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.
In other words, the average age of puberty onset still appears to be in flux.
"In 1997, people said, 'That can't be right; there must be something wrong with the study,' " said Dr. Frank M. Biro, director of adolescent medicine at Cincinnati Children's Hospital Medical Center and the lead author of the new study. "But the average age is going down even further."
Biro's 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.
Even for white and Latina girls, it is too early to declare that puberty age is still falling, said Dr. Joyce Lee, assistant professor of pediatric endocrinology at C.S. Mott Children's Hospital at the University of Michigan, who was not involved in the study. The methodology used in the new study differs somewhat from the one in 1997, making direct comparisons difficult, she said.
But, she added, "it's incredible the difference you see between the two studies."
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.
"Kids today are exposed to plastic much more than they were 10 or 20 years ago," Lee said.
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.
Early development in girls is not inconsequential. Studies have linked it to various health risks including a poor body image, reduced self-esteem, higher rates of eating problems, depression and earlier onset of sexual activity. Early maturation in a large population of girls may also affect future breast cancer rates. Studies have linked a younger age at the first menstrual period to a higher risk of breast cancer after menopause.
Parents may be able to influence the timing of puberty, Biro said, through such lifestyle decisions as encouraging a healthful diet and physical activity, and avoiding lotions, shampoos and other products for children and babies that contain phthalates, which are known endocrine disrupters.
"For younger children and the tweens, they should probably live a little bit greener," Biro said. "People could eat together as families — not avoiding fast food, but minimizing it to once a week — and families could engage in regular physical activity."