earlier puberty are complex.
"This article adds to studies providing the unsettling findings that the age of onset of breast development, in synch with, though not entirely explained by the 'obesity epidemic,' has continued to drop," Marcia Herman-Giddens of the University of North Carolina said in the commentary.
Herman-Giddens said low fibre diets, preschool diets high in meat, dairy products are also associated with earlier development. Again, these haven't been proven.
Biro's team is investigating whether endocrine-disrupting chemicals, such as plasticizers, are involved.
The study was funded by the U.S. National Institutes of Health.Suggest a correction