Girls playing in autumn leaves

Two girls play among autumn leaves in Stockholm October 13, 2013. (Jessica Gow/TT News Agency/Reuters)

The earlier onset of puberty in some girls is linked to obesity, U.S. researchers say.

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, a pediatric endocrinologist at British Columbia's Children's Hospital.

"It's really the parents who are anxious," Chanoine said. "It's not like it's huge breast development, they just notice it and then they come to the clinic."

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 earlier full-blown changes of puberty.

Chanoine said there are many potential implications of early puberty, such as an increased breast cancer risk, changes in bone development, or behaviour such as developing an interest in boys at an earlier age. But these are all still open questions that haven't been proven, he noted.

What should parents be concerned about?

Dr. Chanoine says: if puberty is starting before age of 7 in girls and it's not usual for the family (precocious puberty runs in families), if associated with other findings or if rapidly progressing. In the vast majority of female patients, this is a benign finding. That's not the case in boys.

In females, gonadal puberty is reflected by breast development, not pubic hair.

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.

With files from CBC's Amina Zafar