Shari Roan, Los Angeles Times, August 8, 2010
According to the paper, which appears in the journal Pediatrics, almost 25% of African American girls have reached a stage of breast development marking the onset of puberty by age 7, as had almost 15% of Latina girls and more than 10% of white girls.
Those percentages are significantly higher than in 1997, when a landmark study first reported that girls were beginning puberty much younger than they had in the mid-20th century. In that study, the rate of girls who had begun puberty at age 7 was, on average, 5% for whites, compared with 10.4% in the new study.
In other words, the average age of puberty onset still appears to be in flux.
Biro’s [Dr. Frank M. Biro, director of adolescent medicine at Cincinnati Children’s Hospital Medical Center and the lead author of the new study] study included 1,238 girls ages 6 to 8 who lived in one of three regions: Cincinnati, East Harlem, N.Y., or San Francisco. Puberty was determined by two examiners who worked independently to assess the girls’ breast development. By age 8, 27% of the girls had begun puberty: 18.3% of whites, 42.9% of blacks and 30.9% of Latinas.
Compared with data from the 1997 study, the age at which puberty begins did not fall for African American girls, although they still mature at younger ages than white or Latina girls. It’s not clear why there was no change for black girls. “Perhaps black girls have approached a biologic minimum,” Biro said.
There are numerous potential explanations for why puberty is starting earlier. Chief among them is the increase in average body weight among children over the last three decades, Lee said. Excess body weight, especially body fat, is thought to increase the blood levels of estrogens that promote breast development. Earlier studies, including one by Lee, have linked early puberty to higher body mass index as far back as the toddler years.
But other studies suggest that body fat may not be the only cause. A Danish study released last year in the journal Pediatrics found puberty occurring earlier in children regardless of body mass index at age 7. Factors may include a diet that is increasingly high in sugar and fat, declining physical activity and exposure to endocrine disrupters, chemicals in the environment that act on hormones.
Biro said that his study would continue to follow the girls’ development, and that blood and urine samples were being collected to look at biomarkers that reflect potential environmental exposures.